topic 3 selection of kv

Topic 3 Selection of kV• High kV scattered radiation is energetic. • Angle of rays is same as those of primary

beam and is not deflected very much.• Follows path of primary beam in forward

direction adding to film density.• Use of relatively high grid ratio, e.g. 12:1

or higher, improves image quality as scattered radiation in high kV techniques would be more efficiently absorbed.

Low kV results in high absorption of x rays.

High kV results in less absorption ofx rays. More x rays penetrate the tissue being examined and reach the film. This results in good visualization of structures required in pattern recognition.

Poor patient positioning

Entire knee architecture not included. Lack of penetration of knee joint due to low kV selection.

Topic 4 Selection of mAs

• The higher mAs, the darker the image,the higher the patient dose.*

• Factors affecting mAs selection: Focus film distance: For the same image

density, if new FFD is equal to a factor multiplied by the old FFD, the new mAs equals the old mAs multiplied by a number equal to the square of this factor.**

Selection of mAsSpeed of film: Fast films need less mAs

than slow films.Size, thickness, and type of phosphors in

intensifying screens: Fast screens require less mAs than slow

ones. When changing from a 200 speed system to a 400 speed one, reduce original mAs by 50%. This reduces dose to patient.

Selection of mAs

Intensification factor of intensifying screens determines selection of mAs.

Large phosphors require less mAs than slow ones. Fast screens need less mAs

than slow ones. Single intensifying screen requires more mAs than two screens in cassette.

Selection of mAs

Slow screen systems produce good image detail, but more mAs needed for film blackening resulting in higher patient dose.

Selection of specific film-screen combinations depends on information needed for pattern recognition.

Selection of mAs Use of grid: mAs must be increased

based on grid factor. Degree of collimation: Reducing area of

interest needs an increase in mAs. *Output of generator/capability of unit.**

Using a 3-phase generator instead of a single phase one requires mAs reduction.

Relationship to kV used.Size of focus.

Very black film of distal femur due to high mAs. Patient subjected to unnecessary dose. Film retaken with reduced mAs.

Grossly over exposed radiographAP pelvis of child. Too high mAs. Poor collimation – gonads in primary beam which means dose to reproductive organs. Repeat radiograph done.

Grossly overexposed skull radiograph

Too high mAs. Poor radiographic technique –patient's skull rotated. No evidence of beam restriction – unnecessary dose to patient's eyes and thyroid.

Lecture 8 Radiographic Exposures (Module 6)

• Topic 1 Introduction• Topic 2 Exposure Chart• Topic 3 The step system• Topic 4 Tasks 26 – 28

Topic 1 Introduction

• Objectives:On completion of this module the student

will Understand the effect each exposure

factor has on the resultant radiograph.Be able to use this knowledge effectively.Be able to select the correct exposure

factors required.


Understand and use the step system of exposure calculation.

Be able to manipulate exposure factors using the step system.

Be able to establish and maintain a reliable exposure chart.

Be able to modify an existing exposure chart.

Topic 1 IntroductionExposure: Refers to the quantity of radiation to which

the patient is exposed.In radiography, the term "exposure" is

used as the collective exposure factors kV, mA and time, which together produce the radiation exposure that will give the required penetration, density and contrast on the radiograph.


Other factors influence film quality but here we will only consider kV, mA, time and FFD (SID)

kVControls largely the penetrating power

(better described as beam quality), and to a lesser degree, the radiation intensity and threfore film density and patient dose.

Topic 1 IntroductionAffects contrast. The higher the kV, the

lower the contrast. The lower the kV, the higher the contrast.

mA: Controls radiation intensity and therefore film density and patient dose. The higher the mA, the higher the film density and patient dose.

Topic 1 IntroductionTime: Controls the length of time of

radiation flow, and therefore film density and patient dose. The longer the exposure time, the higher the film density and patient dose.

mAs:The product of mA and time (in seconds).Many modern x-ray units use mAs rather

than mA and time separately.


FFD ( SID):The greater the distance an x-ray beam

travels, the less effective it will be. The less the distance it travels, the more effective it will be.

Exposure compensation is necessary with changes in FFD (SID).

Topic 1 IntroductionNOTE:It is important that the radiographer sets

correct exposure factors required to produce a high quality radiograph which will give the maximum information yet minimises patient dose.

Systems must be in place to help the radiographer select and manipulate exposure factors effectively.

Topic 2 Exposure chart

• Each x-ray unit should have a list of commonly used exposures for easy reference (see Appendix B, page 164).This can serve as a guide.

Methods of recording exposures:Written chart.Notebook.Computerised "chart" within the x-ray unit.


Here we will deal with:Establishing and modifying an exposure

chart.Manipulating exposure factors using the

step system.


Establishing an exposure chart Method:1. Draw up a blank exposure chart (see Fig

6-1 and Appendix B page 168).2. Fill in all anatomical areas to be covered,

listing the views for each.


3. Divide these into groups using the same set of conditions, e.g. all extremities using detail screens, no grid, 100cm FFD(SID).

4. Produce a well exposed set of radiographs of an extremity, e.g. hand. (This can be done in the course of a routine examination.)


5. Record the exposures used against HAND on the chart.

6. Measure the thickness of the patient's hand in all projections used, e.g. PA, oblique, lateral

7. Measure thickness at the level of entry of central ray, for each hand position.


8. Measure all other areas/positions in the same group (these can be obtained from a colleague or friend of a similar size rather than inconvenience the patient).

9. Calculate the exposures for all other areas/positions in the group, based on the following chart (patient thickness related to exposure change using the hand as your base level:


Patient thickness related to exposure change

1.5 cm increase in thickness requires a 25% increase in exposure (+1 step).

5.0 cm increase in thickness requires a 100% increase in exposure(+3 steps).


5.0 cm decrease in thickness requires a 100% decrease in exposure (-3 steps).

1.5 cm decrease in thickness requires a 25% decrease in exposure (-1 step).

NOTE: The reference to "+ or – steps" is explained in the next topic, the step system.


ExamplePA HAND (2 cm thick) Exposure 50kV

6mAs.Calculate an exposure for a lateral WRIST

(7cm thick).Thickness difference between the PA

HAND and lateral WRIST is 5 cm.


For an increase of 5 cm the exposure is increased by 100% ( see previous discussion).

Therefore the lateral WRIST exposure will be 50kV 12 mAs.

The exposures for all other areas of the body can be calculated in this way.

Topic 3 The step system

• A simple standardised form of exposure factor manipulation, designed to remove some of the guess work and make exposure setting more accurate.

• Works on a series of step charts allowing use of standard step changes in exposure.

• Also related to patient thickness and medical condition (see Fig 6-5).


Note: Step changes can be applied to any one of the charts or split between the charts.

The step system provides step charts for kV, mA, mAs, time and FFD(SID), (see Fig 6-2, Fig 6-3, Fig 6-4 & Fig 6-5).

To use the step system for exposure changes, the term "step" is used.


Each step on any of the charts will alter the exposure by approximately 25%.

For a noticeable change in film density, the exposure must be altered by at least 25%.

If a film is considered to be too light or too dark, change one of the exposure factors by at least 3 steps up or down.


Any step change required to modify an exposure can be applied to any of the Step Charts, although other factors will influence this decision (contrast, penetration, patient dose, movement, limitations of the x-ray unit).


Step changes need not be limited to one chart only. Necessary step changes can be split up between charts if necessary, providing that the overall number of required step changes is made.

ExampleOriginal Exposure: 60 kV 20 mAsNew kV = 70 kV (an increase of 10 kV)

Topic 3 The step system An increase of 10 kV = three steps on the kV

step chart.To retain the same density film, the mAs must be

reduced by the same number of steps (three steps) on the mAs chart.

New exposure = 70 kV 10 mAs All Step Charts are from "Course Notes in

Basic Radiography", Radiation Protection Branch, South Australian Health Commission.

Topic 5 Tasks 26 - 28

• The tasks are in the next four slides.

topic 3 selection of kv

Documents

mas selection

high mas

new mas

old mas

mas reduction

reduced mas

original mas

selection of masthe