GE Healthcare 2007

Quality Control Testingof

GE Digital Mammography Systems

John M. Sandrik, Ph.D.GE HealthcareMilwaukee, WI

john.sandrik@med.ge.com

GE Healthcare 2007

Day 2: Overview

• General considerations• The QC tests

– Tests also done by Rad. Tech.• Flat Field and Phantom Image Quality• MTF and CNR• AOP and SNR

– Tests that need no introduction• kVp Accuracy and Reproducibility• Beam Quality Assessment (HVL)• Radiation Output• Mammo. Unit Assembly Evaluation

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Day 2: Overview (cont’d.)

• The QC tests– FFDM-specific, FFDM-modified tests

• Collimation Assessment / “Film-less”• Evaluation of Focal Spot Performance• Sub-system MTF• Ent. Exposure, Avg. Glandular Dose• Artifact Eval., Flat Field Uniformity• View Conditions Check and Setting• Monitor Calibration• Image Quality–SMPTE Pattern• Display Screen Uniformity

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General Considerations

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Protecting the Detector

• Repeated imaging of a stationary object may lead to formation of “ghost” image.– E.g., HVL, kVp tests.

• Cover detector withsteel & lead plate(provided) whenimages not needed.

235mm

(2.5 mm thick)

233 mm

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Order of Performing Tests

1. Flat Field test (RT Test, Chap. 1)– but not Phantom IQ tests.

2. Artifact Evaluation; Flat Field Uniformity (MP Test, Chap. 2)

3. Remaining tests– including Phantom IQ tests, Chap. 1

Minimize artifacts caused by testing.

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Analyzing Images

• Images displayed after acquisition are in logarithmic format.– Labeled “Proces” in Browser.

• Quantitative tests must be performed on “Raw” images.– CNR Test– MTF Measurements– AOP Mode and SNR Check

Automatic image selection for RT Tests on Seno DS and Essential

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Raw or Processed Image?

Annotation Level“Full” or “Partial”

Processing Description • “PROC_0,” “PROC_1,”

etc., then Processed• None, then Raw

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Seno DS & Essential: FineViewFrom the QC Manual:“… a physicist’s measurements (e.g. MTF and noise) performed using methods other than those described in this manual can be affected when FineView processing is applied.”

• FineView compensates for the detector MTF.• With FineView enabled, measurement

procedures differing from the QC Manual or QAP Tool may give unexpected results.

• Disable Fine View for Sub-system MTF.

Check FineView before doing independent tests.

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The QC Tests

Tests also done by Rad. Tech.

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Flat Field TestBrowser Window

Then Select Flat Field from list of tests.

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Flat Field Test

• Image 25 mm thick PMMA plate– To avoid false results, plate must be

clean and free from imperfections.• Automated parameter selection and

analysis• Set up instructions on-screen• Make two exposures• Results, Action Limits, and Pass/Fail

posted at end of test

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Flat Field Test

• Image sampled using 2 x 2 cm2 ROIs at 1 cm intervals

• Mean, Std. Dev. measured for each ROI• Test five image quality measures• Brightness Non-Uniformity (BNU)

– (Max ROI Mean – Min ROI Mean) expressed as % of Average ROI Mean.

• High Frequency Modulation (HFM)– Max (ROI Std. Dev. / ROI Mean) expressed

as percent.

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Flat Field Test

• SNR Uniformity– Second image subtracted from first and std.

dev. calculated for each ROI of difference image.

– SNRs calculated from ROI Means of first image and ROI Std. Devs. of difference image.

– (Max ROI SNR – Min ROI SNR) expressed as percent of Average ROI SNR.

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Flat Field Test

• Bad Pixel– Each pixel signal compared to its ROI Mean.– |Pixel signal – ROI Mean| > threshold means

“bad pixel.”• Bad ROI

– ROI with ≥ 2 bad pixels = “bad ROI.”

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Phantom Image Quality

Phantom

Breast Support

Chest wall edge

• Acquire image in Medical Application• 19 x 23 centered FOV for Essential• Use specified manual techniques• Score Processed phantom image on

acquisition WS and printer– Also on review WS for 2000 D

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Change of CNR Test: 2000 D

• Use image from phantom IQ test.• Analyze largest mass in Raw image

of phantom.• Measurement compared to baseline

established from average of five measurements over five days.

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Change of CNR Test: 2000 D

Second ROI centeredbetween largest massand group of largestspecks.

1

2

First ROI centered over largest mass

• Difference of means measures contrast.

• Std. Dev. of background measures noise.

∆CNR must not exceed 0.2.

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MTF Measurement: 2000 D

• Check of detector MTF– Minimal magnification

• Bar pattern on surface of Bucky– Para. or perp. to chest wall edge.– To avoid false results, the pattern must

be clean and free from scratches.• Remove compression paddle.• Set specified parameters. • Acquire image in Medical Application• Analyze Raw image.

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MTF Measurement: 2000 D

ROI to measure mean of “space” material.

ROI to measure mean of “bar” material.

ROI to measure std. dev. of “2 lp/mm” pattern.

ROI to measure std. dev. of “4 lp/mm” pattern.

••• ••• •••1.

0

1.11

1.23

2.09

3.93

%222MeanMean

.Dev.StdMTFbarspace

×−

=

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CNR & MTF: DS, Essential

• Image of Image Quality Signature Test (IQST) tool.

• Automated parameter selection and analysis

• Select QAP button.• Then select CNR and MTF test.• Set up instructions on-screen

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CNR & MTF: DS, Essential

IQST – Image Quality Signature Test

Automates MTF and CNR Tests

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CNR & MTF: DS, Essential

Resolution Uniformity

Contrast

MTF

Noise Power Spectrum

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CNR & MTF: DS, Essential

• Measurement compared to baseline established from average of five measurements over five days.– Comparison not valid until after fifth

measurement.• Results, Action Limits, and Pass/Fail

posted at end of test.– If failed, ensure that FineView has not

been changed from previous setting.

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AOP Mode and SNR Check

Check for • Correct selection of

– kVp, – anode track, – filter, and – mAs and

• Correct level of SNRusing Automatic Optimization of Parameters (AOP) when varyingphantom thickness.

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AOP Mode and SNR Check

For 2000 D• Acquire images in Medical App.• Select the STD AOP mode.• Acquire images.• Record parameters selected. • Open the Raw images for analysis.• Set the Zoom to “True Size.”• Set ROI, read mean and std. dev.• Calculate SNR.

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AOP Mode and SNR Check

For DS, Essential• Select QAP button.• Then select AOP and SNR Check.• Set up instructions on-screen.• AOP mode selected automatically.• Acquire images.• Parameters compared to specs.• Results, Action Limits, and Pass/Fail

posted automatically at end of test.

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AOP Test Plates

Seno 2000 D Seno DS, Essential

20 x 20 cm2 21 cm10 cm

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AOP Test Plate Change

Improved simulation of breast shape

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AOP Change & Phantom IQ

• Seno 2000 D– Mo / Mo, 26 kVp, 125 mAs– Simulates AOP CNT mode– “film-like”

• Seno DS and Essential– Rh / Rh, 29 kVp, 56 mAs– Simulates AOP STD mode– “digital”

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AOP ChangePredicted Track / Filter Combination Use

Mo/Mo, 1%

Rh/Rh, 79%

Mo/Rh, 20%

N.Shramchenko, P.Blin, C.Mathey and R.Klausz. Optimized exposure control in digital mammography.Medical Imaging 2004: Physics of Medical Imaging, Proc. SPIE 5368, 445-456 (2004)

Optimized for consistent CNR rather than detector exposure

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The QC Tests

Tests that need no introduction

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QC Tests for the Med. Phys.

• kVp Accuracy and Reproducibility• Beam Quality Assessment (HVL)• Radiation Output• Mammographic Unit Assembly

Evaluation

Tests not unique to FFDM.No procedures provided in the QC plan.

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The QC Tests

FFDM-specific, FFDM-modified tests

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Collimation Assessment

Seno 2000 D• Only 24 x 30 film and cassette identified in

equipment list– Will be generalized in next revision

Seno DS and Essential • Identify “auxiliary image receptor”

– 24 x 30 cassette– smaller cassette rotated and/or elevated– general rad. screen-film or CR cassette– multiple, small, distributed detectors

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Collimation Assessment

• Check both Mo and Rh anode tracksSeno Essential• Acceptance, after a major repair (MEE)

– 24 cm x 30.7 cm– 19 cm x 23 cm, centered– 19 cm x 23 cm, offset right– 19 cm x 23 cm, offset left

• Annual QC survey– 24 cm x 30.7 cm

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Collimation Assessment

Moving and sizing FOV for Essential• Two buttons on back of collimator

– lower sets field size– upper sets field position

• Setting size– First press of size button turns

on light.– Second press changes FOV to

next size.– Press repeatedly for desired size.

X-ray tube head,rear view

FOV Position

FOV Size

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Collimation Assessment

Moving and sizing FOV for Essential• Setting position

– First press of position button turns on light.

– Second press moves FOV to next position.

– Press repeatedly for desired position.

X-ray tube head,rear view

FOV Position

FOV Size

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Collimation Assessment

Sliding paddle on Essential• Paddle position must agree with FOV

position to enable exposure.• To slide paddle, press either

paddle release button.– Release button once paddle

has slid from its initial position. • At pre-defined position,

paddle automatically locks.

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Collimation Assessment

• Paddle release for Essential• Turn paddle unlocking knob

clockwise until it points to unlocked padlock symbol on the paddle holder.

• Release knob, slide paddle away. – Knob then returns to its initial position.– For sliding paddle, not necessary to

push sliding release button.

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Collimation Assessment

Suggested procedure for Essential• Set gantry at 0 degrees.• Set 19 x 23 FOV size (lower button).• Set FOV position (upper button).• Mark the light field edges.• Install sliding 19 x 23 paddle to

match FOV position.• Make the exposures.

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“Filmless” Collimation Test

10" x 12" sheet

Use of GAFCHROMIC® XR-QA

3.2 mm Aluminum plate

42 chips per sheet

Light field

4 x 4 cm2 chip

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“Filmless” Collimation Test

Set up• White side up• Better contrast of light fieldExposure• 30 kVp, 200 mAs, Mo/Mo and Rh/Rh• Self-develops “instantaneously”Attenuator• 3.2 mm aluminum• Useful contrast on film at exposure level used• Avoid detector saturation and “ghost” images

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“Filmless” Collimation TestGAFCHROMIC® XR-M

3.2 mm Aluminum plate

Light field

XR-M film strip placed white side up

White paperOrange

side(case marked)

Whiteside

International Specialty Productshttp://www.ispcorp.com/products/dosimetry/content/gafchromic/index.html

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“Filmless” Collimation TestGAFCHROMIC® XR-M

light fieldx-ray field

detector

Light – X-ray deviation = 5 mmX-ray – Detector dev. = 4 mmin film plane.

Then scale to detector plane.

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Focal Spot Performance

• Film-based • Essentially the same as screen-film

test.• Magnification tested at 1.5X.• Recommend to replace with Sub-

System MTF test

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Focal Spot Perf., Essential

For test of small focal spot, cassette will not lie flat on detector cover

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Focal Spot Perf., Essential

Support cassette on 3 cm of AOP test plates or do Sub-System MTF Test

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Sub-System MTF

• Bar pattern at 4.5 cm above breast support– Tests focal spot plus detector sub-system.

• Film-less measurement• Option to replace detector MTF test and

(film-based) Eval. of Focal Spot Perf.• Physicist must provide two bar patterns

– “low freq.” with 2.09 and 3.93 lp/mm– “high freq.” with 5 and 8 lp/mm

• Fine View must be disabled (DS & Ess.).

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Sub-System MTFContact Configuration

• 4.5 cm acrylic block on Bucky• Bar pattern on block• Mo and Rh targets• Perp. and Para. to Anode-Cathode Axis

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Sub-System MTF

Small focal spot • Tested at 1.8X.• For Seno 2000 D,

– edge of bar pattern within 1 cm of chest wall edge of image receptor.

– 9 cm x 9 cm FOV• For DS & Ess.,

– edge of bar pattern 50 mm from chest wall surface of mag. stand.

• Larger value, more reliable measurement• Less sensitive to positioning error

– 13 cm x 18 cm FOV

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Sub-System MTFMagnification Configuration

• Bucky removed• 9x9 FOV for 2000 D, 13x18 for DS, Ess.

• 4.5 cm acrylic block on Mag. Stand at 1.8X• Bar pattern on block• Mo and Rh targets• Perp. and Para. to Anode-Cathode Axis

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Sub-System MTF Test

3.934.37

1.0

1.11

1.23

1.37

1.52

1.69

1.88

2.092.322.582.873.193.54

4.86

2.09

3.93

3

4

1

1

2

2

205

810

1112

1314

1516

1718 19

3 4

1

2

51

82

MeasureMean and Std. Dev.

in ROIs

2.09 and 3.93 lp/mm for Contact

5 and 8 lp/mm for Magnification

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Sub-System MTF

Calculation• Presence of acrylic

block reduces signalcompared to detectorMTF test.

• Noise correction addedto improve accuracy.

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Bar Pattern Accuracy

PatternGroup

Pattern AAvg. Freq.

Pattern BAvg. Freq.

5 5.0 4.56 6.0 5.57 7.0 6.58 7.9 7.59 9.0 8.5

10 10.0 9.411 10.7 10.512 11.8 11.213 13.2 12.5

Bar Pattern Frequencies (lp/mm)

4.5 lp0.9 mm

= 5.0 lp/mm

0.9 mm

4.5 line pairsDef’n. of Avg. Freq.:

Bar pattern calibration is recommended.

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Exposure, Dose, Repro.

Dose in Automatic Optimization ofParameters (AOP) mode measured by• Acquiring image of accreditation

phantom and recording parameters,• Replacing phantom by ion chamber, • Setting AOP-selected parameters in

Manual mode,• Measuring exposure for selected

parameters.

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Exposure, Dose, Repro.

AOP parameter selection based on• Most attenuating “breast” tissue in

AOP sense window.– May reject highly attenuating object.

• Breast composition– Based on attenuation and compression

paddle heightFor accurate dose measurement• No high attenuators in sense window• No objects on phantom top

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Dose Measurement for AOP

X-ray field

Chest wall edge

Ion chamber outside AOP Sense Window

16 / 23 cm

23 / 31 cm

3.5 cm

14 / 19 cm

19 / 24 cm

AOP SenseWindow

Phantom

Phantom rotated 180º.Wide edge of wax insert frame

2000 D & DS / Essential

No protruding screws or disk on phantom

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Exposure, Dose Summary

• AOP parameter selection is more consistent with phantom rotated 180°.

• Acrylic disk on phantom increases dose.• Disk and raised thumbscrews cause

thickness estimate error.• Consistent compression force of 5 daN

improves AOP consistency.• ACR method – uniform phantom – is only

for accreditation application, not QC.

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Artifact Eval., Flat Field Unif.

• Image of uniform acrylic plate• Mo/Mo, Mo/Rh, Rh/Rh at lowest clinical

kVp• Large focal spot with grid in• Small focal spot without grid• Review Raw image• Set window width to 400 - 450• Visual evaluation of uniformity• No artifact or non-uniformity that is

expected to mimic or obscure clinical information

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Artifact Eval., Flat Field Unif.Bands in small FS raw image

Anode cut off, ~ 16 cm from chest wall

Depth of Gain Map19 cm

Collimator edge for 13 cm field

chestwall

Signal boosted by gain map, not radiation.

Area masked in processed image.

Gain map (schematic) to produce flat 19 cm field

Gain

Position

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Viewing Conditions Check and Setting• Set intended illuminance for interpretation of

mammograms.• Darken displays.• Measure illuminance at display screens.• For the RWS, recommend reducing ambient

illuminance upper limit from 50 to 20 lux.• For Seno Advantage, upper limit is 20 lux.• Provide “map” enabling RT to reproduce

illuminance conditions of your measurement.

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Monitor Calibration

• Not a calibration, but a check.• Field Engineer sets black / white levels.

– Uses external, calibrated photometer.• FE performs perceptual linearization.

– Uses internal photometer.• FE records 5 baseline

luminance levels.– Uses internal photometer.

• During QC, physicistcompares lum. levelsto baseline.– Uses internal photometer.

0 50 100 150 200 250Digital Driving Level

Log

Lum

inan

ce

L255L180L120

L60L10

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Monitor Calibration

• For “RWS,” background set to DDL = 180.

• For Seno Advantage, background set to DDL that gives luminance ~ 20% of luminance for DDL = 255.

• Encourage QC RT to ensure that Field Engineer leaves baseline luminance values after re-calibration.

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Image Quality–SMPTE Pattern

• Be sure to use SMPTE pattern from graphics display driver not Browser or elsewhere.– Center of pattern must have bar pattern

not black and white squares.

• Examine image for the listed features.

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Display Screen Uniformity

• Screen set to DDL = 255.• Examine screen for artifacts.• During system acceptance• When necessary to isolate system

artifacts.– Not an annual test

• No artifact or non-uniformity that is expected to mimic or obscure clinical information

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Thank you

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Appendix

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DS & Essential: Auto. RRA

Automated Repeat / Reject Analysis (RRA)• User classifies each image

• Accept (default)• Repeat (extra dose to patient)• Reject (no intention to keep, e.g., QC test)

• User assigns cause to each Repeat and Reject• System maintains data base• Repeat, Reject rates posted at user’s request• Export database for off-line analysis

Replace paper-based record keeping, hand calculations.

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Automated RRA – InputClassify image

• Accept• Repeat• Reject

Assign causeEnter unlisted causeSet all images

to same class and cause

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Automated RRA – OutputCause # Repeats % Rejects# Rejects% Repeats

Exposure TotalRepeat, Reject TotalsRepeat, Reject Rates