the exposure index for digital radiography (iec 62494-1...

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MDACC Imaging Physics1

AAPM Spring Clinical Meeting

2015

The Exposure Index for Digital Radiography

(IEC 62494-1 and AAPM Report 116)

MDACC Imaging Physics

S. Jeff Shepard, MS, DABR, FAAPMImaging Physics DepartmentDiagnostic Imaging Division

The University of Texas M. D. Anderson Cancer CenterHouston, Texas

Acknowledgement:Michael Flynn, PhDHenry Ford Hospital System


Learning Objectives• Recognizing good technique in DR - Noise• Understand causes and solutions to “exposure

creep”• Understand IEC/TG116 Exposure Index• Verifying EI calibration in the clinic• Be aware of additional recommendations in TG116


Identifying Correct Clinical TechniqueRecognizing Bad Images

• In the film-screen world, under- and over- exposures were easily recognized by the appearance of the recorded image – Repeat at a higher or lower technique

based on optical density

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Recognizing Bad Images• With Digital Radiography, under- and

over-exposures are not so easily recognized

– Adequate images over a much wider range of exposure

– Post-processing can hide mistakes– Excellent dynamic range may have down-sides

Identifying Correct Clinical Technique


Exposure Indices


Exposure Indices

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Exposure “Creep”• Under-Exposure

– Higher noise– Detail visibility suffers

• Over-Exposure– Lower noise (“Pretty” – improved SNR)– High patient dose

• Radiologists may complain about noise, but not usually about over-exposure

– Technologists learn quickly how to avoid criticism– If no one is paying attention, exposures will

“creep” up.


Exposure “Creep”Widely known problem that’s been around

for a long time• Freedman M, Pe E, Mun SK, Lo SCB, Nelson M,

“The potential for unnecessary patient exposure from the use of storage phosphor imaging systems,” SPIE 1897:472-479 (1993).

• Gur D, Fuhman CR, Feist JH, Slifko R, Peace B, “Natural migration to a higher dose in CR imaging,” Proc Eighth European Congress of Radiology, Vienna Sep 12-17, 154 (1993).


Vendor-Specific IndicesManuf. Index Lin or

LogExposure relation

Std kV/Filter Cal

Agfa lgM Log Direct 75/1.5 mm Cu 1.96 bel @ 2.5 uGyAlara EIV Log Direct 70/7.1 mm HVL 2000 mbel @ 10 µGyCanon EXP Linear Direct 80/8.2 mm Al 2000 @ 10 µGyCanon REX Linear Direct -- 106 @ 10 µGy*Carestream EI Log Direct 80/0.5 Cu + 1 Al 3000 @ 1 mRFuji S Linear Inverse 80/3 mm Al

“Total”200@1 mR

GE UDExp Linear Direct 80/21 mm Al 2.85 @ 0.5 mAsImaging Dynamics

SE Linear Inverse 80/1 mm Cu 200 @ 10 uGy

Konika S Linear Inverse 80/3 mm Al “Total”

200@1 mR

Philips EI_s Linear Inverse 70/7.1 mm HVL 400 @ 2.53 µGySiemens EXI Linear Direct 70/0.6 mm Cu 100 @ 1 uGy

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Exposure IndicesIEC 62494-1 & AAPM TG116

• Recommendation for a standard detector exposure index for all digital radiography

• Published 2008• Mike Flynn and Jeff Shepard represented the

USA (USNC TAG members) and AAPM on the IEC working group


Exposure Index (EI):• Index is proportional to the air Kerma that the

detector would have received under standard beam conditions for the same raw pixel value in the relevant image region.

• Calibrated for the imaging system over the specified operating range of image receptor air kerma such that:

EI = (100 uGy-1) * KCALwhere KCAL is the image receptor air kerma in μGyunder the calibration conditions.

• For Krel = 10 uGy, EI = 1000• For Krel = 2.5 uGy, EI = 250

Exposure Index – IEC


“Relevant” Image Region• Gray histogram is for the

entire image.• Black histogram is for the

relevant anatomic region or Values of Interest (VOI)

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“Relevant” Image Region• Gray histogram is for the

entire image.• Black histogram is for the

relevant anatomic region or Values of Interest (VOI)


“Relevant” Image Region

• AAPM TG116: “The median is recommended rather than the mean or mode because it is less affected by data extremes and outliers.”

• IEC 62494-1: “The [indicator] shall be calculated using the mean, median, mode, trimmed mean, trimean, or other recognized statistical method for the description of central tendency of the [values] in the relevant image region.”


Region to assess signal indicator

Systems vary in the region used to assess the signal for an image.

• Full Image

• Regular regions

• Anatomic regions (segmentation)

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Deviation IndexBoth standards also call for a “Deviation Index”

DI = 10 × Log10{EI/EIT(b,v)}

• EIT (b,v) is a table of target values storedby body part (b) and view (v)

DI = 0 is a perfect exposureDI = +1 means exposure was high/low by about 28%

(one density or mAs step)

• EIT tables to be customized for each site• If not customized, default value of DI is 0.


Deviation Index• Exposure indices are saved in the DICOM header

– 0018,1411 (EI)

– 0018,1412 (EIT)

– 0018,1413 (DI)

– 0018,1405 (EI) and 0018,6000 (Sensitivity) are old and no longer recommended

• DI Format:– AAPM: Decimal string with one decimal place (tracking

and trend management)

– IEC: Integer

• Both indices change with VOI modification by the tech


Values of interest – VOIPixel values to be filtered/re-scaled for

presentation

Num

ber

of P

ixels

Pixel Value

Values of Interest

EI and DI calculated from

median pixel value

EI = EITDI = 0.0

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VOI Recognition Failure• Gonadal shields, prosthetics, surgical mods• False EI & DI reported

Num

ber

of P

ixels

Pixel Value

CorrectValues of Interest

EI and DI calculated from incorrect median

pixel value

Incorrect Values of Interest

EI ≠ EITDI = -1.3

EI = EITDI = 0.0


EI = EITDI = 0.0

Manual Correction of VOI FailureTech manually returns the VOI to the proper position

by reprocessing

Num

ber

of P

ixels

Pixel Value

Correct EI and DI calculated from new median pixel value

CorrectValues of Interest


RQA5 is the standard beam condition• RQR5 pre-filtered beam (70 kVp, 2.6 mm Al

HVL)• 70 kVp• 21 mm added pure Al filtration• 6.8 mm Al HVL

But …• Pure Aluminum is impractical for field

measurements (expensive, heavy, hard to find).• An alternative is to use copper (cheap, portable

and widely available).• Valid substitution?

Standard Beam

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Beam Quality – spectral shapeReport 116 reported equivalent spectral shape with RQA5 conditions and Cu/Al filtration

72.59 kVp3.02 mm Al pre-filtered HVL

• 21 mm type 1090 Al (99.9%)• Filtered HVL : 6.80 mm Al

+ + + + +• 0.5 mm Cu + 2.8 mm Al• Filtered HVL : 6.80 mm Al

HVL6.8 mm Al

Normalized Spectra,Cu/Al spectrum is about 2X that of RQA5


Standard BeamReport 116 illustrated how the addition of different Al filters to a Cu filter could compensate for differences in the unfiltered beam quality.

Approximate Al to add to 0.5 mm Cu to achieve 6.8 mm HVL at 70 kVp. Averaged from about 25 systems tested.


AAPM Report 116• HVL = 6.8 mm Al• Adjust Al and (if necessary) kVP to get HVL• 70 ± 4 kVP• 0.5 mm Cu + (0 – 4) mm Al (type 1100)

– Brass acceptable as a Cu substitute– 21 mm pure Al acceptable.

IEC Beam condition• 70 kVP• 0.5 mm Cu + 2 mm Al• HVL = 6.8 +/- 0.3 mm Al

Beam Quality – IEC vs Report 116

• Assumes 2.9 mm HVL unfiltered beam

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AAPM Report 116• HVL = 6.8 mm Al• Adjust Al and (if necessary) kVP to get HVL• 70 ± 4 kVP• 0.5 mm Cu + (0 – 4) mm Al (type 1100)

– Brass acceptable as a Cu substitute– 21 mm pure Al acceptable.

IEC Beam condition• 70 kVP• 0.5 mm Cu + 2 mm Al• HVL = 6.8 +/- 0.3 mm Al

Beam Quality – IEC vs Report 116

• Assumes 2.9 mm HVL unfiltered beam

Verifying EI calibration


Verifying EI Calibration• Check generator kV calibration/reproducibility • Set up the Standard Beam HVL (add copper and

adjust Al filtration)• Establish a reference dosimeter away from the CR• Obtain grid and table-top attenuation factors from

the manufacturer• Set up for a 10 µGy exposure to the detector (EI =

1000)• Compare to EI that the system reports

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Beam setup – step 1Prior to any measurements verify that the

x-ray source has acceptable exposure reproducibility (coefficient of variation < 0.03) and kV accuracy (within ± 3%) at the standardized condition.


Beam setup – step 2Collimate the x-ray beam to only

cover the ion chamber with no more than 1 inch margins.

Add 0.5 mm copper filtration at the face of the collimator.

For DR systems, the detector should be covered with a lead apron or similar barrier when making the exposures for HVL determination and adjustment.


Aluminum for HVL

Beam setup – step 3Measure the HVL of the filtered

beam

Adjust the kVP and/or aluminium filtration within the limits specified to obtain a HVL as close as possible to 6.8 mm Al.

70 kVP + 4

0 – 4 mm Al

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Beam setup – step 4 (DR)The detector should be placed as far from the

x-ray source as is practical, at least 100 cm. Collimate to the edges of the detector.

If present, remove the grid and any other components between the ion chamber and the image detector. If any components cannot be removed, obtain the attenuation factors from the DR system or component vendor.


Source

> 25 cm(CR only)

Source to Detector Distance

(maximum possible)

Lab Jack(CR only)

Collimator

Added Filtration

Lead (CR only)

Beam setup – step 4 (CR)For CR, the cassette should be

separated from any surface that may increase backscatter from that surface, as recommended in AAPM Report 93 (TG 10). Use lead behind the plate to further reduce backscatter.

If the detector is not square, the long axis of the detector should be perpendicular to the x-ray tube A-C axis.


Detector housing

Beam setup – step 5Place a calibrated ion

chamber at the center of the beam approximately midway between the source and detector (Position A).

All distances should be measured from the focal spot as indicated on the x-ray tube housing.

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Source


(maximum possible)

Ion chamber(position A)

Source to Chamber Distance

Chamber to Detector Distance

Collimator

Added Filtration

Beam setup – step 6Use lead to protect a DR detector.

Make an exposure. Using an inverse square correction and applying the grid and table attenuation factors (if present) determine the air kerma at the detector (KCal).

Change the mAs setting to obtain a Kcal value that is in the middle of the detector’s response range (suggest 10 uGy).


Source


(maximum possible)

Source to Chamber Distance

Chamber to Detector Distance

Collimator

Added Filtration

Ion chamber(position B)

Beam setup – step 7Move the ion chamber perpendicular to

the tube axis such that it is at the edge of the field of view (Position B).

Ensure that the entire ion chamber is in the radiation beam and is not shadowed by a collimator blade.

Make an exposure using the mAs found earlier and determine the ratio of the air kerma at Position A to that at Position B.


Beam setup – step 8Remove the protective lead and

expose the detector again.

Verify the resulting KCal by monitoring the exposure with the chamber at position B.

Compare the corrected reading to the Exposure Index reported by the system/detector. EI should equal KCal *100 if properly calibrated,

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EI CalibrationEI calibration should be verified

• At acceptance• Annually thereafter• After service events and software

upgrades


Rules for repeatsLittle clinical information on reject

thresholds in literature• Van Metter and Yorkston

–Chest and abdominal imaging –Wide range of patient thicknesses –Most AEC controlled images fell within

the range of DI = ± 1.2.


Rules for RepeatsEmulating film/screen limits

ΔOD = γ*Log10 {E2/E1}, and DI = 10*Log10 {E2/E1}

So: Δ DI = 10*ΔOD / γγ (film gamma): slope of H&D at 1.0 above B+F

For an OD range of +0.3 (0.6 OD total) and γ = 2.5:

Δ DI = 10 * {0.6/2.5} = 2.4Acceptable DI range is + 1.2

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DI action levels (Table 2)DI EI Action

Between -1.0 and +1.0 79% < exp < 126% Check for noise and

clipping (always)

< -1.0 < 79% of targetCheck for noise and

consult with radiologist/management

on need for repeat, investigate cause.

Between +1.0 and +3.0

126% – 200% of target

Repeat only if relevant anatomy is clipped or

“burned out”

> 3.0 > 200% of target

Repeat only if relevant anatomy is clipped or “burned out”, require

immediate management follow-up


DI Action LevelsSites reporting excessive follow-up.

Action levels based on screen/film emulation is admittedly arbitrary.

Little or no published literature.


DI Action LevelsAAPM Report 232

To investigate the current state of practice for CR/DR Exposure and Deviation Indices based on AAPM Report 116 and IEC 62494, for the purpose of establishing achievable goals (reference levels) and action levels in digital radiography. The products of this task group will be a brief report and an updated version of Table 2 from AAPM Report #116.Jaydev Dave and Kyle Jones, Co-Chairs

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Other TG116 Recommendations• VOI Histograms and graphical pixel

overlays• Repeat/Reject logs• Import/Export of EIT tables• Export of for-processing image data• Dependence of EI on kV


AEC and Technique Charts Still important in controlling patient

exposure• AEC may need to be adjusted for CR and

add-on DR– Energy dependence may not be the same as

GdO2S• Technique charts are important for

portables and extremities


AEC Calibration Using EIEI should remain consistent with:

• Varying energy• Varying phantom thickness• Varying AEC cell• Varying dose rate (mA)

• EI should be reproducible over multiple exposures

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What the Exposure Index is NOT for

Patient dose estimation– Need beam HVL, pt. thickness, SSD and

SID, grid atten, AEC pickup atten, detector assy. input atten …

– If you have all these, you don’t need EI!

System intercomparisons– Index says nothing about detector energy

dependence, efficiency. - VOI recognition strategy will dramatically

affect indices.


Exposure Index MonitoringQC programs based on exposure indices

are successful– Seibert JA, Shelton DK, and Moore EH,

“Computed Radiography X-ray ExposureTrends,” Academic Radiology 3, 313-318(1996).

Katie Hulme will cover this next.


SummaryExposure Indices

• Noise, not density• Exposure creep• Exposure indices

– AAPM Report 116– IEC 62494-1

• Calibration• Rules for repeats

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THANK [email protected]


References:Fredman M, Pe E, Mun SK, Lo SCB, Nelson M, The

potential for unnecessary patient exposure from the use of storage phosphor imaging systems, SPIE 1897:472-479 (1993).

Gur D, Fuhman CR, Feist JH, Slifko R, Peace B, Natural migration to a higher dose in CR imaging, Proc. Eighth European Congress of Radiology, Vienna,Sep 12-17, 154 (1993).

Yorkston J. Flat-panel DR detectors for radiography and fluoroscopy. In: Specifications, Performance Evaluations, and Quality Assurance of Radiographic and Fluoroscopic Systems in the Digital Era,Goldman LW and Yester MV eds. Madison, WI: Medical Physics Publishing (2004)177-228.


References:Willis CE, Thompson SK and Shepard SJ. Artifacts

and Misadventures in Digital Radiography. Applied Radiology 33(1):11-20, January 2004.

R.E. Alvarez, J.A. Seibert, and S. K. Thompson,Comparison of dual energy detector system performance, Medical Physics31(3), 556-565 (2004).

JA Seibert, DK Shelton, and EH Moore, Computed Radiography X-ray Exposure Trends, Academic Radiology 3, 313-318 (1996).

J A Seibert, et al, AAPM Report #93, “Acceptance Testing and Quality Control of PhotostimulableStorage Phosphor Imaging Systems: Report ofAAPM Task Group 10.” AAPM (2006)

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References:Richard S., Siewerdsen J. H., Jaffray D. A., Moseley

D. J. and Bakhtiar B., Generalized DQE analysis of radiographic and dual-energy imaging using flat-panel detectors, Med. Phys. 32 (5), May 2005, 1397 – 1415

Lehman L. A., Alvarez R. A., Macovski A., Brody W. R., Pelc N. J., Reiderer S. J., and Hall A., Generalized image combinations in dual KVP digital radiography, Med. Phys. 8 (5), Sept/Oct 1981, 659 – 667

Shepard S.J., et al, AAPM Report 116, An Exposure Index for Digital Radiography (Executive Summary), Med. Phys., 2009


References:IEC 62494-1, Medical electrical equipment- Exposure

index of digital X-ray imaging systems Part 1: Definitions and requirements for general radiography, International ElectrotechnicalCommission, 2008

the exposure index for digital radiography (iec 62494-1...

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