Quality Control Testing
of
GE Digital Mammography
Systems
John M. Sandrik, Ph.D.
GE Healthcare
Milwaukee, WI
john.sandrik@med.ge.com
GE Healthcare
2007
Day 2: Overview
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General considerations
The QC tests
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GE Healthcare
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
2007
Day 2: Overview (cont’d.)
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The QC tests
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GE Healthcare
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
2007
General Considerations
GE Healthcare
2007
Protecting the Detector
•
Repeated imaging of a stationary
object may lead to formation of
“ghost” image.
(2.5 mm thick)
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E.g., HVL, kVp tests.
Cover detector with
steel & lead plate
(provided) when
images not needed.
235
mm
233 mm
GE Healthcare
2007
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
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including Phantom IQ tests, Chap. 1
Minimize artifacts caused by testing.
GE Healthcare
2007
Analyzing Images
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Images displayed after acquisition
are in logarithmic format.
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Labeled “Proces” in Browser.
Quantitative tests must be performed
on “Raw” images.
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CNR Test
MTF Measurements
AOP Mode and SNR Check
Automatic image selection for RT
Tests on Seno DS and Essential
GE Healthcare
2007
Raw or Processed Image?
Annotation Level
“Full” or “Partial”
Processing Description
• “PROC_0,” “PROC_1,”
etc., then Processed
• None, then Raw
GE Healthcare
2007
Seno DS & Essential: FineView
From 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.
GE Healthcare
2007
The QC Tests
Tests also done by Rad. Tech.
GE Healthcare
2007
Flat Field Test
Browser Window
Then Select Flat Field from list of tests.
GE Healthcare
2007
Flat Field Test
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Image 25 mm thick PMMA plate
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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
GE Healthcare
2007
Flat Field Test
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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)
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(Max ROI Mean – Min ROI Mean)
expressed as % of Average ROI Mean.
High Frequency Modulation (HFM)
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GE Healthcare
Max (ROI Std. Dev. / ROI Mean) expressed
as percent.
2007
Flat Field Test
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SNR Uniformity
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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.
GE Healthcare
2007
Flat Field Test
•
Bad Pixel
Each pixel signal compared to its ROI Mean.
– |Pixel signal – ROI Mean| > threshold means
“bad pixel.”
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Bad ROI
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ROI with ≥ 2 bad pixels = “bad ROI.”
GE Healthcare
2007
Phantom Image Quality
Breast Support
Phantom
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
GE Healthcare
2007
Change of CNR Test: 2000 D
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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.
GE Healthcare
2007
Change of CNR Test: 2000 D
Second ROI centered
between largest mass
and group of largest
specks.
2
1
• Difference of means
measures contrast.
• Std. Dev. of background
measures noise.
First ROI centered
over largest mass
∆CNR must not exceed 0.2.
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2007
MTF Measurement: 2000 D
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Check of detector MTF
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Bar pattern on surface of Bucky
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Minimal magnification
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.
GE Healthcare
2007
MTF Measurement: 2000 D
•••
ROI to measure mean
of “space” material.
3.93
2.09
1.23
ROI to measure std. dev.
of “4 lp/mm” pattern.
1.11
1.0
ROI to measure mean
of “bar” material.
•••
•••
ROI to measure std. dev.
of “2 lp/mm” pattern.
Std. Dev.
× 222 %
MTF =
Meanspace − Meanbar
GE Healthcare
2007
CNR & MTF: DS, Essential
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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
GE Healthcare
2007
CNR & MTF: DS, Essential
IQST – Image Quality Signature Test
Automates MTF and CNR Tests
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2007
CNR & MTF: DS, Essential
Resolution Uniformity
Noise Power Spectrum
MTF
Contrast
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2007
CNR & MTF: DS, Essential
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Measurement compared to baseline
established from average of five
measurements over five days.
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Comparison not valid until after fifth
measurement.
Results, Action Limits, and Pass/Fail
posted at end of test.
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GE Healthcare
If failed, ensure that FineView has not
been changed from previous setting.
2007
AOP Mode and SNR Check
Check for
• Correct selection of
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kVp,
anode track,
filter, and
mAs and
Correct level of SNR
using Automatic Optimization of
Parameters (AOP) when varying
phantom thickness.
•
GE Healthcare
2007
AOP Mode and SNR Check
For 2000 D
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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.
GE Healthcare
2007
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.
GE Healthcare
2007
AOP Test Plates
20 x 20 cm2
Seno 2000 D
GE Healthcare
10 cm
21 cm
Seno DS,
Essential
2007
AOP Test Plate Change
Improved simulation of breast shape
GE Healthcare
2007
AOP Change & Phantom IQ
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Seno 2000 D
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Mo / Mo, 26 kVp, 125 mAs
Simulates AOP CNT mode
“film-like”
Seno DS and Essential
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GE Healthcare
Rh / Rh, 29 kVp, 56 mAs
Simulates AOP STD mode
“digital”
2007
AOP Change
Predicted Track / Filter Combination Use
Rh/Rh, 79%
Mo/Rh, 20%
Mo/Mo, 1%
Optimized for consistent CNR
rather than detector exposure
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)
GE Healthcare
2007
The QC Tests
Tests that need no
introduction
GE Healthcare
2007
QC Tests for the Med. Phys.
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•
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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.
GE Healthcare
2007
The QC Tests
FFDM-specific,
FFDM-modified tests
GE Healthcare
2007
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
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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
GE Healthcare
2007
Collimation Assessment
Check both Mo and Rh anode tracks
Seno Essential
• Acceptance, after a major repair (MEE)
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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
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GE Healthcare
24 cm x 30.7 cm
2007
Collimation Assessment
Moving and sizing FOV for Essential
• Two buttons on back of collimator
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lower sets field size
upper sets field position
Setting size
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First press of size button turns
on light.
Second press changes FOV to
next size.
Press repeatedly for desired size.
GE Healthcare
X-ray tube head,
rear view
FOV Position
FOV Size
2007
Collimation Assessment
Moving and sizing FOV for Essential
X-ray tube head,
• Setting position
rear view
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First press of position button
turns on light.
Second press moves FOV to
next position.
Press repeatedly for desired
position.
FOV Position
FOV Size
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2007
Collimation Assessment
Sliding paddle on Essential
• Paddle position must agree with FOV
position to enable exposure.
• To slide paddle, press either
paddle release button.
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Release button once paddle
has slid from its initial position.
At pre-defined position,
paddle automatically locks.
GE Healthcare
2007
Collimation Assessment
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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.
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Knob then returns to its initial position.
For sliding paddle, not necessary to
push sliding release button.
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2007
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.
GE Healthcare
2007
“Filmless” Collimation Test
Use of GAFCHROMIC® XR-QA
Light field
3.2 mm Aluminum plate
42 chips per sheet
GE Healthcare
10" x 12" sheet
4 x 4 cm2 chip
2007
“Filmless” Collimation Test
Set up
• White side up
• Better contrast of light field
Exposure
• 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
GE Healthcare
2007
“Filmless” Collimation Test
GAFCHROMIC® XR-M
XR-M film strip
placed white side up
Light field
Orange
White
side
side
(case marked)
White paper
3.2 mm Aluminum plate
International Specialty Products
http://www.ispcorp.com/products/dosimetry/content/gafchromic/index.html
GE Healthcare
2007
“Filmless” Collimation Test
GAFCHROMIC® XR-M
light field
x-ray field
detector
Light – X-ray deviation = 5 mm
X-ray – Detector dev. = 4 mm
in film plane.
Then scale to detector plane.
GE Healthcare
2007
Focal Spot Performance
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Film-based
Essentially the same as screen-film
test.
Magnification tested at 1.5X.
Recommend to replace with SubSystem MTF test
GE Healthcare
2007
Focal Spot Perf., Essential
For test of small focal
spot, cassette will not lie
flat on detector cover
GE Healthcare
2007
Focal Spot Perf., Essential
Support cassette on 3 cm
of AOP test plates or do
Sub-System MTF Test
GE Healthcare
2007
Sub-System MTF
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Bar pattern at 4.5 cm above breast
support
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Film-less measurement
Option to replace detector MTF test and
(film-based) Eval. of Focal Spot Perf.
Physicist must provide two bar patterns
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Tests focal spot plus detector sub-system.
“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.).
GE Healthcare
2007
Sub-System MTF
Contact Configuration
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4.5 cm acrylic block on Bucky
Bar pattern on block
Mo and Rh targets
Perp. and Para. to Anode-Cathode Axis
GE Healthcare
2007
Sub-System MTF
Small focal spot
• Tested at 1.8X.
• For Seno 2000 D,
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edge of bar pattern within 1 cm of chest wall edge of
image receptor.
9 cm x 9 cm FOV
For DS & Ess.,
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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
GE Healthcare
2007
Sub-System MTF
Magnification Configuration
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• 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
GE Healthcare
2007
Sub-System MTF Test
Measure
Mean and Std. Dev.
in ROIs
3
4
1.23
1
1
2
5
2.09
1.37
1
1.52
1.69
1
1.88
2.09
2.32
2
3.93
2.09 and 3.93 lp/mm
for Contact
GE Healthcare
2
8
2
2.58
2.87
3.19
3.54
3.93
4.37
4.86
8 10 11 12 13 14 15 16 17 1819 20
1.11
4
5
1.0
3
5 and 8 lp/mm for
Magnification
2007
Sub-System MTF
Calculation
• Presence of acrylic
block reduces signal
compared to detector
MTF test.
• Noise correction added
to improve accuracy.
GE Healthcare
2007
Bar Pattern Accuracy
Bar Pattern Frequencies (lp/mm)
Pattern
Group
Pattern A
Avg. Freq.
Pattern B
Avg. Freq.
5
5.0
4.5
6
6.0
5.5
7
7.0
6.5
8
7.9
7.5
9
9.0
8.5
10
10.0
9.4
11
10.7
10.5
12
11.8
11.2
13
13.2
12.5
GE Healthcare
Def’n. of Avg. Freq.:
4.5 line pairs
0.9 mm
4.5 lp
0.9 mm
= 5.0 lp/mm
Bar pattern calibration
is recommended.
2007
Exposure, Dose, Repro.
Dose in Automatic Optimization of
Parameters (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.
GE Healthcare
2007
Exposure, Dose, Repro.
AOP parameter selection based on
• Most attenuating “breast” tissue in
AOP sense window.
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May reject highly attenuating object.
Breast composition
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Based on attenuation and compression
paddle height
For accurate dose measurement
• No high attenuators in sense window
• No objects on phantom top
GE Healthcare
2007
Dose Measurement for AOP
Phantom
rotated 180º.
Wide edge of
wax insert
frame
2000 D & DS / Essential
23 / 31 cm
X-ray field
AOP Sense
Window
16 / 23 cm
19 / 24 cm
3.5
cm
14 / 19 cm
Ion chamber
outside AOP
Sense Window
GE Healthcare
Phantom
Chest wall edge
No protruding
screws or disk
on phantom
2007
Exposure, Dose Summary
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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.
GE Healthcare
2007
Artifact Eval., Flat Field Unif.
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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
GE Healthcare
2007
Artifact Eval., Flat Field Unif.
Bands in small FS raw image
Anode cut off, ~ 16 cm from chest wall
Collimator edge for 13 cm field
chest
wall
Signal boosted by gain
map, not radiation.
Area masked in
processed image.
Gain
Depth of Gain Map
19 cm
Gain map (schematic) to
produce flat 19 cm field
Position
GE Healthcare
2007
Viewing Conditions Check
and Setting
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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.
GE Healthcare
2007
Monitor Calibration
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Not a calibration, but a check.
Field Engineer sets black / white levels.
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FE performs perceptual linearization.
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Uses internal photometer.
FE records 5 baseline
luminance levels.
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Uses external, calibrated photometer.
Uses internal photometer.
During QC, physicist
compares lum. levels
to baseline.
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Log Luminance
•
L180
L255
L120
L60
L10
0
50
100
150
200
250
Digital Driving Level
Uses internal photometer.
GE Healthcare
2007
Monitor Calibration
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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.
GE Healthcare
2007
Image Quality–SMPTE Pattern
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Be sure to use SMPTE pattern from
graphics display driver not Browser or
elsewhere.
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Center of pattern must have bar pattern
not black and white squares.
Examine image for the listed features.
GE Healthcare
2007
Display Screen Uniformity
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Screen set to DDL = 255.
Examine screen for artifacts.
During system acceptance
When necessary to isolate system
artifacts.
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Not an annual test
No artifact or non-uniformity that is
expected to mimic or obscure clinical
information
GE Healthcare
2007
Thank you
GE Healthcare
2007
Appendix
GE Healthcare
2007
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.
GE Healthcare
2007
Automated RRA – Input
Classify image
• Accept
• Repeat
• Reject
Assign cause
Set all images
to same class
and cause
GE Healthcare
Enter unlisted
cause
2007
Automated RRA – Output
Cause # Repeats % Repeats # Rejects % Rejects
Exposure Total
Repeat, Reject Totals
Repeat, Reject Rates
GE Healthcare
2007