Acceptance Testing
and QC of Digital
Radiography Units
Douglas Pfeiffer, MS, DABR
Boulder Community Hospital
Outline
Status of Task Groups
Acceptance testing
Quality Control
Technologist
Physicist
Relevant Task Groups
TG 150
“Acceptance Testing and Quality Control of
Digital Radiographic Imaging Systems”
Physicist stuff
TG 151
“Radiographic System Quality Control”
Technologist stuff
TG 116
“DR Exposure Index Task Group”
Task Group Charges
TG 150: This group will outline a set of tests to be used in the
Acceptance Testing and Quality Control of Digital
Radiographic Imaging Systems.
TG 151: Task Group to recommend consistency tests (one or
more) designed to be performed by a Medical Physicist, or a
radiologic technologist under the direction of a medical
physicist, to identify problems with an imaging system that
need further evaluation by a medical physicist.
TG 116: To identify a method of providing feedback, in the
form of a standard index, to operators of DR systems, which
reflects the adequacy of the exposure that has reached the
image receptor (IR) after every exposure event.
TG 150 schedule
Should be competed in about 2 years
May have educational session
RSNA 2008
AAPM 2009
Preliminary guidelines may
be provided via MedPhys
TG 150
Working in concert with TG 151
Have sent to vendors a survey regarding the QC
recommendations for their systems
Comprehensive list of product specifications is
being developed (living)
Concern regarding access to “for processing” or
earlier images
TG 150
Determining what corrections are allowed
in a “for processing” image (IEC 62220-1)
Flat field
Geometric distortion
Reference list being compiled
TG 151
Pretty much the same schedule as TG150…
Compiling a summary of QC tools/programs
provided by manufacturers of radiographic
systems or by third parties
TG 116
Draft written 11/2007
The goal of TG 150 is to
0%
81%
6%
13%
0%
1.
2.
3.
4.
5.
write code for the automated evaluation of digital
QC images
establish standardized tests for digital imaging
equipment
compel digital imaging system manufacturers to
implement standardized QC in their systems
validate the QC programs currently provided by
digital imaging system manufacturers
define a technologist-level quality control
program for digital imaging equipment
The goal of TG 150 is to
1.
2.
3.
4.
5.
write code for the automated evaluation of digital
QC images
establish standardized tests for digital
imaging equipment
compel digital imaging system manufacturers to
implement standardized QC in their systems
validate the QC programs currently provided by
digital imaging system manufacturers
define a technologist-level quality control
program for digital imaging equipment
Charge of AAPM TG 150, http://aapm.org/org/structure/default.asp?committee_code=TG150,
and Ranger, private communication.
Acceptance Testing
Inventory: did you get what you ordered
(features, accessories)
Installation: is it correctly placed in the
room
Function: does it work as advertised
Clinical use: are the images acceptable
Inventory
Hardware
Grids (40”, 72”)
Cassettes
Test tools
Software
Special processing
QA tools
Installation
According to plan
Any unforeseen impediments
Shielding (x-ray, after all)
Function
Get performance specifications (tee hee!)
Laser printer
Workstation calibration
System interfaces
Beam characterization / Generator calibration
Exposure index calibration
Exposure index linearity
Limiting resolution / MTF
Noise and low contrast
Geometric accuracy
Throughput
Plate erasure (CR)
Automatic exposure control calibration
Artifacts
Laser Printer
Values may be specified by modality
manufacturer
LUT curve is modality-specific
End result should look like workstation
Typical values and ranges
Dmin: 0.05 ± 0.03
Low density: 0.45 ± 0.07
Mid density: 1.20 ± 0.15
High density: 2.20 ± 0.15
Workstation Calibration
AAPM Task Group 18
Calibrated to DICOM Gray Scale Display
Funtion
Luminance and illuminance
measurements are required
TG-18QC Test Pattern best for overall
check
System Interfaces
Verify PACS, RIS
communications
NOW, prior to clinical
use
No worky later – you
have VERY grumpy
staff!
Standard Radiographic Testing
Light field / radiation field
Electronic
Radiochromic film
Image receptor / radiation field
Direct: beam centering
Bead at center of receptor
Collimate smaller than detector area
CR: same as screen film
Standard Radiographic Testing
Generator calibration
Output
mR/mAs linearity
HVL
kVp accuracy
Reproducibility
PBL function (CR)
Light field intensity
SID
Beam Characterization
Exposure index calibrations are exposure dependent
Agfa CR: 75 kVp, 1.5 mm Cu
Fuji CR: 80 kVp, no added filtration
Kodak CR: 80 kVp, 0.5 mm Cu + 1 mm Al
TG 116: 80 kVp, 1 mm Al + 1 mm Cu + 1 mm Al
HVL requirements
0.1, 1.0, 10 mR
Tight tolerances: 10.0 mR ± 0.2 @ 80 kVp ± 1
SID (heel effect)
mAs
60 kVp, 5 mR, unfiltered
From:
TG116_v9c
Exposure Index Calibration
TG 116
Avoid “speed class” – different world now
(“acts sort of like a 200 speed system”)
KIND – Indicated Equivalent Air Kerma: “an indicator of
the x-ray beam air kerma, expressed in µGy, that is
incident of the digital detector and used to create the
radiographic image”
DI – Deviation Index: the relative deviation from the
value targeted (KTGT) by the system for a particular body
part and view
K
DI = 10 × Log10
[
IND
K TGT ( b ,ν )
]
Exposure Indexes
From:
TG116_v9c
Exposure Index Linearity
Should match manufacturer equation over
at least 3 orders of magnitude
Recommend 0.1 mR, 1 mR, 10 mR
Wait 10 minutes before processing CR
plates
Manufacturer-specific tolerances
Kodak: nominal EI ± 100
The exposure index
94%
1.
Varies from manufacturer to manufacturer
6%
2.
Is required for FDA approval of a system
3.
Is automatically flagged when an exposure is
out of range
4.
Allows a patient to see what his dose was
5.
Is prominent in the DICOM header
0%
0%
0%
The exposure index
1.
2.
3.
4.
5.
Varies from manufacturer to
manufacturer
Is required for FDA approval of a system
Is automatically flagged when an
exposure is out of range
Allows a patient to see what his dose
was
Is prominent in the DICOM header
Williams, et al, Digital Radiography Image Quality: Image Acquisition, J
Am Coll Radiol 2007;4:371-388.
Limiting Resolution / MTF
MTF is preferable
Difficult in the field
Need manufacturer software
60 kVp, 5 mR
Patterns up to 5 lp/mm, slight angle (2-5°)
No edge enhancement
Visual performance within 10% of manufacturer
specification
Verify at center and periphery
mammography contact test tool
Measure for all matrix/plate sizes
Limiting Resolution
Noise and Low Contrast
Low contrast is easiest field measurement
“UAB”, CIRS Model 903
Noise and Low Contrast
Low contrast performance should improve with
increased exposure
Kodak CR950
SN
18 x 24 GP 9101061800
24 x 30 GP 9102064558
35 x 43 GP 9104078405
Minimum detectable contrast
0.1 mR
1.0 mR
10 mR
2.0%
1.5%
1.5%
1.1%
1.1%
1.1%
0.6%
0.6%
0.6%
Noise and Low Contrast
Noise should vary predictably for the system
being tested
Use Linearity images (0.1 mR, 1 mR, 10 mR)
Can extend to improve fit
Plot STDEV vs Exposure (or some function thereof)
For example: Fuji CR 5000
plot (stdev)2 vs. 1/exposure
least squares fit
slope = 1.8 – 2.0
r2 > 0.98
Noise and Low Contrast
Kodak CR 975
Pixel value ∝ log(E)
SD(ROI) ∝ E-0.5
Noise vs Input Exposure
100
-0.3311
Noise (SD)
y = 9.2215x
2
R = 0.9853
10
1
0.1
1
Exposure (mR)
10
Geometric Accuracy
Image should not be morphed in any way
Two radiopaque objects of known length (or one
2D object of known dimensions)
Measured should be
accurate to within 1%
(Never found a problem)
Nominal = 152.4 mm
Throughput
System may be bogged down due to faulty
controlling computer, hardware or
electronic malfunction
Multiple readouts in rapid succession
10 mR
Direct: 5 exposures
Single plate CR: 5 plates
Multiplate CT: max feed loading
Throughput
Measure time from beginning of first readout to
end of last readout
Direct: end of first exposure to able to make a sixth
exposure
CR: first plate into reader to last plate out of reader
Throughput (readouts/hour) = 60 x N / t
Should be within 10% of manufacturer
specification
Plate Erasure (CR)
Exposure IP of each size to 10 mR
Erase
Reprocess
Compare exposure index to manufacturer
specifications
Kodak: EI < 100
AEC Calibration
CR response is different from S-F
Energy dependence
Determines typical exposure index and
patient dose
Proper gain setting may take some time
Lowest possible for NOISE tolerance
AEC Calibration
Uniform phantom
2” – 12” equivalent
PMMA (oofdah!)
60 – 120 kVp
Cell selection
Cell balance
Artifacts
CR
Screens
Dust, etc in optical path
Mechanical motion of slow, fast scan mechanisms
Image processing glitches
Direct
Flat fielding errors
Detector faults
Image processing glitches
Use large SID to limit heel effect
Artifacts
Ghost!
Ghost image was left after one hearty
exposure
Imagine the result after generator testing
Always cover the detector with at least 0.5
mm lead (more is better)
Artifacts
Screen Cleaner!
Recommend against Kodak Premoistened wipes
Clean only when needed
Use lint-free cloth, barely damp with water
(at least for Kodak)
Artifacts
EI = 250
EI = 430
2 days post-erasure
4 days post-erasure
Background Fog!
Natural background radiation is ~0.5
mR/day
(Plates not sensitive to much of this)
Typical diagnostic plate exposure ~ 1 mR
Recommend using or erasing cassettes
every 2-3 days
Clinical
Phantom image quality
DOSE
Exposure index ranges
Will take some time to develop
Exam-specific
Can use range of about factor of 1.75 (sort of
equivalent to the S-F ± 0.3 OD variation)
Phantom Image Quality
System performance
Clinical-esque
Anthropomorphic
Use your favorite
Clinical technique
Regarding comprehensive acceptance testing of
digital imaging systems, all of the following are true
EXEPT:
1.
75%
19%
2.
3.
0%
4.
6%
5.
0%
It is not needed due to the automatic
calibration and self-monitoring of modern
systems
It verifies correct installation and
calibration
It sets baseline values to be used for
future testing
It provides an opportunity to train
technologists in appropriate QC
It is the time when radiation shielding
should be verified
Regarding comprehensive acceptance testing of
digital imaging systems, all of the following are true
EXEPT:
1.
2.
3.
4.
5.
It is not needed due to the automatic
calibration and self-monitoring of
modern systems
It verifies correct installation and
calibration
It sets baseline values to be used for
future testing
It provides an opportunity to train
technologists in appropriate QC
It is the time when radiation shielding
should be verified
Williams, et al, Digital Radiography Image Quality: Image Acquisition, J
Am Coll Radiol 2007;4:371-388.
Quality Control
Technologist
TG 151
Physicist
TG 150
Technologist QC (TG 151)
Daily
Look for artifacts
Monthly
Laser printer sensitometry
Self-calibration module may fail!
Quarterly
Workstation QC (monthly?)
TG-18QC, SMPTE
Repeat analysis
Per manufacturer
Test object
Flat field
Repeat Analysis
Use ranges established at acceptance period
Most repeats should be positioning, motion, etc.
“Odd” images can often be saved with window
width / level or reprocessing
Check with radiologist if Exposure Index too low
NEVER repeat for Exposure Index too high
(unless saturated)
one facility had it as a policy!
Physicist (TG 150)
Laser printer – review QC
Workstation calibration
Beam characterization
Exposure index calibration
Exposure index linearity
Limiting resolution / MTF
Noise and low contrast
Physicist (TG 150)
Plate erasure (CR)
Artifacts
Phantom
Exposure Index review
Generator calibration?
Dose Creep
Not the guy who monitors the doses!
Recall that images just get better with
higher dose
“I’ll just up my mAs a bit to be sure”
AEC systems
calibration
check regularly
Monitor the indexes!
Generator Calibration?
Self-calibrating during the exposure
HVL change??
Anything funky will show up in dose
IMAGE QUALITY
DOSE
Dose creep, the gradual increase in patient doses,
17%
8%
75%
0%
0%
1.
2.
3.
Was a problem with screen-film imaging
also
Exists only in computed radiography
departments
Is due to the decrease in noise with
increased detector exposure
4.
Is easily recognized by the radiologist
5.
Is not impacted by the presence of
technique charts
Dose creep, the gradual increase in patient doses,
1.
2.
3.
4.
5.
Was a problem with screen-film imaging
also
Exists only in computed radiography
departments
Is due to the decrease in noise with
increased detector exposure
Is easily recognized by the radiologist
Is not impacted by the presence of
technique charts
Williams, et al, Digital Radiography Image Quality: Image Acquisition, J
Am Coll Radiol 2007;4:371-388.
Helpful Tools
Some manufacturers provide proprietary
tools to assist…
Fuji
GE
Flat field
images taken
prior to other
image quality
tests, as
driven by the
software.
GE
Flat Field
for processing
pretty darn nice!
GE
GE
Kodak
Kodak
Kodak
References
AAPM Monograph 20: Specification, Acceptance Testing and
Quality Control of Diagnostic X-ray Imaging Equipment (1991)
AAPM Report 74: Quality Control in Diagnostic Radiology (2002)
AAPM Monograph No. 30: Specifications, Performance Evaluations
and Quality Assurance of Radiographic and Fluoroscopic Systems
in the Digital Era (2004)
IPEM Report 91 Recommended Standards for the Routine
Performance Testing of Diagnostic X-Ray Imaging Systems (2005)
Frey, Changes in Equipment Acceptance Testing, J Am Coll Radiol;
2005; 2:639-641
AAPM Report 93 CR Acceptance Testing and QC (2007)
Williams, et al, Digital Radiography Image Quality: Image
Acquisition, J Am Coll Radiol 2007;4:371-388.
Exposure Indicators
The Digital Imaging Chain
Diagnostic
Exposure
Detection
(Lat. Image)
mAs
too
high
Capture
(Sampl./Quant.)
Processing
Output/Display
(Hardcopy/Softcopy)
Signal
CV
Image
Processing
mAs
optimal
CV
logE
mAs
too low
Imaging System
Code
Values
Code
Values
Code
Values
Film-Screen World
Adequate film optical density guaranteed
adequate dose:
Papa Bear: mAs too high, dose too high, film
too dark.
Mama Bear: mAs too low, dose too low, film
too light.
Baby Bear: mAs OK, film OD OK, dose OK.
The film-screen characteristic (H&D) curve
defined the proper exposure range.
What’s the Fuss About?
Since 1897, we have gotten by just
fine in film-screen without a universal
dose parameter for every image.
Why is this all of the sudden a
problem for digital imaging?
In The Digital World:
The good news:
Digital systems produce diagnostically useful
images over a very wide exposure range.
Images always have adequate brightness.
The bad news:
Digital systems produce diagnostically useful
images over a very wide exposure range.
Images always have adequate brightness.
In the digital world:
Since the always-acceptable brightness
masks the dose (mAs) used, the chance
for producing images with too little or too
high dose is increased.
Docs will either:
suffer too much noise (i.e. missed diagnosis if
dose too low),
or will be very pleased with absence of noise (but
patients suffer high dose).
Film/Screen
Above
optimal
exposure
Optimal
exposure
Below
optimal
exposure
Digital
Effects of Dose
on Digital Image Quality
Image brightness does not change
with dose: Double the mAs still gives
same brightness.
What DOES change with dose are
the noise properties of the image.
0.1 mR
0.3 mR
1.0 mR
Not Your Daddy’s S-F
Image brightness is always adequate,
regardless of dose.
Image brightness does not depend on mAs
(an mAs range over 100x greater than the
acceptable mAs range for film/screen).
Adequacy of dose manifests as adequacy
of image quality – dose does not manifest
as brightness.
The Answer My Friend
Film/screen was able to get by without
any dose parameter to accompany
each image, because film/screen was
a self-regulating system.
Digital is not self-regulating, and the
chance of missed diagnosis, or
excessive dose, is too great without
having a dose parameter
accompanying each image.
The Way We Were
Each hospital selected a film-screen
system based on their desired operating
speed class
Speed = 200 / (mR required for OD=1)
if 0.5 mR radiation exposes film to OD=1:
Speed = 200/(0.5) = 400
if 1 mR radiation exposes film to OD=1:
Speed = 200/(1) = 200
if 2 mR radiation exposes film to OD=1:
Speed = 200/(2) = 100
New World Symphony
In digital:
Each hospital decides on desired speed class
for each exam type (AEC calibration /
technique chart)
Noise
Dose
The manufacturer calibrates so that digital
exposure value relates to the incident
exposure
If vendor does not use a radiation meter to calibrate, then
dose indicator is worthless.
DR Patient Exposure Indicators
Canon
Reached Exposure Value REX
GE
Dose Area Product DAP, dGy*cm2 & skin dose mGy
Hologic
DAP & Exam Factor, Center of Mass of log E Histogram
Imaging Dynamics Corporation, IDC
Dose parameter, median exposure histogram f # ~ dose/target
Philips/Siemens/Thompson (Trixel)
Dose Area Product DAP, dGy*cm2
also Exposure Indicator or EI, ~ Speed Class
SwissRay
mA, sec, field size, kVp, no exposure indicator
What is Dose Area Product
(DAP)?
DAP = (entrance dose R, Gy) * (field size, cm2)
If you double the patient’s skin entrance dose, e.g.
double the mAs, the DAP also doubles.
If you double the x-ray field size (e.g. open the
collimators), the DAP will also double.
Measured by detector inside collimator, so DAP
could be part of patient’s image record in DR.
But defining a proper value of DAP is
problematic because DAP depends on
collimation.
CR Exposure Indicators
•Agfa
•LgM, Logarithm of the Median of the histogram
•+0.3 LgM = 2x exposure, –0.3 LgM = 1/2x exposure
•Fuji
•S number, Sensitivity Number
•200/S proportional to exposure
•Kodak
•EI, Exposure Index
•+300 EI = 2x exposure, –300 EI = 1/2x exposure
•Konica
•S value, similar to Fuji
Appropriate Values for Digital Dose
Indicators
(for a 200 speed class exam)
Exposure
(mR)
Fuji
(S)
Kodak
(EI)
Agfa
(LgM)
IDC/DR
f#
Low
0.5x optimal
0.5
400
1700
1.9
-1
Optimal
1
200
2000
2.2
0
High
2x Optimal
2
100
2300
2.5
+1
AEC Calibration
200 speed appropriate?
Can modify for diagnostic requirements
Energy response is different from screenfilm
Each manufacturer has specific calibration
methods
Quality Control
Three levels of system performance quality control
1. Routine: Technologist
- no radiation measurements
2. Full inspection: Physicist
- radiation measurements and non-invasive adjustments
3. System adjustment: Vendor service
- hardware and software maintenance
Periodic Quality Control (RT)
Daily
Inspect CR system interfaces
Erase image receptors
Weekly / Biweekly
Calibrate monitors (SMPTE)
QC phantom test image performance
Check / clean PSP screens and cassettes
Quarterly
Review image retakes and exposures
Update QC log. Review out-of-tolerance issues
Periodic Quality Control
Annually (Physicist)
Perform linearity / sensitivity / uniformity
Inspect / evaluate image quality
Re-establish baseline values
Review retakes, service records
What is needed?
Computer friendly phantoms
Objective quantitative analysis methods
System performance tracking and database logs
Exposure monitoring tools with database
Quality Control
Tracking the indexes
Not “Too dark or too light?”
“Is the index appropriate?”
May shift techniques from what you’re used to
overall higher (200ish vs. 400ish)?
increase kVp?
Quality Control
Control Limits
Know what is appropriate for your facility
work with radiologists, RT’s, manufacturer
Know what a change in your index means
a change of 300 for Kodak is a factor of 2 in
exposure
Set appropriate limits
what’s a reasonable, acceptable variation?
what impacts the number (collimation, etc.)
DO NOT REPEAT IF THE DOSE IS TOO HIGH!!!
Quality Control
Repeat analysis
DO NOT REPEAT FOR HIGH DOSE (no dark
films)
Light films
Low (or high) exposure index
Collimation
Positioning
Artifacts (junk on screen, mainly)
Frequency Components
sin(x)
sin(3x)
sin(5x)
sin(7x)
sin(9x)
sin(x)+sin(3x)
+sin(5x)
+sin(7x)
+sin(9x)
MTF Comparison
Image Quality Descriptors
“Resolution”
MTF
Noise
DQE
“Resolution”
Pixel size?
Misleading
MTF?
Better, but incomplete
Resolution and digital sampling
MTF of pixel (sampling) aperture
Modulation
1
0.9
0.8
0.7
0.6
0.5
50 µ m
100 µ m
0.4
0.3
0.2
0.1
200 µ m
0
0
2
4
6
8
10
12
14
16
18
20
22
Frequency (lp/mm)
Detector
pitch
Detector
aperture
Cutoff frequency = 1 / ∆x
MTF of sampling aperture
Nyquist (max usable) frequency = 1/2∆x, when pitch = aperture
Line Pair Resolution
MTF: object signal
MTF at 4.5 cm above stand
Scan
Perfect resolution: 50 µm pixel
a-Se
70 µm pixel
Subscan
CsI
100 µm pixel
Microcalcifications??
Noise
mAs
Electronic
Putting It Together
Detective Quantum Efficiency
almost universally regarded as the best overall
indicator of the image quality of digital radiography
systems
NPS(f) = Noise Power Spectrum: noise is frequency
dependent
G = a magic number