AAPM REPORT NO. 25

PROTOCOLS FOR THE RADIATIONSAFETY SURVEYS OF

DIAGNOSTIC RADIOLOGICALEQUIPMENT

Published for theAmerican Association of Physicists in Medicine

by the American institute of Physics

AAPM REPORT NO. 25

PROTOCOLS FOR THE RADIATIONSAFETY SURVEYS OF

DIAGNOSTIC RADIOLOGICALEQUIPMENT

A REPORT OF THE DIAGNOSTICX-RAY IMAGING COMMITTEE

TASKGROUP

Pei-Jan Paul Lin (Chairman), Northwestern University, MedicalSchoolKeith J. Strauss (Co-Chairman), The Children’s Hospital, BostonBurton J. Conway, Center for Devices and Radiological HealthJane R. Fisher, Polyclinic Medical Center, HarrisburgRobert J. Kriz, University of Illinois, Hospitalat ChicagoMary E. Moore, Cooper Hospital, University Medical Center, CamdenDenny Dean, Illinois Department of Nuclear SafetyLincoln B. Hubbard, FGHB, Inc., Downers GroveKenneth L. Miller, Pennsylvania State University, M.S. Hershey

Medical Center

May1988

Published for theAmerican Association of Physicists in Medicine

by the American Institute of Physics

DISCLAIMER: This publication is based on sources andinformation believed to be reliable, but the AAPM and theeditors disclaim any warranty or liability based on or relat-ing to the contents of this publication.The AAPM does not endorse any products, manufac-turers, or suppliers. Nothing in this publication should beinterpreted as implying such endorsement.

Further copies of this report may be obtained from

Executive OfficerAmerican Association of Physicists in Medicine

335 E. 45 StreetNew York, NY 10017

Library of Congress Catalog Card Number: 88-071433International Standard Book Number: 0-88318-574-1

International Standard Serial Number: 0271-7344

Copyright © 1988 by the American Associationof Physicists in Medicine

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmittedin any form or by any means (electronic, mechanical,photocopying, recording, or otherwise) without the

prior written permission of the publisher.

Published by the American Institute of Physics, Inc.,335 East 45 Street, New York, New York 10017

Printed in the United States of America

Contents

Introduction and General Clarifications

Introduction ------------------------------- 1

General Clarifications --------------------- 1

Part I. Medical Radiographic Installations

I-l. Introduction ------------------------- 3

I-2. System Information ------------------- 3

I-3. Beam-on Control Indicators ----------- 4

I-4. Shielding Devices -------------------- 4

I-5. The Source-to-Image ReceptorDistance ----------------------------- 5

I-5-A. Fixed Radiographic Unit ------------ 5I-5-B. Mobile/Portable Unit --------------- 6

I-6. Radiation Beam Restrictors andLight Localizers --------------------- 6

I-6-A-C. X-ray/Light Field Congruence,X-ray Field/Image ReceptorCentering, and Numerical IndicatorAccuracy --------------------------- 6

I-6-D. Positive Beam LimitingCollimators ------------------------ 8

I-7. Primary Radiation BeamCharacteristics --------------------- 11

I-7-A. Determination of HVL -------------- 13I-7-B. Simplified Filtration Test--------- 14I-7-C. Radiation Exposure and Expected

output ----------------------------I-7-D. Instrumentation ------------------- 15

I-8. Radiographic Technique Chart -------- 17

I-9. Identification of the PersonConducting the Evaluation ----------- 17

I-10. Recommendations and Suggestions ----- 17

Radiographic Installation Checklist -------- 19

Part II. Medical Fluoroscopic Installations

II-l. Introduction---------------------------- 22

II-2. Exposure Switches and Interlocks ------- 22

II-3. Fluoroscopic Timer --------------------- 22

II-4. Lead Protective Devices ---------------- 23

II-5. Minimum Source to Skin Distance(SSD) ---------------------------------- 23

II-5-A. Measurement of SSD ------------------- 24

II-6. Radiation Beam Restriction andAlignment ------------------------------ 25

II-6-A. Test Method -------------------------- 25Fluoroscopic Mode ------------------ 25Spot Film Mode Alignment ------------- 26Spot Film Field Sizing --------------- 27

II-6-B. Evaluation of Congruence BetweenX-ray Field/Field of View ofImaging Chain ------------------------ 27

II-6-C. Evaluation of SpotfilmAlignment ---------------------------- 28

II-6-D. Evaluation of Spotfilm FieldSizing ------------------------------- 28

II-6-E. Alternate Test Methods --------------- 29

II-7. Radiation Output and BeamCharacteristics ------------------------ 29

II-7-A. Determination of HVL ----------------- 29Units with Manual ERC ---------------- 30Units without Manual ERC ------------- 30

II-7-B. Entrance Exposure Rate --------------- 31II-7-C. Entrance Exposure Rate

Measurements ------------------------- 32II-7-D. Qualitative Evaluation of Automatic

Exposure Rate Control Devices -------- 33

Fluoroscopic Installation Checklist ---------- 34

Part III. Dental Radiographic Installations

III-l. Introduction ------------------------- 38

ii

III-2. Beam-on Controls and Indicators ------ 38

III-3. Minimum Source-Skin Distance:Intraoral Unit ----------------------- 38

III-4. Source to Image Receptor Distance:Cephalometric Unit ------------------- 39

III-5. Radiation Beam Restrictors ----------- 39III-5-A. Intraoral System ------------------- 39III-5-B. Panoramic System ------------------- 40III-5-C. Cephalometric System --------------- 40III-5-D. Labeling of Cones or Apertures ----- 40

III-6. HVL Measurement ---------------------- 41

III-7. Patient Exposure --------------------- 41

III-8. Multiple Tube Configuration ---------- 43

III-9. Mechanical Support ------------------- 43

III-10. Shielding Devices -------------------- 44

Dental Radiographic Installation Checklist --- 45

Part IV. Measurement of Area Radiation Levels

IV-l. Introduction ------------------------- 48

IV-2. Leakage Radiation -------------------- 48

IV-3. In-room Scattered RadiationMeasurement -------------------------- 50

IV-4. Protective Barrier/ShieldingAssessment --------------------------- 50

IV-S. Instrumentation ---------------------- 51

Area Radiation Level Checklist --------------- 53

References ------------------------------------------ 54

iii

Introduction and General Clarifications

Introduction

The task group: "Protocols on The Radiation SafetySurvey of Diagnostic Radiological Equipment" was formedby the Diagnostic X-ray Imaging Committee to provide aunified approach to radiation surveys of x-ray imagingequipment conducted by radiologic physicists. Stationaryand mobile radiographic, fluoroscopic, dental x-ray, andmammographic equipment are covered. Due to itsspecialized requirements computed tomography (CT)equipment is not.

The protocols described are intended to verifycompliance with the radiation safety regulations outlinedin the Code of Federal Regulations and the majority ofstate agencies. State regulations are not uniform fromstate to state, and federal are revisedperiodically.

regulationsTherefore, it is recommended that the

appropriate state and federal radiation controlregulations be reviewed prior to conducting surveys.Also, in view of the complexity and costequipment, advice should be solicited

of x-rayand/or the

operator's manual referred to when the surveyor is notfamiliar with a given type of equipment.

The objective of the report is to achieve a unifiedapproach to routineequipment and was

radiation surveys of x-ray imagingreflected in the task

membership, which included:group's

(a) Representatives from regulatory agencies,(b) Medical (diagnostic radiologic) physicists, and(c) Medical health physicists.

The CRCPD (Conference of Radiation ControlDirectors,

ProgramInc.) has published radiation survey protocols

for use by state-employed inspectors which may be usefulin certain situations.

General Clarifications

"Good Radiological Practices" set forth by the JointCommission on Accreditation of Hospitals (JCAH) requirethat patients and personnel are protected from radiation,electrical, and mechanical hazards associated with x-rayequipment and that quality imagesradiation survey protocols

are produced. Thedescribed in this report

address only the radiation hazards question.To assist hospitals in their quality assurance

efforts, most diagnostic radiologic physicists also:1. Check the x-ray generator calibration (kV, mAs

linearity, phototimer, etc.),

1

2. Determine typical patient exposure levelsfor standard radiographic exams and fluoroscopy,

3. Assess fluoroscopic contrast and spatialresolution,

5.4. Evaluate radiographic image quality, and

Review processor quality control procedures.These quality assurance efforts are beyond the scope ofthis report.

The performance levels cited from Reference 1 do notapply to non-certified components. In the absence ofspecific state regulations for thesecertified component performance levels

components,should be

considered "ideal"may not be

goals for this type of equipment, butachievable due to design limitations.

In addition to survey data, records should also bemaintained on:

(a) the manufacturer, model number, and serialnumber of the transformer/generator andcontrol console,

(b) the manufacturer, model type and serial numbersof the x-ray tube housing and insert,

(c) the model type and the serial number of thecollimator, and

(d) the manufacturer, model type and serial numberof the x-ray image intensifier.

Outlines of these records and survey data are at the endof each "part" of this report.

Unless specifically noted in the text or outline,each parameter should be checked at least annually orafter machine repairs orreplacement) which could affectused equipment

adjustments (e.g., x-ray tubeperformance. Heavily

checks.may require more frequent (semi-annual)

Measurements of parameters such as the half-valuelayer (HVL), the source-to-image receptor distance (SID),etc. that are common to all four radiologicalare discussed in detail in Part I -- Medical

systems

Installations. Recommended modificationsRadiographic

in themeasurement methods for unique types of radiologicalequipment are listed in the section where the unique typeof equipment is discussed.

2

Part I: Medical Radiographic Installations

I-l. Introduction

Pages 19-21 contain a checklist of the physicalparameters to be evaluated and the information pertinentto the radiation protection survey of radiographicinstal-lations. The record keeping informationidentified in the checklist normally has practical valuein identifying the radiographic unit under evaluation.If information concerning the certification status of theunit or warning labels affixed to the control panel,etc., is desired, additional space should be provided onthe form developed by the radiologic physicist.

I-2. System Information

Items A-D of Part I-2 of the checklist are "recordkeeping' items to identify the survey of a specific roomin an institution and the date the survey is conducted.The generator/control manufacturer, model type or number,and serial number should be recorded. The model type maybe needed to verify technical specifications of the unit, such as

(a) Single phase or three phase,(b) Falling load and/or constant load,(c) Minimum switching time, etc.The maximum tube potential and the maximum tube

current of the generator are useful in setting up aradiographic technique chart for various examinations aswell as for radiation protection considerations whenrequired. Depending on the needs of thehospital, the assumed

specific"maximum tube potential and

current" may be the maximum technique factors employed inthe radiographic examination room.

The model numbers of the x-ray tube insert andhousing allow the identification ofcharacteristics of the x-ray

thermaltube. The serial number of

an x-ray tube is important since x-ray tubes are replacedupon failure. The radiation output varies from one x-raytube to another especially when the comparison is madebetween an old and new x-ray tube. The nominal focalspot sizes identified by the manufacturer, not the actualmeasured sizes, should be recorded. Any added filtrationin the x-ray beam designed to be selected by the operatorshould be recorded since it affects the machine'sexposure rate.

I-3. Beam on Controls and Indicators

Verify the availability of a positive indicator ofx-ray production when the x-ray tube is energized, suchas:

(a) mA-Meter,(b) X-ray "on" light, and/or(c) Audible signal during or at end of exposure.

Test their function by making x-ray exposures. Any ofthese three warning systems which are provided should befunctioning. The x-ray exposure switch should be adeadman type switch; the exposure must terminate whenpressure on the switch is released.verified on a radiographic tube

This can only be

time is set toif a l-2 second exposure

allow the operator time to release theswitch prior to termination of the exposure by the timer.(When conducting this testloading.) A deadman switch is

avoid excessive tubenot used nor required in

conjunction with rapid film changers; a separate switchis provided to stop the exposure series. Unsteadypressure on a deadman switch in this application couldprematurely terminate the exposure series after theinjection of contrast media.

If the exposure switch is attached to a cord, thecord or switch should be fastened to the control consoleor be short enough to prevent the radiographer fromextending any part of his/her body outside the protectivebarrier. If the exposure switch is a built-in type, thecontrol panel should be installed so that theradiographer is confined behind the protective barrier.

For a mobile/portable x-ray unit, the radiographermust be able to stand at least 6 feet away from the x-raytube/beam during the actual exposurescattered radiation reaching him/her.

to minimize anyThis normally is

accomplished by attaching the exposure switch to the unitwith at least a six foot long cord.

I-4. Shielding Devices

Lead or Lead equivalent protective aprons and glovesof at least 0.25 mm Pb (Reference 12) should be readilyavailable for the protection of occupationally exposedpersonnel or parents of children in the procedure roomduring exposures. Appropriate gonadal shields should bepresent without exception for use with children andpatients under forty-five. At least one apron andgonadal shields should be kept with each mobile unit.Other appropriate shielding devices should be readilyavailable for use in fluoroscopic andfacilities.

angiographic

The surveyorshielding devices.

should verify the presence and use ofThe surveyor should check or monitor

the checking of protective apparel. The shieldingapparel should be examined fluoroscopically. The resultsshould be documented as required by the JCAH. Thesechecks should be completed at least annually.

I-5. The Source-to-Image Receptor Distance

Most general radiographic units have a variable SID(source-to-image receptor distance). Head units oftenhave a fixed 32" (81 cm) or 36" (91 cm) SID. Chest unitsusually have a fixed SID of 72" (180 cm). On variableSID units, the SID indicator should indicate the distancebetween the focal spot and the film cassette in thecassette holder in the table or on the wall.

Four basic types of SID indicators are available.The first type is interfaced with the telescopic motionof the x-ray tube hanger assembly; the SID distance isdisplayed on the tube hanger carriage or on the x-raytube positioning handle. The second-type of indicator isa linear scale or single marks labeled 40" (100 cm), 72"(180 cm), etc., attached to the telescopic support arm.A magnetic or a mechanical lock/detent is the third typeof indicator. A marking on the overhead tube rail and ora detent should be provided for all wall cassetteholders.

A tape measure is the fourth type of SID indicator.It should be available to determine the SID whenever thefilm cassette is placed in locations other than the tableor wall cassette holder.

The indicated SID on any and all of the typesemployed on a unit should be within 2% of the measuredSID (Reference #l).

The range of SIDs used should be within the focaldistance range of the grid. For example, if a wallcassette holder with a 72" (180 cm) focused grid designedfor chest imaging is used for 40" (100 cm) uprightabdomens, grid cut off occurs which needlessly increasespatient exposure,

I-5-A. Fixed Radiographic Units

To evaluate any of the above SID indicatoraccuracies, the location of the focal spot must be known.On some x-ray tube housings, the focal spot location ismarked with a dot or a line at the factory. Thus the SIDindicator accuracy can be determined by directmeasurement,

If the focal spot location istriangulation can be used to locate it.

not marked,Several simple

5

devices have been developed to expedite this. Hendee andRossi, for example, employ a plastic cylinder formeasuring SID (Reference #3). Localization of the focalspots on tubes should be performed as part of acceptancetesting of newly installed x-ray tubes. A repeat of thismeasurement usually is not necessary until the x-ray tubeis replaced.

i-5-B. Mobile/Portable Unit

For (non-dental) mobile or portable x-ray units theminimum x-ray source-to-skin distance (SSD) shall be noless than 12" (30 cm) (Reference #1). This can bemeasured directly with a tape measure provided thelocation of the focal spot is known.

I-6. Radiation Beam Restrictors and Light Localizers

This section describes the evaluation ofcollimators. These devices restrict the x-ray beam andnormally use a light source and mirror arrangement toindicate the direction and extent of the x-ray field.The following parameters need to be evaluated:

(a) Congruence (alignment and size) of x-rayand light localization fields.

(b) Alignment of the central ray and the centerof the image receptor.

(c) Agreement of the collimator dial field sizeindication and the actual x-ray field size.

(d) Agreement of x-ray field size and imagereceptor size on units equipped withPositive Beam Limiting (PBL) collimators.

I-6-A-c. X-ray/Light Field Congruence, X-ray Field/ImageReceptor Centering, and Numerical IndicatorAccuracy

The first three collimator parameters listed inSection I-6 (a-c) can be evaluated at the table using tworadiographs.

1) Align the x-ray tube assembly to the centerof the table or wall cassette holder at anSID that is clinically used. Most unitsare equipped with a light mounted on the x-raytube support assembly to indicate when theradiation field is centered.

2) Center a 35 x 43 cm (14 x 17") cassette withfilm in the cassette holder.

6

3)

4)

5)

6)

8)

The

Set a standard x-ray field size, 18 x 24 cm(8 x 10"), using the appropriate numericalindicator scale on the collimator.Turn on the collimator light field and placeradiopaque markers on the table top todelineate the edges of the light field.Expose the cassette with a technique thatresults in an optical density range of l-l.4on the film.Open collimator blades and without movingthe markers or cassette make a secondexposure using l/5 the mAs used in step 5.This produces an image of the opaque markers.

7) Process the exposed film.Repeat steps 2-7 using a larger x-ray fieldsize, 30 x 35 cm (11 x 14").

two field size evaluation is necessary since thecollimator blades on most units do not move in a linearmanner. Any locks routinely used during this test whichdo not hold properly should be noted in the"Recommendations and Suggestions" at the end of thesurvey report.

X-ray/Light Field Congruence: I-6(a)

The size of the delineated light field and the sizeof the x-ray field on the exposed radiographs shouldagree within 2% of the SID (Reference #l). The shiftbetween the centers of the light field and x-ray fieldshould also be no greater than 2X of the SID (Reference#l). Both x-ray films should be evaluated. Thisparameter should be measured at least annually or afterrepairs which involve the removal of the collimator fromthe x-ray tube housing.

After the x-ray/light field congruence at the tablecassette holder has been verified, the light field of thecollimator may be used to verify the alignment of the "x-ray field" and center line of any wall mounted cassetteholders within the room.

X-ray Field/Image Receptor Alignment: I-6(b)

Draw two sets of diagonals on each radiograph. Oneset should connect the corners of the x-ray field on eachradiograph; this represents the x-ray field center. Theother set should be drawn to the corners of the x-rayfilm; this indicates the center of the Bucky tray. Thecross points of each set of diagonals should not beseparated by more than 2% of the SID (Reference #1).This parameter should be evaluated at least annually orafter any repair which could affect the alignment light

7

on the x-ray tube assembly.

X-ray Field/Numerical Indicator Size: I-6(c)

Measure the length and width of the x-ray field oneach film. These dimensions should agree with thenumerical values originally set in step 3 above to within2% of the SID (Reference #l). Most units have more thanone numerical indicator scale. Each of these scalesreferences a specific SID. The units displayed on thescales (metric vs English) should match the units used tosize the typically used cassettes. This parameter shouldbe checked-at least annually or after repairs to thecollimator blade assembly or dial indicator assembly.

Other Test Methods

Hendee and Rossi (Reference #3) and Gray et al.(Reference #4) have described similar collimator tests.The X-ray Field/Image Receptor Alignment can also beevaluated by closing one pair of the collimator blades toobtain a slit beam and making an exposure. Prior tomoving the cassette, a second exposure is made with theopposing pair of collimator blades now open and theoriginal pair closed. This double exposure techniqueproduces a radiographic cross hair which indicates thecenter of the x-ray field.

The X-ray/Light Field Congruence of the collimatorcan also be measured without exposing x-ray film if filmprocessing is unavailable. Four fluorescent strips (2 x10") with appropriate reticule lines drawn in thelengthwise direction can be used. The lines are placedat the edge of the collimator light field. This methodis faster than the above method, but may require anobserver at table side to view the fluorescent emissionduring the radiographic exposure. (Appropriate shieldingshould be used.) A misalignment of l/8" can be detecteddepending on how the reticule lines are drawn on thefluorescent strips. With care one can also directlymeasure the distance between the reticule lines to checkthe numerical field accuracy.

I-6-D. Positive Beam Limitation (PBL) Collimators

Both PBL and manual collimators must be testedaccording to Section I-6-A-C. In addition, PBLcollimators must confine the radiation beam size to thesize of the image receptor. The automatic setting of thex-ray field size should also be accurate for any distancebetween the x-ray tube focal spot and cassette which iswithin the SID range of the PBL system.

8

Three different test methods which require nospecial instrumentation are described below.

Test Method I1) Place the collimator in automatic mode.2) Center a small cassette, 18 x 24 cm

(8 x 10"), in the cassette tray.

4)3) Center a larger cassette on the table top.

5)Make an exposure.Process the film. If the x-ray fieldsize exceeds the film size in the cassettetray, use triangulation and the radiographfrom the table top to determine the actualx-ray field size at the cassette tray.

6) Repeat steps 2-5 with the cassettes rotated90 degrees in the cassette tray and on thetable top.

7) Repeat steps 2-6 with a larger cassette inthe cassette tray.

Test Method II1) Place the collimator in automatic mode.2) Center a small cassette 18 x 24 cm (8 x 10")

in the cassette tray.3) Disable the automatic PBL feature.4) Remove the small cassette and place a

larger cassette with film in the cassettetray.

5) Make an exposure and measure the x-rayfield size on the film.

6) Repeat steps l-5 with the two cassettesrotated 90 degrees in the cassette tray.Repeat steps 1-6 with a larger cassette'

7) 30 x 35 cm (11 x 14") in cassette trayinitially.

Test Method IIISince the Numerical Indicator accuracy was verified

in I-6-A-C, these indicators can be used to verify PBLsizing.

2)1) Place the collimator in automatic mode.

Center a small cassette, 18 x 24 cm (8 x 10"))in the cassette tray.

3) Record the dimensions of the x-ray fieldfrom the numerical indicators.

4) Repeat steps l-3 with cassette rotated 90degrees in the cassette tray.

5) Repeat steps l-4 with a larger cassette, 30 x35 cm (11 x 14") in the cassette tray.

9

Caution: Some PBL collimators do not have a key designedto-disable their automatic sizing as required in step 3of Method II. If this key is not provided and theautomatic collimation feature cannot be readily disabled,method I or III must be used. Method I has the sameprecision, but less accuracy than Method II. Method IIIis both less precise and less accurate than Method II.

Regardless which method is used, the PBL sizingevaluation should be completed by placing the appropriatesize cassettes in each cassette holder (eg. table, wallunit, other, etc.) which is interfaced to the PBLcollimator. If any of these cassette holders are used atmore than one SID, the PBL sizing for each commonly usedSID should be checked.

The above methods cannot evaluate the PBL sizing fora 35 cm x 43 cm cassette without additional effort.Special PBL testing devices have been fabricated toaddress this and the numerous films which must be made.Lin, Kriz and Storzum (Reference #5) have described onedevice. Hendee and Rossi (Reference #3) and Gray et. al.(Reference #4) describe test methods similar to Method I.

The measured length or width of the x-ray field inthe plane of the image receptor compared to the length orwidth respectively of the film in the cassette tray mustagree to within 3% of the SID used. In addition, theabsolute value of the sum of the length and widthdifferences must not exceed 4% of the SID (Reference #l).

If the collimator is designed to allow the operatorto override the PBL function, a key must be ided forthis function.

provWhen the PBL function is overridden, the

collimator must not allow removal of the key (Reference#1).

PBL collimators allow the operatorreduce the x-ray field size with

to manually

receptor in the cassette holder without respect to the image

override key,using the

When this is done, the collimator shouldrevert to the normal PBL mode when the cassette or SID ischanged (Reference #l). Both functions should be checked.

Most units are designed to bypass the PBL functionwhen the central x-ray is not perpendicular to the imagereceptor. As above, the collimator should revert to thenormal PBL mode when the centralperpendicular to the image receptor.

ray is realignedThis function

should also be checked.Some units are equipped with semi-automatic PBL

collimators which do not automatically adjust the x-rayfield size to the image receptor size. Instead, they

10

lock out exposures until the operator has manuallyadjusted the x-ray field to be smaller or equal to thesize of the image receptor.

The function and sizing accuracy of the PBL shouldbe checked at least annually or after any repairs to thePBL sensors in the cassette holder, drive motors in thecollimator, or electronics of the PBL.

I-7. Primary Radiation Beam Characteristics

The penetrating quality of an x-ray beam is characterizedby its half value layer (HVL). Undesirable low energy x-rays are edliminated from the beam by adding a filterwhich increases the HVL.specified in Reference #1.

Minimum half-value layers areThese specifications are

based on recommendations for single phasefound in Reference #2.

generatorsThe minimum required half value

layers for diagnostic x-ray equipment are listed in TableI below (Reference #l).

The minimum HVL requirements can be met if theequipment is installed with the proper amount offiltration in the x-ray tube housing and collimator. Thetotal equivalent Aluminum filtrations that will meet theminimum HVL requirements of Table I are listed in TableII (Reference #2).

No maximum total Aluminum filtration values areestablished. However, excessive filtration needlesslyreduces the x-ray intensity with little gain in theeffective beam energy and possible shortened x-ray tubelife.

The HVL is a function of x-ray tube age (use), thetube voltage waveform, and the kVp. As the x-ray tubeages, the target's surface pits and becomes rough whichincreases the inherent filtration of the tube.Therefore, if the kV calibration of the generator and theadded filtration remain constant during the lifetime ofthe tube, the HVL will increase and the radiation outputwill decrease. Thus, a modest change in HVL from onesurvey to the next does not necessarily mean that the kVcalibration has changed.

The values for minimum HVL in Table I under the"Other X-ray systems" heading for the kVpapply to dedicated mammographic units

range of 30-50(Reference #6).

The radiologic physicistadded filtration is

should verify that Molybdenumused with anode

mammographic x-ray tubes if theMolybdenum

machine allows theoperator to choose,

Materials other than Aluminum (Copper,and rare earth elements),

Molybdenum,or specialty filters (trough

filters, and wedge filters) are occasionally used. Ifsuch a filter is not permanent, it must be removed during

11

the HVL measurement.The test protocols of the following two sections

describe methods to measure the HVL to determinecompliance with minimum filtration requirements. Sincethe HVL is a function of the actual high voltage appliedto the x-ray tube, the protocols assume the unit's kVcalibration is correct. If such is not the case, the

measured HVL may lead to an erroneous conclusion

12

concerning compliance with minimum filtrationrequirements. For example. if the actual kV is lowerthan indicated on the operator's console, the measuredHVL might not meet the minimum HVL specifications eventhough adequate filtration is in the beam. Likewise, ifthe kV is higher than indicated, the measured HVL mightappear to be adequate even though the filtration in thebeam is inadequate.

I-7-A. Determination of HVL

The HVL of the x-ray beam should be measured atleast annually, after replacement of the x-ray tubeassembly, or change in the added filter. The measuredHVL is affected by the amount of scatter radiation at theionization chamber (Reference #7). In three phase unitsthe choice of tube current and exposure time setting alsoaffect the measured HVL due to the effects of capacitanceof the high voltage cables (Reference #21). Theseinfluences can be minimized by using "good geometry"(Reference #7), tube currents greater than 200 mA, andexposure time settings greater than 0.050 sec (Reference#21). Comparison of measured HVLs from survey to surveyare meaningless unless the chosen geometry, tube current,exposure time settings, and kVp remain constant.

Test Method I: Manual Timing1) Disable the Automatic Exposure Control (ARC)

and operate the unit in the manual mode.2) Position the ionization chamber (which should

be calibrated for the diagnostic x-ray energyrange) 100 cm from the focal spot. If 100 cm isnot practical, use the largest distance allowed.The ionization chamber should be free standing,a few inches away from the table top or wallcassette holder to minimize back scatter.

3) Collimate the x-ray field to a size slightlylarger than the ionization chamber.

4) Select an appropriate tube potential(e.g. 80 kVp for radiographic, 30 kVp formammographic), a typical clinically used tubecurrent (eg. 200-400 mA for radiographic, 50-100mA for mammographic), and an exposure timegreater than 50 msec. Make output measurementswith no additional attenuating material betweenthe focal spot and the ion chamber.

5) Make additional output measurements with varyingthicknesses of 1100 alloy Aluminum filterslocated near the face of the collimator.

6) Use interpolation to estimate the HVL.

13

7) Compare the measured HVL against "Table I-Minimum HVL Requirement" to determinecompliance.

Test Method II: Units With No Manual TimingPlace the unit in Automatic Exposure1)

2)Control (AEC).

3)

4)

5)

6)

8)

10)

11)12)

Place the ionization chamber a few inchesaway from table top or wall cassette holderto reduce backscatter. The chamber should bepositioned over the center of the AEC sensor.Collimate the x-ray field to a size slightlylarger than the larger of the ionizationchamber or AEC sensor.Select 80 kVp or the closest available kVsetting.Achieve an exposure in the 10-20 mAs rangeby placing an-appropriate thickness ofAluminum or Copper attenuator in front ofAEC sensors, but behind the ionization chamber.Place an additional 4-5 mm Aluminum(1100 alloy) in front of the AEC sensors at thelocation chosen in step #5.

7) Record the exposure for this 0 mm data point.Remove 1 mm of the 1100 alloy Aluminum frombehind the ionization chamber and place it infront of the chamber near the face of thecollimator.Record the exposure.Repeat steps 8-9 with additional thicknessesof Aluminum removed from behind the chamberand placed in front of it.Use interpolation to estimate the HVL.Compare the measured HVL against Table Iminimum HVL's to determine compliance.

I-7-B. Simplified Filtration Test

If one is interested only in compliance and not inan accurate measurement of the HVL, a two exposuretechnique can be employed.

In step (5) of Test Method I, Section I-7-A, useTable 2 "Half-value layers as a function of filtrationand tube potential for diagnostic units",Reference #8 to determine

on page 17 in

the selected tube potential.the minimum required HVL for

Place this thickness ofAluminum in the beam. If the output of the unit isgreater than one half of the output measured without anyadditional filters in the radiation beam, the minimumfiltration requirement is met. If the exposure is lessthan half the zero filter measurement, additional

14

filtration may be necessary as discussed in Section 1-7-A. This abbreviated test method can also be applied tostep #8 in Test Method II in section I-7-A.

I-7-C. Radiation Exposure and Expected Output

After verifying correct filtration of the unit, theradiation output of the x-ray unit should be measured atseveral tube potentials with the tube current stationsroutinely employed clinically. The ionization chambercan be placed in air 24" (61 cm) from the focal spot forthe 40" (100 cm) SID (clinical) geometry. Use tubepotentials of 60, 80, and 100 kVp. These encompass themost often employed x-ray tube potentials in DiagnosticRadiology except for mammography and high kV chestradiography. If the unit operates only in the AutomaticExposure Control (AEC) mode, the mAs can be adjusted byplacing attenuators directly in front of the AECdetectors and noting the indicated mAs of each exposure.These measurements may be used to estimate the patientexposure when such a need arises.

The expected radiation output in mR/mAs [(1000/kg =(µC/kg)/mAs] of diagnostic x-ray units with differenttypes of kV waveforms has been measured or calculated(References 9, 21, 22, 23, and 24). These expectedvalues are listed in Table III. The final column ofvalues applies to three phase units, constant potentialgenerators, mid frequency units, and battery invertermobile units. All the output data assumes a totalfiltration of 2.5 mm Al and a 24-inch distance betweenthe ionization chamber and x-ray source. The outputvalues in each column can be mathematically predicted by:

Output = C x kVpn [1]

(Reference #24) using the fitted values for the constantsn and C listed in Table III. If the total filtrationexceeds 2.5 mm Al, the expected output values in TableIII should be reduced. For example, at 80 kVp, 15% or10% reductions to the listed values, single or threephase respectively, should be applied for every halfmillimeter of Aluminum filtration added (Reference #24).Technically, radiation output (mR/mAs) is a function of:

(a) High Voltage (kVp) calibration,(b) Tube current (mA) calibration,(c) Exposure timer accuracy,(d) The total filtration in the x-ray tube housing-

collimator assembly,(e) The electrical phase, method of rectification,

and capacitance of high tension cables,(f) The distance from the focal spot, and

15

(g) X-ray tube age.If the measured radiation output deviates more than

40-50% from the appropriate value suggested in Table III,the generator may need a complete kV, mA, and timecalibration. However, previous generator calibration andradiation output measurements should be consulted. Thevalues in Table III are not applicable to falling kVgenerators such as capacitor discharge units which do nothave constant mR/mAs values at different mAs stations.For detailed discussion of x-ray generator performancetesting, refer to AAPM Report No. 14, "PerformanceSpecifications and Acceptance Testing forGenerators and Automatic Exposure Control

X-rayDevices"

(Reference #10).

On three-phase units (excluding grid controlledmachines) residual charge due to the capacitance of the

16

high voltage cables contributes to the actual output ofeach exposure (Reference #21). While this contributionto the measured mR/mAs is less than 5-10% for tubecurrents above 200 mA and exposure time settings greaterthan 0.05 seconds, exposure measurements obtained withtube currents less than 25 mA and exposure time settingsless than 0.005 seconds can be two to three times higherthan the predicted values in the last column of Table III(Reference #21). Due to the influence of timer settingand tube current on expected radiation output, the sametechnique settings should be used from one survey to thenext to track the consistency of radiation output overthe lifetime of the tube.

I-7-D. Instrumentation

A good quality ionization chamber and electrometercapable of integrating the collected charge is necessaryfor HVL and output measurements. Since both theintensity and effective energy of the x-ray beam changesas a function of added filtration, the ion chamber shouldhave a relatively constant energy response between 10-120keV. The collection efficiency should be 95% or greaterfor the peak x-ray intensities to be measured. Inaddition, no linear dimension of the sensitive volume ofthe chamber should exceed 6 cm. This allows the x-rayfield cross section to approximate good geometry duringHVL measurements.

I-8 Radiographic Technique Chart

Verify the availability of a radiographic techniquechart. The chart should contain t he followinginformation:

(a) Exam and projection,(b) Radiographic technique factors as a

function of anatomical size,(c) Type and size of image receptor,(d) SID, and(e) Type and placement of gonadal shielding.

I-9 Identification of the Person Conducting theEvaluation

The report should contain a section that identifiesthe radiologic physicist conducting the survey, his/hertitle, and any appropriate professional certifications.

I-10. Recommendations and Suggestions

A section which lists identified non-compliance

17

items should be included on the survey forms. Arecommendation or a suggestion should accompany each itemin an effort to bring the unit into full compliance withthe effective radiation rules and regulations. It mayalso be necessary to contact service engineers to rectifyidentified problems.

The radiological physicist should also bear in mindthat x-ray equipment operators may not have received anyformal radiological training concerned with the safeoperation of a particular piece of equipment. Therefore,incorrect operating procedures may be found and mayrequire correction.

18

Checklist Outline forRadiographic Installations

I-2. SYSTEM INFORMATIONA. Installation

2.1. Date of survey

Room number

3.Department/Building

4.InstitutionUnit identification number

B. Generator1. Manufacturer

3.2. Model type, model number, serial number

Maximum high voltage (kVp)4. Maximum tube current (mA)

C. X-ray Tube Insert

2.1. Manufacturer

Model type

4.3. Serial number

Nominal focal spot sizesa. Largeb. Small

5. Leakage technique factorsD. X-ray Tube Housing

1. Model number

3.2. Serial number

Added filtration

I-3. BEAM-ON CONTROLS AND INDICATORSA. Beam On Indicators

2.1. mA meter: present and functional?

Warning light: present and functional?3. Audible signal: present and functional?

B. Exposure Switch

2.1. Deadman type

Locationa. Fixed unit: Location within control booth

b. Mobile unit: Length of exposure cord

I-4. SHIELDING DEVICES AND APPARELA. Aprons

2.1. Available

EmployedB. Gloves

2.1. Available

EmployedC. Gonadal Shields

1. Available

D. Mobile Shield2. Employed

19

E. Other Specialized Devices

I-5. SOURCE-TO-IMAGE RECEPTOR DISTANCEA. Focal Spot Localization (when x-ray

tube is replaced)B. SID indicators

2.1. Accuracy of numerical indicators3. Accuracy of detents

C.Accuracy of tape measure

Mobile or portable SSD accuracyI-6. RADIATION BEAM RESTRICTORS AND LIGHT LOCALIZERS

X-ray Field/Light Field Congruence (at leastA.

B.C.

D.

annually or after removal of collimator)2.1. Large cassette

Small cassetteX-ray Field/Cassette Tray Alignment (at leastannually or after repair to alignment light)X-ray Field/Numerical Indicator Accuracy (atleast annually or after repair to collimatorblades or dial indicators)2.1. Large cassette

Small cassettePositive Beam Limitation (PBL) System (atleast annually or after repairs to cassettetray or collimator drive motors orelectronics)1. Table cassette holder

a. Small cassetteb. Large cassettec. Key override functiond. Auto return from manual reductione. Repeat a-d for other clinically used

SIDs2. Wall Cassette Holder

a. Small cassetteb. Large cassettec. Key override functiond. Auto return from manual reductione. Repeat a-d for all clinically usedSIDs

I-7. PRIMARY RADIATION BEAM CHARACTERISTICSA. Half Value Layer at Specified kVp (at least

annually or after replacement of X-ray tube,generator recalibration, or filter thicknesschange)

B. Appropriate Type of Added Filter onMammographic Units

C. Radiation Output: mR/mAs at specified kVp,total filtration, and distance from focal spot

20

(at least annually or as in I-7.A)

I-8. RADIOGRAPHIC TECHNIQUE CHARTA. AvailabilityB. UseC. Completeness

I-9. IDENTIFICATION OF PERSON CONDUCTING EVALUATIONA. NameB. TitleC. Professional Certification

I-10. RECOMMENDATIONS AND SUGGESTIONSI-11. POSSIBLE ADDITIONS

A. In-room Stray and Scatter Radiation Levels(see Section IV-3)

B. Protective Barrier/Shielding Survey (SeeSection IV-4)

C. Leakage Radiation (See Section IV-2)D. Use and Presence of Personnel Monitoring

Devices

21

Part II. Medical Fluoroscopic Installations

II-l. Introduction

Pages 34-37 contain a checklist of the physicalparameters which require evaluation and the informationimportant to the radiationfluoroscopic installations.

protection survey of

information are similar toSome of these parameters and

those ofinstallations. While this part's

radiographic

only the parameterschecklist is complete,

and information unique tofluoroscopic installations are discussed here.

II-2. Exposure Switches and Interlocks

Hand or foot-operated fluoroscopic exposure switchesshall be deadman switches. The exposure switch forradiographic spot filming, full size cassette film, orphotofluorographic camera should be hand operated.However, some units are hand or foot operated. Specialprocedure examination rooms may employ either hand orfoot operated switches for photofluorographicusing a photospot camera or a cine camera.

imaging

The image intensifier spot film device and towermust be interlocked to prevent fluoroscopic exposureswhen the image intensifier tower does not intercept theentire useful x-ray beam cross section (Reference #1).If the system allows the removal of the image intensifierfrom its tower, an additional interlock is necessary toprevent fluoroscopic exposures when the image intensifieris detached, regardless of the tower position withrespect to the x-ray beam. If the unit allows the x-raytube to be taken out of alignment,positioning must also be interlocked.

the x-ray tubeThese interlocks

are easily verified with fluorescentdosimeter intercepting the position

screen orof the useful beam

while simultaneously activating the exposure switch andpositioning the image receptor.

Fluoroscopic x-ray tubes may also be interfaced toproduce radiographic exposures(en.

other than spotfilming35 cm x 35 cm cut film changers). In this case.

the comments made in Section I-3 concerning beam onindicators, non dead-man type exposure switches, andexposure switch location and cord length apply.

II-3. Fluoroscopic Timer

A timer shall be present which presets thefluoroscopic cumulative "ON" time to maximum limit of 5minutes. An audible signal shall warn the operator of

22

the completion of the elapsed preset time. Re-initiationof the fluoroscopic exposure without resetting the timershall restart the audible warning (Reference #l). Older.non-certified units and some certified units may termi-nate the exposure as an alternate to the audible warning.

The operation of the timershould be checked.

and audible warningOne should verify that the timer

activates the signal (or shuts off the-radiation) at thetermination of the preset time. Since this timer may beused to measure the total fluoroscopic exposure time percase, the radiologic physicist should verify the elapsedexposure time accuracy of the timer (exposure time vsindicated time), if exposure time is indicated.

II-4. Lead Protective Devices

Three types of lead protective devices are unique tofluoroscopic equipment. A lead drape hanging from thefluoroscopic tower or other image intensifier supportintercepts scatter radiation from-the patient. It shouldbe in good condition. The operator should be able tomove it so it can be positioned between the operator andpatient at all times. A lead drape normally is not foundin special procedure rooms because its presence mightviolate sterile fields on the surface of the patient.

The shield of the slot for the cassette holder is asecond type of protective device; it intercepts leakageradiation from under table x-ray tubes. The Bucky slotshield may consist of folding steel arms, a hinged steeldoor, and/or a lead or steel erectable panel. Verifythat it is operating properly. One or more of thesedesigns may be found on the 'same unit.

The table-end shield is a third type of protectivedevice which intercepts leakage radiation. While newerfluoroscopic tables have end shields, some older unitsmay not because they are modified radiographic tables.The table end shields may simply be the steel covers ofthe table skirt panels.

II-5. Minimum Source to Skin Distance (SSD)

A minimum allowed Sourcefocal spot to

to Skin Distance (SSD,skin distance) has been established

(Reference #l) within Federal Standards to minimizeentrance skin exposure during fluoroscopic examinations.The minimum SSDs are:

(a) Stationary fluoroscopes - 38 cm (15")(b) Mobile fluoroscopes - 30 cm (12")

"Stationary" means any fixed installation.. Thus, the 38cm minimum SSD applies to remote control over-table x-raytube fluoroscopic systems with or without tomographic

23

capabilities. The 38 cm SSD also applies to lateralplane x-ray tubes found in biplane fluoroscopic systems.Since automatic collimators required on certifiedstationary fluoroscopic units are usually large in size,the minimum SSD in these cases usually cannot beviolated. In fact, many current under table fluoroscopicunits are designed with an SSD of 46 cm (18").

The Federal Standard makes one exception to themobile image-intensified fluoroscopic SSD when used forspecific surgical application. The minimum SSD may bereduced to as little as 20 cm (8") (Reference #l).Please note that this standard applies only to specificsurgical situations. Therefore, the reduced SSD shouldbe allowed only if the surgeon can demonstrate that a 30cm (12”) SSD would render the specific procedureimpossible. Be aware that the surgeon is concerned withsurgical procedures and may be unaware of the highpatient skin exposure at short SSDs.

II-5-A. Measurement of SSD

On a mobile C-arm type fluoroscopic systems, the SSDcan be directly measured if the location of the focalspot is known. If this location is not marked on the x-ray tube housing, triangulation (Section I-5-A) can beused to locate the position of the focal spot.Triangulation may be used to measure the SSD on anundertable X-ray tube fluoroscope. One radiopaque ruleris placed on the table top and another is taped on theunderside of the spot film device (Reference #5). Theirrelative magnification can be varied by raising orlowering the image intensifier tower. When thefluoroscopic image of the markings on the tabletop rulerare twice those on the ruler taped to the spotfilmdevice, the vertical distance between the rulers is equalto the SSD. This method can be used if the SSD exceedsthe maximum vertical distance between the tabletop andspotfilm device by using rulers with appropriately spacedmarkings and an appropriate multiplier. If this test iscompleted when the equipment is new, it should berepeated only if the type of x-ray tube or its mountingis changed.

CAUTION: The image intensifier must be protected fromexcessive radiation levels during the test to protect theTV camera. This can be achieved by using the automaticbrightness control mode and by placing l-2 mm copper orl-2 in Aluminum sheets of appropriate cross section onthe table top.

24

II-6. Radiation Beam Restriction and Alignment

This section describes the evaluation of thecollimator's ability to automatically restrict the x-rayfield size to the size of the selected portion of theimage receptor. This parameter should be evaluated atleast annually or after any repair or adjustment to 1)collimator drive motors or blades, 2) collimatorelectronics, 3) image intensifier tower SID sensingdevice, or 4) cassette holder within spot film device.In addition, this section evaluates the alignment of thex-ray beam central ray with the center of the imagereceptor. Misalignment can occur due to:

(a) Incorrect positioning of the fluoroscopictower with respect to the focal spot,

(b) Incorrect positioning of the imageintensifier within the fluoroscopic tower.

(c) Incorrect positioning of the cassette withinthe conventional spot film device,

(d) Incorrect centering of the photospot camera, or(e) Incorrect centering of the television camera

or monitor.Alignment should be evaluated at least annually or afterany repair or adjustment which could affect alignment ina-e above.

II-6-A. Test Method

The followingused to evaluate

test method (Reference #11) may bex-ray beam alignment and beam

restriction. The outline is listed here to illustratethe scope of this testing procedure. The only testequipment required are commercially available ready packdirect exposure film and a beam restriction test tool.This test-tool simply consists of a 1/2" Aluminumattenuating block 8" x 8" in cross section on 1/2" legs.An orthogonal set of slide channels are cut into theblock to accept radiopaque bars made of brass.

Fluoroscopic Mode1) Adjust the tower vertically to result in the

minimum SID.2) Place the test tool on the tabletop and

select the image intensifier's largest fieldof view. Record the distance from the topof the test tool to the level of the cas-sette in the spotfilmer. Record thedistance from the tabletop to the top ofthe test tool.

25

3)

4)

5)

6)

7)

8)

9)

10)

11)

12)

During fluoroscopy, center the test tool.This process is normally easier if one ofthe two pair of collimator blades is closedto give a "slit image" with edges parallelto one of the orthogonal channels.Lock the fluoroscopic tower in place whencentering is complete. Tape the test toolto the tabletop.Place the collimator in the automatic modeof operation. If automatic mode is notpresent, open collimator completely usingthe manual mode.Insert the brass radiopaque slides in theirchannels of the test tool.During fluoroscopy, adjust the slidesuntil their edges are just visible at theedge of the fluoroscopic image.Place a penny on the test tool within thefluoroscopic field of view. Place a readypack film on ton of the test tool andpenny. The penny is used to determineorientation on the processed film.Make a fluoroscopic exposure of approximately100 mAs at 80 kVp.Process the test film: verify sufficientdensity on the film to image-the test tooland radiation field.If the image intensifier has more than onefield of view (eg. 9"-6"-4"), repeat steps6-10 for each remaining field of view.If the system is equipped with automaticcollimator sizing as a function of SID,repeat steps 1-11 with the tower positionedvertically at the maximum SID.

Spot Film Mode Alignment13) Repeat steps l-4 above.14) Place a loaded cassette in the spot film device.15) Select the full size spot film mode.16) With the collimator in the manual mode reduce

the size of the x-ray field until the edges ofthe collimator blades are visible on thefluoroscopic image.

17) Expose, process, and verify density on thespot film.

18) Repeat steps 14-17 for each available spotfilm format. For each format make enoughexposures to fill the image receptor (eg.four exposures when the 4 on 1 mode isselected).

26

19) If any question concerning alignment as afunction of SID exists, steps 13-18 shouldbe repeated with the tower positioned verticallyat the maximum SID.

Spot Film Field Sizing20) Repeat steps l-4 above.21)

22)

23)

24)

25)

26)

27)

Place an unloaded cassette in the spot filmdevice.Place the collimator in the automatic modeand select the full size spot film mode.Place a ready pack film on top of the testtool on the table top.Make three exposures using the technique ofstep 17.Process the film.density.

Verify the correct

Repeat steps 21-25 for each availablespot filmer formatRepeat steps 20-26 with the tower positionedvertically at the maximum SID.

The above test method is used when the x-ray tube isunder the tabletop. The test method, steps l-12 and 20-27, also assumes automatic collimation is present and isthe routine choice for clinical work. If this is not thecase, the unit should be evaluated in the manualcollimator mode. If the x-ray tube is overhead, (eg.remote unit) the ready pack film must be placedunderneath the test tool directly on the tabletop.Normally, a mobile fluoroscope can be evaluated with thex-ray tube below the image intensifier. A stretcher canbe used in this case to provide a tabletop.

II-6-B. Evaluation of Congruence Between X-ray Field/Field of View of Imaging Chain

Steps l-12 of Section II-6-A allow evaluation of thecongruence between the x-ray field and each field of viewof the imaging chain as a function of the SID. On theimages, the inside edges of the radiopaque slides markthe field of view of the imaging chain. The darkenedarea on the film defines the actual radiation field sizeand location. The center of the radiation field isdetermined by drawing diagonals from the corners of thedarkened area. The center of the test tool marks thecenter of the field of view of the imaging chain.

For certified equipment, the congruence of theradiation field size with respect to the imaging chain'sfield of view shall be within an accuracy of 3 percent ofthe SID. The sum, without regard to sign, of thesedifferences along any two orthogonal dimensions

27

intersecting at the center of the field of view shall notexceed 4 percent of the SID (Reference #1).

II-6-C. Evaluation of Spotfilm Alignment

Steps 13-19 of Section II-6-A allow evaluation ofthe alignment of the central ray of the x-ray field andthe center of each selected portion of the image receptorwithin the spotfilm device as a function of the SID. Oneach image, the diagonals drawn from the corner of eachdarkened area mark the center of the x-ray field. Thecenter of the selected portion of the film can bedetermined from diagonals and measurements on the film.The alignment of the x-ray field center and center of theselected portion of the image receptor in the spotfilmdevice should agree to within 2% of the SID (Reference#1).

II-6-D. Evaluation of Spotfilm Field Sizing

Steps 20-27 of Section II-6-A allow evaluation ofthe size of the x-ray field with respect to the size ofeach selected portion of the image receptor within thespotfilm device as a function of the SID. On the imagesone can measure the x-ray field size at the location ofthe test tool. The magnification factor can becalculated since the following distances are known:

(a) From focal spot to tabletop (Section II-5),(b) From tabletop to test tool top, and(c) From test tool top to image receptor.

The magnification factor is applied to the x-ray field atthe test tool to calculate the field size at the imagereceptor level in the spotfilmer. Either the length orthe width of the x-ray field shall not differ by morethan 3% of the SID with respect to the length or thewidth of the selected portion of the image receptor. Thesum of the absolute values of these length and widthdifferences shall not exceed 4% (Reference #1).

CAUTION: This parameter cannot be measured by evaluatingthe degree of overlap of individual exposure fields madeon a loaded cassette placed in the spot film device.Many units contain a second square or rectangulardiaphragm between the patient and image receptor in thespotfilm device. If present, this diaphragm reduces thex-ray field to the appropriate size at the image receptorin the spotfilm device if the x-ray field size is toolarge at the patient position.

28

II-6-E. Alternate Test Methods

The test method briefly described in II-6-A is onlyone of many. While the test equipment required for thismethod is minimal, many images must beprocessed to completely verify

exposed and

device and imaging chain.a conventional spot film

Lin has described in detailthe design and use of more expensive test tools whichdrastically reduce the time required to complete thetesting (References #5 and #12). Hendee and Rossi(Reference #11) also suggest alternate test tools whichreduce testing time. Reference #4 by Gray et. al. isanother source of alternate test methods.

II-7. Radiation Output and Beam Characteristics

With the exception of the paragraph on mammographyunits, all the general comments in Section I-7 apply alsoto fluoroscopic x-ray beams.

II-7-A. Determination of HVL

The HVL of the fluoroscopic x-ray beam should bemeasured at least annually,ray tube assembly,

after replacement of the x-after generator recalibration, or

change in the added filtration. The measurement shouldbe made in "good geometry" (Reference #7). The effectsof high voltage cable capacitance on the measured HVL onthree phase units (Reference #21) can be eliminated bycollecting data with the rate mode of the dosimeter.

On single phase units the measured HVL decreases asthe fluoroscopic tube current increases due to the cablecapacitance of the high voltage cables (Reference #22).HVL measurements obtained with the dosimeter in the ratemode on these units do not avoid this effect; these HVLcompliance measurements should use the maximumfluoroscopic tube current setting.

Comparison of fluoroscopic measured HVLs from surveyto survey are meaninglesskVp, and on

unless the chosen geometry,single phase units, tube current remain

constant.

CAUTION: During measurement of the HVL of a fluoroscopicbeam, an appropriate attenuator should be attached to theface of the image intensifier avoid excessiveradiation rates at its input. Failure to do this coulddamage the image intensifier television chain during HVLmeasurements. See Section I I - 5 - A f o rthicknesses of this attenuator.

suggested

29

HVLs for Units With Manual Exposure Rate ControlsFluoroscopic systems with manual exposure rate

controls allow use of Test Method I in Section I-7-A withsome minor geometry changes. The image intensifiershould be set at its maximum SID on under table x-raytube systems. The added Aluminum test filters are put onthe table top while the ion chamber is placed halfwaybetween the table top and spotfilm device. (On overtable x-ray tube systems, the geometry suggested inSection i-7-A does not require change.) Fluoroscopy canbe used to center the dosimeter, filters, and x-ray beamand to minimize the x-ray field size. An exposure ratemeasurement during routine fluoroscopy (set kV and mA) oran integrated exposure measurement using the spot filmmode (set kVp, mA, and time) can be used.

HVLs for Units Without Manual Exposure Rate ControlsChanging the thickness of test filtration in the

primary beam of most Automatic Exposure Rate Controlledsystems (AERC) results in a change in the kVp whichchanges the HVL. Therefore, in the AERC mode ofoperation, the total thickness of added filtrationbetween the focal spot and image intensifier must remainconstant.

HVL Test Method I: Noninvasive, 80 kVp Test Point1) Extend the imaging chain tower to its

2)

3)

4)

5)

6)

maximum SID.Place the ionization chamber halfway betweenthe spot film device and tabletop on undertablex-ray tube systems. Place it halfwaybetween the tabletop and focal spot onovertable x-ray tube systems.Tape two or three 0.8 mm (0.03") copper sheetsand 4-5 mm 1100 alloy Aluminum sheets ontothe underneath side of the image inten-sifier tower for undertable systems.Place these attenuators on the tabletopon overtable systems.Adjust the collimator during fluoroscopyto minimize the x-ray field size at theionization chamber.Add or subtract Copper until 80-85 kVpis obtained during fluoroscopy.adjustment in kVp can be achieved byreducing the SID. (77-83 kVp is ac-ceptable for this measurement.)Record the exposure rate; thisis the 0 mm Al data point.

30

7) Complete steps #8-#12 of Test Method IIfound in Section I-7-A.

HVL Test Method II: Noninvasive, Maximum kVp Test Point1) Use identical geometry described in steps

1 and 2 in Test Method I, Section II-7-A.2) Place an Aluminum or Copper attenuator

(1" or 1 mm thickness respectively) inthe beam to protect the image intensi-fier from unattenuated x-rays.

3) Adjust the collimator during fluoroscopyto minimize the x-ray field size at theionization chamber.

4) Remove attenuators of step 2 and placeLead attenuator directly in front of imageintensifier to drive the system tomaximum kVp during fluoroscopy.

5) Record the kVp indicated and exposureduring fluoroscopy.

6) Complete steps 5 through 7 of Test MethodI, Section I-7-A.

The two test methods described above are noninvasive.The following method requires the AERC circuitry to beoverridden so the kV and mA can be controlled manually.

Test Method III: InvasiveMost fluoroscopic systems with only AERC contain an

override that enables manual control of the fluoroscopictechnique factors. In the override manual mode, TestMethod I of Section I-7-A may be used.override is usually a

However, theservice and maintenance feature

which is not readily accessible to the operator.

II-7-B. Entrance Exposure Rate

After verifying that the total filtration of theunit is in compliance with Federal Standards. the patiententrance exposure rate must be measured. The entranceexposure rate of fluoroscopic equipment with automaticexposure rate control (AERC) shall not exceed 10 R/min(2.6 mC/(kgmin)) (at the point where the center of theuseful beam enters(Reference #l). If an optional

the patient during fluoroscopyhigh exposure rate

control is present, the entrance exposure rate shall not

rate control is activated (Reference #l). Continuousexceed 5 R/min (1.3 mC/(kgmin)), unless the high exposure

manual pressure by the operator shall be required toactivate the high exposure rate control (Reference #l).During the activation of this mode, a continuous audiblesignal to the fluoroscopist shall be available. FederalStandards do not specify a maximum entrance exposure rate

31

for the high exposure rate mode.considered good practice to

However, it islimit this mode to 10 R/min

(2.6 mC/(kgmin)) unless a specific clinical need has beenidentified.

The entrance exposure rate of fluoroscopic equipmentwithout AERC, shall not exceed 5 R/min (1.3 mC/(kgmin))during fluoroscopy unlessoptional high exposure rate

the unit is provided with ancontrol (Reference #l). If

the unit is provided with this option the comments abouthigh exposure rate control in the previous paragraphapply.

The Federal Standards also specify the location ofthe entrance plane of the patient at which the abovemaximum exposure limits apply (Reference #l). Theionization chamber shall be positioned one cm above thetabletop or cradle if the x-ray source is installed underthe tabletop. If the source is installed above thetable, the ionization chamber shall be positioned 30 cm(12") above the tabletop with the end of the beam-limiting device positioned as closely as possible to theionization chamber. If the fluoroscope has C-arm typegeometry, the ionization chamber should be positioned 30cm (12") from the input surface of the image intensifierassembly.

While the entrance plane for lateral fluoroscopes isnot defined in the Federal Standard (Reference #l), an"FDA Compliance Policy G u i d e " w a s issued tomanufacturers, assemblers,1977.

and field test personnel inThis guide (Reference #13) states that the

ionization chamber shall be located 15 cm (6") from thecenter line of the table in the direction of the lateralx-ray source with the end of the beam limiting device orspacer positioned as close as possible to the chamber.Any movable table ton shall be positioned as close aspossible to the lateral x-ray source, with the end of thebeam-limiting device or spacer no closer than 15 cm (6")to the table top centerline.

II-7-C. Entrance Exposure Rate Measurements

To test compliance, geometrical arrangement of theionization chamber, x-ray tube, and image intensifierspecified in the previous section should be followed. Ifit is not practical to do this, the geometry employedshould be as close as possible to that recommended, withinverse square law corrections applied to the exposurerate readings. Any clinically used grid should be inposition during these measurements.

Measurement of the maximum entrance exposure cannormally be achieved by placing a Lead plate of at least3.2 mm (l/8") at the face of the image intensifier

32

assembly. When the maximumis measured, the

entrance exposure of a unitlead plate protects the

intensifier from excessiveimage

entrance exposure rates.Also, it is good practice to avoid long exposures at themaximum factors minimizing the chance of exceeding thethermal rating of the x-ray tube. These measurementsshould be completed at least annually or afterreplacement of the x-ray tube, generator recalibration,filter thickness change or adjustment to the AERC.

II-7-D. Qualitative Evaluation of Automatic Exposure lateControl Devices

Normally, one should ensure that the automaticexposure rate control device is functioning at leastannually or after repairs.is a simple qualitative

The procedure described belowcheck. It should not be

considered a substitute for the quantitative check whichshould be completed during the original acceptancetesting of the automatic exposure rate control device andsubsequent calibrations.

In general, the fluoroscopic system's AERC shouldselect the fluoroscopic techniques listed in Table IV.The various design strategies of accomplishing this arediscussed in Reference # 14. These suggested techniquefactor combinations are only rough guidelines, EquipmentkV and mA selections in practice depend on the gain ofthe image intensifier, AERC sensor f-stop, the SID, theFOV, and the AERC sensor gain adjustment and referencevoltage settings. Thus, generally one should not passjudgement on the AERC function unless the operationallogic of the AERC is understood.

33

II-l.

II-2

Checklist Outline forFluoroscopic Installation

SYSTEM INFORMATION (See Section I-2)A. Installation

1. Date of survey2. Room number3. Department/Building4. Institution5. Unit identification number

B. Generator1. Manufacturer2. Model type, model number, serial number3. Maximum high voltage (kVp)4. Maximum tube current (mA)

C. X-ray Tube Insert1. Manufacturer2. Model type3. Serial number4. Nominal focal spot sizes

a. Largeb. Small

5. Leakage technique factorsD. X-ray Tube Housing

1. Model number2. Serial number3 added filtration

EXPOSURE SWITCHES AND INTERLOCKSA.

B.

C.

D.

E.

F.

Fluoroscopic Switches1. Deadman type2. Hand or foot operatedRadiographic Spotfilming Switches1. Deadman type2. Hand operatedSpecial Procedure Room Spotfilming Switches1. Deadman type2. Hand or foot operatedRadiographic Exposure (eg. Cut Film Changers)Switches (See Section I-3)1. Non-deadman type2. LocationFluoroscopic Interlocks1. Position interlock on tower2. Image intensifier attachment interlockRadiographic Exposure (eg. Cut film changers)Beam on Indicators (See Section I-3)1. mA meter2. Warning light3. Audible signal

34

II-3. FLUOROSCOPIC TIMERA. Five Minute Maximum SettingB. Audible Warning FunctionalC. Accuracy of Elapsed Exposure time (at least

annually or after repairs or replacement)

II-4. SHIELDING DEVICES AND APPARELA.

B.

C.D.

E.

F.

H.

Lead Drape1. Presence2. ConditionBucky Slot Shield1. Presence2. Working condition3. AdequacyTable End shield PresenceAprons (See Section I-4-A)1. Available2. EmployedGloves (See Section I-4-B)1. Available2. EmployedGonadal Shield (See Section I-4-C)1. Available2. Employed

G. Mobile Shield (See Section I-4-D)Other Specialized Devices

II-5. MINIMUM SOURCE-TO-SKIN DISTANCE (SSD)A. Minimum Fluoroscopic SSD

1. X-ray tube below table (after a changein type or mounting)

2. X-ray tube above table3. C-arm

II-6. RADIATION BEAM RESTRICTION AND ALIGNMENT(Sizing checks should be performed at least an-nually or after repairs or adjustment to thecollimator, its electronics, image intensifiertower SID sensing device, or cassette holderwithin spotfilm device. Alignment checksshould be performed at least annually or afteradjustments or repairs to the cassette holderof the spot film device, to the photospot camera,to the television camera, or to the televisionmonitor.)A. Fluoroscopic Mode Alignment and Sizing

(Minimum SID, auto and/or manual mode)1. Large FOV (14", 13", 12", 11", 10", or 9")2. Medium FOV (10", 9", 7", or 6")3. Small FOV (6", 5", or 4 l/2")

35

B. Fluoroscopic Mode Alignment and Sizing(Maximum SID, auto and/or manual mode)1. Large FOV (14", 13, 12", 11", 10", or 9")2. Medium FOV (10", 9", 7", or 6")3. Small FOV (6", 5", or 4 l/2")

C. Spot Film Alignment (minimum SID)

2. 2 on 1a. Longitudinalb. Transverse

4. 9 on 15. etc.

D. Spot Film Alignment (maximum SID)

2. 2 on 1a. Longitudinalb. Transverse

4. 9 on 15. etc.

E. Spot Film Field Sizing (minimum SID, autoand/or manual mode)

2. 1 on 1a. Longitudinalb. Transverse

4. 9 on 15. etc.

F. Spot Film Sizing (maximum SID, auto and/ormanual mode)1. 1 on12. 2 on 1

a. Longitudinalb. Transverse

3. 4 on 14. 9 on 15. etc.

II-7. RADIATION OUTPUT AND BEAM CHARACTERISTICSA. Half Value Layer at Specified kVp (at least

annually or after replacement of x-ray tube,generator recalibration, or filter thicknesschange)

B. Maximum Entrance Exposure Rate (at leastannually or after replacement of x-ray tube,generator recalibration, filter thicknesschange, or adjustment to AERC)1. Automatic exposure rate control (AERC)2. Manual exposure rate control

36

C. Qualitative Evaluation of AERC Device (samefrequency as II-7.B

II-8. IDENTIFICATION OF PERSON CONDUCTING EVALUATIONA. NameB. TitleC. Professional Certification

II-9. RECOMMENDATIONS AND SUGGESTIONS (See Section I-10)

II-10. POSSIBLE ADDITIONSA. In-room Stray and Scatter Radiation Levels

(See Section-IV-31B. Protective Barrier/Shielding Survey (See

Section IV-4)C. Leakage Radiation (See Section IV-2)D. Use and Presence of Personnel MonitoringDevices

37

III. Dental Radiographic Installations

III-l. Introduction

Many items discussed in the Medical RadiographicInstallation Section, Section I, are applicable to dentalx-ray units. Pages 45-47 contain a checklist of thephysical parameters which evaluation and theinformation important to the

requireradiation protection survey

of dental radiographic installations. While thissection's checklist is complete, only information andparameters unique to dental installations are discussedhere.

Dental radiographic equipment can be divided intothree major categories: intraoral, cephalometric, andpanoramic systems. The panoramic x-ray system is atomographic unit unique to dentistry. It usually has afixed SID in the range of 18" to 22" (45-56 cm) and anexposure time of 15 to 25 seconds. The radiation beam isusually collimated with two slit collimators, one at theend of a short cone attached to the x-ray tube and theother in front of the image receptor.

The intraoral system uses small image receptors whichare held in place by the patient's teeth during theexposure. Therefore, this unit has no attached imagereceptor holder. Normally, the x-ray beam is collimatedby a cylindrical cone.

The cephalometric unit may be dedicated or mayconsist of an intraoral x-ray tube assembly attached to amechanical device which supports the image receptor. Theimage receptor normally is 8" x 10" (18 cm x 24 cm) insize; the SID is from 60" to 65" (152 to 165 cm).

III-2. Beam-On Controls and Indicators

With the exception of the comment on cut filmchangers, all the statements ofdental radiographic installations.

Section I-3 apply toIn addition to

surveying the unit, the physicist should verify that theoperators are usingcord properly.

the exposure time and the exposureManually terminating the

releasing the deadman exposure switch or notexposure by

far away fromstanding as

possible error.the x-ray source as possible are two

III-3. Minimum Source Skin Distance: Intraoral Unit

The minimum x-ray source to skin distance (SSD) of adental unit used with intraoral image receptors shall be18 cm (7") if the unit is operable above 50 kVp (Reference#1). If the unit operates at or below 50 kVp, the minimum

38

SSD shall be 10 cm (4") (Reference #L). The distancebetween the end of the cone attached to the tube headassembly and focal spot usually can be measured with atape measure. If the location of the focal spot is notindicated or if the tube has been replaced since theprevious survey, see Section I-5-A for measurementdetails.

Federal standards do not specify an SSD forcephalometric or panoramic units. The geometricalrequirements of these two units result in SSDs in excessof the specifications for intraoral units.

III-4. Source to Image Receptor Distance: CephalometricUnit

The measured Source to Image Receptor Distance (SID)should be within 2% of the SID indicated thecephalometric unit (Reference #1). This distance usuallycan be measured with a tape measure. The location of thefocal spot, if unmarked, can be located usingtriangulation (Section I-5-A). Since this SID requirementdoes not apply to fixed SID units, it does not apply toPanoramic units. It is also not applicable to intraoralunits because operationally the SSD is fixed and the SIDis variable due to varying patient anatomy.

III-5. Radiation Beam Restrictors

Beam restriction on a dental unit usually consists ofa circular, rectangular, or slit diaphragm mounted at thex-ray tube/cone junction of an intraoral, cephalometric orpanoramic unit, respectively. Since light localizers arenot used, beam restrictor tests are straight forward. Themethod is discussed in III.5.A, B, and C. These testsshould be completed at least annually or after removaland/or adjustment of the diaphragm.

III-5-A. Intraoral System

For a unit with an SSD equal to or greater than 18 cm(7"), the x-ray field size at the plane of the SSD must becontained within a maximum diameter circle of 7 cm (2.8")(Reference #l). If the SSD is less than 18 cm (7"), themaximum diameter circle must be less than 6 cm (2.4")(Reference #l). "Contained" in this context means that nopart of the cross-sectional area of a different shaped x-ray field (eg. square, rectangle, etc.) falls outside theboundary of the circle,is defined as the "edge"

The boundary of the x-ray fieldof the field where the exposure

rate is 25% of the maximum exposure rate within the field(Reference #1).

This parameter is measured by making an exposure on

39

an appropriate image receptor which is larger than the x-ray field placed at the distal end of the cone. The imagereceptor can be film, fluorescent screen, etc.

III-5-B. Panoramic System

The rectangular slit within the cone mounted on thex-ray tube assembly should restrict the size of the x-raybeam to a size smaller than the slit opening on the imagereceptor support housing. The alignment of the x-ray tubeassembly with respect to the image receptor supporthousing shall ensure that the entire x-ray beam fallswithin the slit opening on the image receptor supporthousing. Vertical beam misalignment results in either topor bottom cone cut on the film. Horizontal misalignmentrequires increased radiographic technique factors due toattenuation of the x-ray beam by the edge of the slit infront of the image receptor. Both horizontal and verticalmisalignment of the slits results in unnecessary patientexposure. The accuracy of the slit's size and alignmentcan be verified by exposing a film (with appropriatemarks) or a fluorescent screen taped on the slit of theimage receptor support housing.

III-5-C. Cephalometric System

The alignment of the central ray of the x-ray beamwith respect to the center of the image receptor shall bewithin 2 percent of the SID (Reference #1). The x-rayfield shall be restricted so that each dimension does notexceed the image receptor dimension by more than 2 percentof the SID (Reference #1). Since the cassette holdergenerally permits the image receptor to move horizontallyto compensate for different patient mandibular and cranialprofiles, the image receptor should be placed in thecenter of the cassette holder when the alignment ischecked.

To verify the alignment and size of the x-ray field,place a 14" x 17" (35 cm x 43 cm) cassette distal to thecassette holder. Measure the source to film distance.Expose the film. The field size in the image is scaled tothe field size at the cassette holder. A shadow image ofthe cassette holder should also be present on the film.This may be used to verify x-ray beam to image receptoralignment.

III-5-D. Labeling of Cones or Apertures

Cephalometric units may use more than one size ofimage receptor. In this case the system usually usesremovable cones or apertures to restrict the x-ray beam tomeet the federal requirements listed in III-5-C. Each of

40

these interchangeable cones. or apertures shall havepermanent labels which indicate the image receptor sizeand SID for which the beam restrictor is designed(Reference #1).

III-6. HVL Measurement

Measurement of radiation beam quality onCephalometric and Intraoral units is a straight forwardprocess. The comments of Sections I-7, I-7-A, and I-7-Bapply.

Special care must be used to make HVL measurements onPanoramic units. These x-ray units have long exposuretimes. In general, one must wait 5 minutes betweenPanoramic exposures of 15-25 seconds to ensure that theheat load of the tube's anode is not exceeded. This heatload can be minimized by making exposure rate measurementsand terminating the exposure- immediately after theelectrometer reading stabilizes. IN EMPLOYING EITHERMETHOD, CARE SHOULD BE EXERCISED AND THE MANUFACTURER'STUBE RATING CHARTS SHOULD BE CONSULTED TO INSURE THAT TUBELOADING LIMITATIONS ARE NOT EXCEEDED.

One desires stationary geometry of the Panoramic unitwhen making HVL measurements. Depending on themanufacturer, the rotating motion of the panoramic unitcan be disabled by removing the appropriate fuse. Therecommended method for this is usually noted in theinstallation manual. This allows one to position theionization chamber on a stand in front of the post patientslit aperture.

One more geometry concern for HVL measurements ispresented by the slit aperture of the x-ray tube cone.The slit aperture produces a typical x-ray field size of13 cm x 0.6 cm (5" x 0.25"). The 0.6 cm dimension resultsin approximately the same partial coverage of the typicalionization chamber regardless of the thickness of testfilter in front of the chamber. While each measuredexposure is too small, the same test filter thickness isrequired to reduce the measured value to one half of itsmeasured zero filter value. Therefore, provided theposition of the ionization chamber "within" the x-rayfield does not change, a sufficiently accurate measurementshould result.

Half value layer measurements should be completed atleast annually or after replacement of x-ray tube,generator recalibration, or filter thickness change.

III-7. Patient Exposure

Patient exposure is greatly influenced by filmprocessing conditions. The Dental Exposure NormalizationTechnique (DENT) program, an ongoing study by the Center

41

for Devices and Radiological Health (CDRH) of the USDepartment of Health and Human Services (USDHHS), hasdocumented that dental film development is a major problem(Reference #15). In a typical full mouth intra oralradiographic examination, a patient is subjected to 14 to20 exposures. The entrance exposure of overlapped areasis high and can be excessive unless the film is properlydeveloped. Dental radiographic exposure guidelines havebeen established by the FDA for a routine intraoralbitewing examination for "D" and "E" Speed Dental Film(Reference #15). These are listed in Table V.

Using the facility's clinical techniques measure theexposure with an ionization chamber positioned at theappropriate SSD. The obtained value should be compared tothe appropriate values in Table V. If the exposures perfilm are higher than the appropriate range, the film isprobably being under developed.

If the measured exposures are low and the HVLmeasured in Section III-6 is not excessive, the film isprobably being over developed. This leads to fogging ofthe processed films which diminishes radiographic contrastand overall image quality. Over or under developmentnormally is caused by an improper combination of film,chemistry, development time and/or temperature.

42

The general comments of Section I-7-C on measuringradiation exposures apply to dental x-ray units. Theionization chamber and electrometer described in SectionI-7-D are appropriate for these measurements onCephalometric or intraoral units. Most dental units aresingle phase halfwave or full wave rectified units. Theexpected exposures at 24" from the x-ray source as afunction of kV are listed in Table III, Section I-7-C.

Due to the geometry of the Panoramic unit, thepatient tissue exposed to radiation is continuallychanging. Therefore, the patient entrance exposure(mR/mAs) is less than the expected radiation output(mR/mAs) within the slit beam. The patient entranceexposure per film can be measured by placing theionization chamber at the location of the patient's skin(at chin support) and by scanning across it using clinicalkV and mA settings. The manufacturer's tube rating chartshould be consulted to avoid overheating the tube whenmaking multiple exposures.

To measure the exposure within the slit x-ray field(mR/mAs) for comparison with the values in Table III,section I-7-C, use the ionization chamber geometrydescribed in section III-6 and use short exposure times tocollect exposure rate date. Correct these readings tomR/mAs values and for error due to partial coverage of theionization chamber. The correction factor for partialcoverage error is the ratio of the total volume of thechamber to the radiated chamber volume. The actual widthof the x-ray slit field should be known from measurementsin Section III-5.B.

Patient entrance exposures should be measured atleast annually or after replacement of the x-ray tube,generator recalibration, filter thickness change or achange in film or development techniques.

III-8. Multiple Tube Configuration

Multiple tube configurations are commonly found inmany dental offices and clinics. One generator may beinstalled to power as many as four x-ray tubes. Only oneexposure switch may be present to initiate an exposurefrom any one of the x-ray tubes, or each x-ray tube may beequipped with its own exposure switch. In suchconfigurations, the x-ray tubes must be interlocked toprevent exposure from more than one x-ray tube at a time.The x-ray tube selected for the exposure should be clearlyidentified (Reference #1).

III-9. Mechanical Support

Certain states require checks of mechanical supports.

43

The mechanical support of the x-ray tube on Cephalometricand Panoramic units shall be designed so that the x-raytube housing assembly remains stable during an exposureafter positioning.units by placing

This should be evaluated on intraoralthe x-ray tube assembly in the

positions/angles commonly usedcephalometric unit should

clinically. Thebe set up

geometry to evaluate mechanical stability.in its clinical

While the x-ray tube assembly is not stationary on Panoramic unitsduring exposures; it should bemaintain the vertical and

rigidly supportedhorizontal slit

toalignment

discussed in Section III-5-B.

III-10. Shielding Devices

The comments made in Section I-4 also generally applyto dental installations. Typically a lead apron is placedon the patient to provide ‘whole body” andgonadal shielding.

specificallyThyroid shields are also effective at

reducing thyroid doses As Low As Reasonably Achievable(ALARA) for dental patients.

44

Checklist Outline ForDental Radiographic Installations

III-1. SYSTEM INFORMATION (See Section I-2)A. Installation

2.1. Date of survey

Room number

4.3. Department/Building

5.InstitutionUnit identification number

B. Generator1. Manufacturer

3.2. Model type, model number, serial number

Maximum high voltage (kVp)4. Maximum tu e current (mA)b

C. X-ray Tube Insert

2.1. Manufacturer

Model type3. Serial number4. Nominal focal spot sizes

a. Largeb. Small

5. Leakage technique factorsD. X-ray Tube Housing

1. Model number

3.2. Serial number

Added filtration

III-2. BEAM-ON CONTROLS AND INDICATORS (See Section I-3)A. Beam On Indicators

1. mA meter: present and functional?2. Warning light: present and functional?3. Audible signal: present and functional?

B. Exposure Switch

2.1. Deadman type

Locationa. Fixed unit: location outside of exam

roomb. Mobile unit: length of exposure cord

III-3. MINIMUM SOURCE-SKIN DISTANCE: INTRAORAL UNITA. Focal Spot Localization (when x-ray tube

is replaced)B. SSD Accuracy

III-4. SOURCE-TO-IMAGE RECEPTOR DISTANCE:CEPHALOMETRIC UNITA. Accuracy of DetentsB. Accuracy of Tape MeasureC. Accuracy of Markings

45

III-5.

III-6.

III-7.

III-8.

III-9.

RADIATION BEAM RESTRICTORSA. Intraoral System (at least annually or after

removal of cone)1. X-ray field diameter at SSD

B. Panoramic System (at lest annually or afterremoval or adjustment of slit diaphragm)

2.1. Slit size

Verticle alignment of slit3. Horizontal alignment of slit

C. Cephalometric System (at least annually orafter removal or adjustment of diaphragm)1. 8 x 10" (18 x 24 cm) receptor

a. X-ray beam alignmentb. X-ray field size

2. Other receptor sizea. X-ray beam alignmentb. X-ray field size

D. Labeling of Removable Cones or Apertures

HALF VALUE LAYER AT SPECIFIED KVP (at leastannually or after replacement of x-ray tube,generator recalibration, or filter thicknesschange)

PATIENT ENTRANCE EXPOSURE (at least annually or asin III-6)A. mR/mAsB. Intraoral Entrance Exposure/Film

MULTIPLE TUBE CONFIGURATIONA. InterlocksB. Light Indicators

MECHANICAL SUPPORTA. Lack of Tube Drift

III-10. SHIELDING APPAREL (See Section I-4)A. Aprons

1. Available2. Employed

B. Mobile ShieldC. Thyroid Shield

2.1. Available

Employed

III-11. RADIOGRAPHIC TECHNIQUE CHART (See Section I-8)A. AvailabilityB. UseC. Completeness

46

III-12. IDENTIFICATION OF PERSON CONDUCTING EVALUATION(See Section I-9)A. NameB. TitleC. Professional Certification

III-13. RECOMMENDATIONS AND SUGGESTIONS (See Section I-10)

III-14. POSSIBLE ADDITIONSA. In-room Scatter Radiation (See Section IV-3)B. Protective Barrier/Shielding Survey (See

Section IV-4)C. Leakage Radiation (See Section IV-2)D. Use and Presence of Personnel Monitoring

Devices

47

IV. Measurement of Area Radiation Levels

IV-l. Introduction

Parts I, II, and III, have concentrated on equipmentrelated radiation safety matters. In addition to theseconcerns, a radiation survey of the facility should beperformed. The following three types of radiation levelsin the vicinity of the machine may be of interest:

(a) Scattered radiation inside the examination room,(b) Stray radiation outside the examination room,(c) Leakage radiation from the x-ray tube housing.Measurement of scattered radiation inside the

examination room should be checked annually. Themeasurement of radiation levels outside the procedure roomis necessary prior to first clinical use and followingroom modifications.

IV-2. Leakage Radiation

All certified diagnostic medical and dentaldiagnostic source assemblies must meet the FederalStandard for the diagnostic source assembly (ReferenceCl). This standard results from the definition of a"diagnostic-type protective tube housing" by the NCRP(Reference #2):

"An x-ray diagnostic source assembly mustbe so constructed and assembled that theleakage radiation measured at a distance of 1meter from the source does not exceed 100 mR (25.8µC/kg) in 1 hour when the tube is operated at itsmaximum continuous rated current for the maximumrated tube potential."As stated in NCRP report No. 33 (Reference #2), "in

general, modern diagnostic tube housings incorporatesufficient attenuating material toradiation to that permitted in the

limit the leakagedefinition of a

diagnostic-type protective housing and it is probablyunnecessary to perform leakage tests in the field onmodern x-ray machines". Unless one suspects a faulty x-ray tube housing is causing excessive leakage radiation asa result of the housing's age, appearance, history, highmeasured radiation levels inside the room as noted inSection IV-3, or unless it is required by StateRegulations, this survey is generally not necessary.

In general, leakage radiation measurement conditionscannot be precisely met and various technical problemsmust be overcome. Examples are:

(a) The geometry of "1 meter from the source"vs. actual physical limitations in theexamination room,

48

(b) The lowest tube current station availableon the control panel exceeds the maximumcontinuous rated current for the maximumrated tube potential, and

(c) Dental x-ray tubes may have a specifiedduty cycle in addition to the maximumcontinuous rated tube current.

If the ionization chamber cannot be positioned one meterfrom the source in certain radial directions the readingscan be corrected using the inverse square law.

Radiographic x-ray tubes normally have a maximumcontinuous rated tube current of 3-5 mA at maximum kVp.This value must be obtained from the unit's tube ratingchart. If, for example, the lowest mA station on the unitis 50 mA and the maximum "continuous" tube current is 5mA, an exposure rate leakage reading must be scaled byl/10 (5 mA/50mA) to obtain the leakage rate at the maximumcontinuous rated tube current. If the mA station and tuberatings remain the same, but an integrated leakage readingusing 100 mAs is measured, multiply the measured readingby 180/hr to obtain the correct leakage rate.

[2]

An integrated leakage reading is preferred on radiographicx-ray tubes to prevent long exposures and excessive heatloading to the tube.

If the lowest mA station available 'exceeds themaximum continuous rated tube current on a dental x-raytube, the correction factor is calculated as described inthe previous paragraph. However, many dental x-ray tubesalso have a specified duty cycle, eg. 6 seconds/minute.The leakage reading in this case must also be multipliedby this duty cycle, 1/10, to obtain the leakage radiationrate at the tube's maximum continuous tube current.

The following recommendations should be followedduring leakage radiation measurements:

2)

3)

4)

5)

1) Select the lowest tube current station.Select an exposure time that is appropriatefor the ionization survey meter's responsetime. (Most survey meters have a responsetime of 4 to 5 seconds.)Select the highest tube potential allowable.(Consult the tube rating chart.)Do not exceed the total heat capacity ofthe anode and the x-ray tube housingduring the survey.Close-the collimator blades and blockthe collimator port with at least 10HVL equivalent of lead.

49

6) Select positions on the surface of animaginary sphere of 1 meter radius withits-center located at the focal spot.Include points at a height equal tothe plane including the tube housingand the collimator junction.

7) Measure the leakage radiation at theselected positions.

8) Correct any data which could not becollected at 1 meter,

The average reading over 100 square centimeters at 1meter should not exceed 100 mR in one hour, normalized tothe maximum current for continuous operation at themaximum (or the maximum allowable) tube potential. Nolinear dimension of the 100 square cm area should begreater than 20 cm (Reference #l).

IV-3. In-room Scattered Radiation Measurement

In room stray and scattered radiation levelmeasurements are often necessary for routine fluoroscopicsystems or special procedure suites. Measurements aroundstationary radiographic systems and portable x-ray unitsmay also be necessary to assure that the non-radiationworkers are not subjected to excessive amounts ofradiation.

The technique factor requirements are similar tothose employed in leakage radiation level measurements. Awater, plastic, or pressed wood scattering phantom shouldbe placed in the primary beam. It should be approximately30 cm x 30 cm x 25 cm (width x length x thickness) tosimulate an average adult abdomen. Suitable sizes shouldbe employed to simulate other parts of the body, eg. 15 cmx 19 cm x 15 cm for an adult head. Aluminum or Copper areunacceptable as scattering materials.

The equipment should be arranged to simulate theclinical situation. Measurements should be made at thefluoroscopist's, angiographer's, radiographer's, and otherancillary personnel's locations.

(Reference #16). The actual thickness of shielding

To assess the totalscattered radiation delivered to each point of interest,the measured exposures should be scaled to a weeklyexposure using appropriate workload information.

IV-4. Protective Barrier/Shielding Assessment

Shielding provided by room barriers must be adequateto reduce radiation levels to personnel, patients, and thegeneral public to meet the guidelines established by NCRPReport No. 39, "Basic Radiation Protection Criteria"

required is a function of the following:(a) Type of material in the barrier,

50

(b) Orientation of the x-ray beam in the room,(c) Workload of the x-ray unit,(d) Size of the room and the equipment layout, and(e) Degree of occupancy in the adjoining areas.

One cannot arbitrarily assume that a given thickness ofLead will be appropriate on all barriers.

Existing records of room barrier design and thereport of the shielding evaluation should be reviewed.Any one of the following conditions should cause a newbarrier evaluation to be Initiated.

(a) Records of previous room barrier certifi-cation by a qualified expert as definedby NCRP Report No. 49 cannot be found(Reference #17)

(b) Previous room barrier certification isincomplete.

(c) It cannot be established with certaintythat changes in equipment, its operation,or the room barriers have not been madesince the last barrier certification.

If the facility is new, a complete survey of thefacility during and after the construction should be done.This includes visual inspections of shielding integrityduring the construction of the room barriers, relativemeasurements to detect any voids in the barrier with anappropriate radioisotope prior to installation of theradiographic equipment; and quantitative measurements ofstray radiation levels outside the room after installationof the radiographic equipment. In this case,"quantitative" means the determination of exposure levelsper week outside the room using the installed equipment asthe source of radiation. Actual readings must be scaledto reflect the projected workload of the Installation. Ifthe facility's shieldinn has been modified, spot checksshould be made.

Actual procedures in the evaluation of protectivebarriers have been discussed inReference #18) and

NCRP Report No. 57,more recently by K.J. Strauss

(Reference #19). NCRP Report No. 49 (Reference #17) andNCRP Report No. 35 (Reference #20) contain information onthe design of barriers for routine x-ray rooms and dentalx-ray rooms respectively.

IV-5. Instrumentation

The quantitative survey meter used to measure leakageradiation, scattered radiation within the room orradiation levels outside the room barriers must be chosencarefully. An ionization chamber with a sensitive volumeof one liter coupled to an electrometer with integratingcapabilities is one type of instrument suitable formeasuring weak radiation fields of short duration. The

51

following factors must be considered when quantitativemeasurements are completed:

(a) Meter's energy response(b) Meter's directional response(c) Meter's intensity response(d) Ion chamber cross sectional area corrections(e) Meter's calibration(f) Meter's response time in the rate mode on

its most sensitive scales.The selection and use of survey meters to measure low

level radiation fields have been discussed in more detailin References #18 and #19.

52

Checklist OutlineMeasurement of Area Radiation Levels

IV-2. LEAKAGE RADIATION (prior to first patient use oranytime damage to the tube housing is suspected)A. Measurement #lB. Measurement #2, etc.

IV-3. IN-ROOM SCATTERED RADIATION MEASUREMENT (annually)A, Operator Position in Front of Table

2.1. Table upright

Table horizontalB. Head End of Table

D.C. Foot End of Table

Back Side of TableE. Location of Physiological Monitoring Equipment

G.F. Other Occupied Locations in Room

Control Booth: Waist High at Operator'sLocation During Exposure

H. Behind Control Window

IV-4. PROTECTIVE BARRIER/SHIELDING ASSESSMENT (prior tofirst patient use or as needed, see Section IV-4)A. Visual Inspections During Construction

1. Barrier thicknesses

3.2. Joint integrity

"Wrap" integrity at electrical bores, etcB. Relative Measurements With Radioisotope, Prior

to Installation of Equipment1. Control booth2. View windows3. Each Wall4. Floor5. Ceiling6. Location of voids

a. Wrapped electrical boxesb. Joints

7. Room entrancea. Doorb. Maze

C. Quantitative Measurements With X-ray Unit

2.1. Control booth

View windows3. Each wall4. Floor5. Ceiling6. Location of voids

a. Wrapped electrical boxesb. Joints

7. Room entrancea. Doorb. Maze

53

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

REFERENCES

"Part 1020-Performance standards forRadiation Emitting Products", 21 Code of

Ionizing

Federal Regulations, Parts 800-1299 (1984).NCRP Report No. 33; "Medical X-ray and Gamma-Ray-

Protection for Energies up to 10 MeV," NCRPPublications, P.O.(1968).

Box 30175, Washington, D.C. 20014

W.R. Hendee and R.P. Rossi, "Quality Assurance forradiographic X-ray Units and Associated Equipment,HS Publication (FDA) 80-8094 Rockville, Maryland(1979)J.E. Gray, J.T. Winkler, J. Stears, E.D. Frank.

Park Press,Quality Control in Diagnostic Imaging, University

Baltimore, 1983, 244P.P. Lin, R.J. Kriz,

pages.

Test Device forand R. Storzum, "An Alignment

Compliance of PBL AutomaticCollimators," pp 183-196, in AAPM Proceeding SeriesNo. 1, Acceptance Testing of Radiological Imaging

Institute of Physics, New York, N.Y. 10017, (1982).Equipment, Published for AAPM by the American

NCRP Report No. 66, "Mammography", NCRPPublications, P.O. Box 30175, Washington, D.C. 20014(1980).E.D. Trout, J.P."Determination of

Kelley and A.C. Cucas,Half-value Layers", Am. J.

Roentgenology, Radium Therapy and Nuclear Medicine,Vol. 84, No. 4, pp 729-740. (1960).NCRP Report No. 54, "Medical Radiation Exposure ofPregnant and Potentially Pregnant Women," NCRPPublications, P.O. Box 30175, Washington, D.C. 20014(1977).E.C. McCullough and J.R. Cameron, "Exposure Ratesfrom Diagnostic X-ray units," Br. J. Radiol. 43, pp448, (1970).AAPM Report No. 14, "Performance Specifications andAcceptance Testing for X-ray Generators and AutomaticExposure Control Devices," Published for AAPM by theAmerican Institute of Physics, New York, N.Y. 10017(1985).W.R. Hendee and R.P. Rossi, "Quality Assurance forFluoroscopic X-ray Units and Associated Equipment",HHS Publication (FDA), 80-8095,(1980).

Rockville, Maryland

P.P. Lin, "A Device for Checking Alignment ofFluoroscopic Systems," Optical Engineering, Vol. 16,No. 3, pp 302-304, (1977).

54

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

24.

Personal Communication with Thomas Mosely concerningFDA Compliance Policy Guide issued to manufacturers,assemblers, and field test personnel:of Entrance Exposure

"MeasurementRate for Lateral

Fluoroscopes-21 CFR 1020.32(d)" (1977).Type

P.P. Lin, "Acceptance Testing of Automatic ExposureControl Systems and Automatic Brightness StabilizedFluoroscopic Equipment,"No. 4.

pp 10-27, in AAPM MonographQuality Control in Diagnostic Radiology

Published for AAPM by the American Institute ofPhysics, New York, N.Y., 10017 (1980)."Dental Exposure Normalization Technique (DENT) Ins-truction Manual", HHS Publication (FDA) 80-8035.NCRP Report No. 39, "Basic Radiation Protection Cri-teria", NCRP Publications, P.O. Box 30175,Washington, D.C. 20014. (1971).NCRP Report No. 49, "Structural Shielding Design andEvaluation for Medical Use of X Rays and Gamma Raysof Energies up to 10 MeV," NCRP Publications, P.O.Box 30175, Washington, D.C. 20014 (1976).NCRP Report No. 57, "Instrumentation and MonitoringMethods for Radiation Protection", NCRP Publications,P.O. Box 30175, Washington, D.C. 20014 (1978).K.J. Strauss,

Equipment, Published for AAPM by the American

"X-ray Room BarrierTesting", pp 255-274, in AAPM

AcceptanceProceeding Series No.

1, Acceptance, testing of Radiological Imaging

Institute of Physics, New York, N.Y. 10017, (1982)NCRP Report #35, "Dental X-ray Protection", NCRPPublications, P.O. Box 30175, Washington, D.C. 20014(1970).J.P. Kelley and E.D. Trout, "PhysicalCharacteristics of the Radiations From 2-Pulse, and1000-Pulse X-Ray Equipment", Radiology, Vol. 100, pp653-661, (1971).E.D. Trout, J.P. Kelley, and A.C. Lucas, "Influenceof Cable Length on Dose Rate and Half Value Layer inDiagnostic X-Ray Procedures,' Radiology, Vol. 74, pp255-263, (1960).

23. Personal communication with R.P. Rossi.E.D. Trout, J.P. Kelley, and R.E. Gross, "BeamQuality Measurements in Diagnostic Roentgenology,"Am. J.(1968).

Roentgenology, Vol. 103, No. 3, pp 681-688,

55