Download Computed Radiography: Acceptance Testing and Quality Control

Computed Radiography:
Acceptance Testing
and
Quality Control
Introduction
CR is the primary means to capture 2D
images in a PACS environment
• Acceptance testing validates performance
• Quality control verifies optimal operation
J. Anthony Seibert, Ph.D.
University of California Davis
Department of Radiology
Sacramento, California
Considerations:
• Knowledge of CR attributes and operation
• Understanding the tests
• Determining appropriate results
Presentation Outline
Computed Radiography (CR)
...is the generic term applied to an imaging system
comprised of:
Overview of CR
How does it work? What are the issues?
Photostimulable Storage Phosphor
to acquire the xx-ray projection image
Acceptance test procedures
What tests?
Why?
Quality control
When ?
CR Reader
How?
to extract the electronic latent image
Digital electronics
to convert the signals to digital form
What?
How often?
CR Image Acquisition
1. X-ray Exposure
Patient
5.
unexposed
2.
Image
Reader
X-ray
system
2. Display
3.
Image
Scaling
Computed
Radiograph
1. Acquisition
Digital to Analog
Conversion
Transmitted xx-rays
through patient
Digital
processing
4.
Image
Record
Analog to Digital
Conversion
exposed
Phosphor plate
Digital Pixel
Matrix
Charge
collection
device
X-ray converter
x-rays → electrons
3. Archiving
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
1
CR Networking
Film laser printer
CR
Reader
• PACS and DICOM
– Digital Imaging COmmunications in Medicine
– Provides standard for modality interfaces,
storage/retrieval, and print
• Modality Worklist Input (from RIS via HLHL-7)
DICOM
• Technologist QC Workstation
– Image manipulation and processing
– Processed / Unprocessed images
CR - QC Workstation
• DICOM image output
PACS SoftSoft-copy review
Stimulation and Emission Spectra
CR: How does it work?
Photostimulated Luminescence
t
recombination
4f 6 5d
F/F+
PSL
3.0 eV
t
e-
Laser
stimulation
Energy Band
BaFBr
8.3 eV
2.0 eV
Eu
Emission
Optical
Barrier
Conduction band
tunneling
Relative intensity
t
phonon
BaFBr: Eu2+
Stimulation
1.0
0.5
Diode
680 nm
4f 7
Eu
3+
/ Eu 2+
Incident
x-rays
e
Valence band
PSLC complexes (F centers) are created in
numbers proportional to incident xx-ray
intensity
0.0
800
1.5
PSL
Signal
PMT
Exposed
Imaging
Plate
Light
Scattering
Photostimulated
Luminescence
Protective Layer
2
f-θ
lens
Laser
Source
Polygonal
Mirror
Laser beam:
Scan direction
Phosphor Layer
Laser Light Spread
"Effective" readout diameter
1.75
Reference
detector
Light guide
600
500
2.5
400
300
3
4
λ (nm)
Energy (eV)
CR: Latent Image Readout
Photostimulated Luminescence
Incident Laser Beam
700
Base Support
Cylindrical mirror
Light channeling guide
Output Signal
PMT
ADC
ADC
x= 1279
To1333
image
y=
processor
z= 500
Plate translation:
SubSub-scan direction
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
2
Phosphor Plate Cycle
PSP
SubSub-scan Direction
Typical CR resolution:
Base support
x-ray exposure
Plate translation
reuse
35 x 43 cm -- 2.5 lp/mm (200 µm)
24 x 30 cm -- 3.3 lp/mm (150 µm)
18 x 24 cm -- 5.0 lp/mm (100 µm)
plate exposure:
create latent image
laser beam scan
plate readout:
extract latent image
light erasure
Screen/film resolution:
Laser beam deflection
plate erasure:
remove residuals
7-10 lp/mm (80 µm - 25 µm)
Computed Radiography
• Acquisition, Display and Archive are separate
functions
• Variable speed detector
– 20 to 2000 speed
• Wide dynamic range
– 0.01 to 100 mR
Exposure Latitude: Dynamic Range
Film
Signal output
Scan Direction
CR
100:1
10000:1
• Image processing is a crucial requirement
Log relative exposure
Raw image
Processing the Image
• Image prepre-processing
• Inherent subject
contrast displayed
– Find pertinent image information (histogram analysis)
– Scale data to an appropriate range
• Contrast inverted
(to screenscreen-film)
• Contrast enhancement
• PSL signal amplitude
log amplified
• Spatial frequency enhancement
– Anatomy specific grayscale manipulation
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
3
Histogram analysis
Finding the Image Location
• Frequency distribution of pixel values within a
defined area in the image
• Image recognition phase
– Collimation (Agfa)
– EDR, automatic mode (Fuji)
• Shape is anatomy specific
– Segmentation (Kodak)
• Sets minimum and maximum “useful” pixel values
• Finding collimation borders and edges
Histogram:
Frequency
1
4
4
4
1
4
2
2
2
4
4
2
3
2
4
4
2
2
2
4
1
4
4
4
1
Frequency
Histogram Distribution
frequency distribution of pixel values in an image
16
14
12
10
8
6
4
2
0
0
1
2
Value
Pixel value
Useful signal
The shape is dependent on radiographic study,
positioning and technique
1023
Frequency of
Digital Number
Direct
x-ray
area
Frequency
Anatomy
5
Histogram Distribution
Histogram Distribution
Collimated
area
3 4
Value
Q2
Digital
value
511
S1
0.01mR 0.1mR
SK
1mR
Q1
0
S2
10mR
100mR
Latitude (L)
20000
2000
200
20
2
Sensitivity (S)
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
4
Data conversion
Input to output digital
number
1,000
102
101
100
10-1
511
1023
10-1 100 101 102 103 0
Raw Digital Output
Exposure input
min
600
400
200
0
600
400
200
200
600
1,000
Input digital number
0
0
max
1. Find the
signal
2. Scale to
3. Create film
range
to 8323
to 9368
800
800
Frequency
Output digital number
Relative PSL
Exposure into digital number
Histogram
Histogram: pediatric image
Grayscale transformation
200
400
600
800
1000
Digital value
Useful image range for anatomy
looklook-alike
Data conversion for overexposure
Exposure into digital number
Relative PSL
Reduce overall gain
102
101
100
10-1
Exposure
input
PrePre-processed
“raw” image
Scaled and inverted:
“unprocessed” image
10-1 100 101 102
0
511
1023
Raw Digital Output
overexposure
Screen-Film
Underexposed
103
min
max
(scaled and log amplified)
Computed Radiography
Overexposed
Underexposed
Overexposed
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
5
Data conversion for wide latitude
ScreenScreen-Film
80 kVp, 18 mAs
Exposure into digital number
CR
80 kVp, 64 mAs
80 kVp, 18 mAs
Change gradient
(auto mode)
Relative PSL
102
101
100
10-1
Exposure
input
10-1 100 101 102 103
low kVp
(wide range)
0
511
1023
Raw Digital Output
(scaled and log amplified)
min
400 speed screen - film
max
L=4, wide latitude
LookLook-upup-table transformation
Contrast Enhancement
1,000
Output digital number
• Optimize image contrast via nonnon-linear
transformation curves
• Unprocessed images display linear “subject contrast”
– “Gradation processing” (Fuji)
– “Tone scaling” (Kodak)
– “MUSICA” (Agfa)
M
E
L
A
800
600
GT
Gradient Type
Fuji System
Example LUTs
400
200
0
0
200
400
600
800 1,000
Input digital number
Spatial Frequency Processing
“Edge Enhancement”
Response
Solid:
Original
originalMTF
response
Edge
Enhanced:
Difference:
Dash:
low
pass
filtered
Difference
Original
Original - +
filtered
Raw
Unprocessed
Contrast Enhanced
Sum
low
low
low
Original
Blurred
high
high
high
Spatial frequency
Difference
Edge enhanced
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
6
Fuji CR Parameter Settings
LUT shape parameters
Anatomy
Anatomical region
General chest (LAT)
General chest (PA)
Port Chest GRID
Port Chest NO GRID
Peds chest NICU/PICU
Finger
Wrist
Forearm
Plaster cast (arm)
Elbow*
Upper Ribs*
Pelvis*
Pelvis portable
Tib/Fib
Tib/Fib
Foot
Foot*
Os Calcis
Foot cast
C-spine
T-spine
Swimmers
Lumbar spine
Breast specimen
GA
1.0
0.6
0.8
1.0
1.1
0.9
0.8
0.8
0.8
0.8
0.8
0.9
0.9
0.9
0.8
1.2
0.8
0.8
1.1
0.8
1.2
1.0
2.5
GT
B
D
F
D
D
O
O
O
O
O
O
O
O
N
O
N
O
O
F
F
J
N
D
GC
1.6
1.6
1.8
1.6
1.6
0.6
0.6
0.6
0.6
0.6
1.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
1.8
0.9
0.9
0.6
Frequency enhancement
parameters
GS
-0.2
-0.5
-0.05
-0.15
-0.2
0.3
0.2
0.3
0.4
0.4
0.0
0.2
0.2
0.25
0.3
-0.05
0.4
0.5
0.5
-0.05
0.3
0.4
0.35
RN
4.0
4.0
4.0
4.0
3.0
5.0
5.0
5.0
5.0
7.0
5.0
6.0
4.0
5.0
5.0
7.0
5.0
5.0
5.0
4.0
5.0
5.0
9.0
RT
R
R
T
R
R
T
T
T
T
T
R
T
T
F
T
T
F
F
P
T
T
T
P
Recommended Acceptance Tests
• Physical Inspection–
Inspection–Inventory–
Inventory–PACS Interfaces
RE
0.2
0.2
0.2
0.5
0.5
0.5
0.5
0.5
0.5
1.0
1.0
1.0
0.5
0.5
0.5
0.5
1.0
0.5
0.5
0.2
0.5
1.0
1.0
Acceptance Test / QC considerations
• Image acquisition
• ElectroElectro-Mechanical readout
• Image processing
• PACS / RIS interfaces
• Image handling
Recommended Acceptance Tests
• Noise / LowLow-Contrast Response
• Imaging Plate Uniformity and Dark Noise
• Distortion
• Signal Response: Linearity and Slope
• Erasure Thoroughness
• Signal Response: Exposure calibration and
beam quality
• Artifact Analysis: Hardware/Software
• Laser Beam Function
• Positioning and collimation robustness
• High Contrast Resolution
• Imaging Plate Throughput
Acceptance test tools required
CR: Spatial Resolution
• Phosphor plate sizes: impact on resolution
• Exposure meter/dosimeter
• Spatial resolution phantom
• Low contrast phantom
• Vendor QC phantom (periodic tests)
• SMPTE test pattern
• Anthropomorphic phantom
• Documentation log / spreadsheet / instructions
35x43 (14x17)
24x30 (10x12)
18x24 (8x10)
0.2 mm pixels
0.14 mm pixels
0.1 mm pixels
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
7
High Contrast (Spatial) Resolution
18 x 24 cm
MTF Curves
35 x 43 cm
1
MTF
0.8
PrePre-sampled
MTF
ScreenScreen-film
0.6
Scan
Subscan
0.4
Sampled MTF:
Standard CR
0.2
2K x 2K matrix
35 x 43 cm
0
Photon absorption fraction
0
2
4
6
8
10
Spatial Frequency (lp/mm)
Low Contrast Response: Leeds TOTO-16
X-ray Absorption Efficiency
1
Hi res CR
Standard CR
BaFBr, 100 mg/cm²
0.8
Gd2O2S, 120 mg/cm2
0.6
0.4
0.2 BaFBr, 50 mg/cm²
0
0
20
40
60
80
100
120
140
Energy (keV)
3.5 mR
70 kVp
0.5 mR
Uniformity
498
508
537
490
497
10 mAs
480
513
505
544
487
20 mAs
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
8
Radiation Dose for CR
Sensitivity number, S
• Variable Speed Detector
• Estimate of the incident exposure on the IP
• Optimal dose (UC Davis)
– Adult chest image:
– Neonates/pediatrics:
– Extremities:
• Comparable to screenscreen-film “speed”
S=200S=200-300
S=400S=400-600
S= 7575-100
• Amplification required to map median value of
histogram to 511 (0 to 1023 grayscale)
– Lower detection efficiency, luminance and readout noise
• Dependent on histogram shape and examination
selected
• AntiAnti-scatter grids needed
Date:
7/10/98
Date:
7/10/98
Medical
Medical Physicist:
Physicist: Anthony
Anthony Seibert,
Seibert, Ph.D.
Ph.D.
Location:
Location:
System
System Identification:
Identification:
UCDMC,
UCDMC, ACC,
ACC, 33
CR
CR unit
unit 33
Guidelines for QC based on Exposure
UC
UC Davis
Davis Medical
Medical Center
Center
CR
CR Reader
Reader and
and Screens
Screens
Signal
Signal Response:
Response: Calibration
Calibration and
and Beam
Beam Quality
Quality
Sensitivity number
Note:
Note: Use
Use mAs
mAs values
values to
to provide
provide an
an approximate
approximate exposure
exposure of
of 11 mR
mR to
to the
the IP.
IP.
Menu
Menu == TEST
TEST
IP
ST14x17
14x17
IP Type:
Type: ST
IP
IP SN:
SN:
Exposure
Exposure Conditions
Conditions
SubMenu
SubMenu==Ave
Ave2.0
2.0
Focal
Focalspot
spot
Time
Timedelay
delay
SID
SID(cm)
(cm)
SMD
SMD(cm)
(cm)
LL == 2,
2, EDR
EDR == semi
semi
1.2
1.2 mm
mm
~2
~2 min
min
140
140
130
130
kVp
kVp Dependency
Dependency
kVp
kVp
Filtration
Filtration
m
mAs
As
m
mR-m
R-meter
eter
60
60
80
80
115
115
11 Al/0.5
Al/0.5 Cu
Cu
11 Al/0.5
Al/0.5 Cu
Cu
11 Al/0.5
Al/0.5 Cu
Cu
15.00
15.00
4.5
4.5
1.13
1.13
1.06
1.06
1.06
1.06
1.14
1.14
Filtration
Filtration
m
mAs
As
m
mR-m
R-meter
eter
80
80
80
80
80
80
none
none
11 Al/0.5
Al/0.5 Cu
Cu
1Al/2.5Cu
1Al/2.5Cu
0.50
0.50
4.50
4.50
60.00
60.00
0.96
0.96
1.06
1.06
0.99
0.99
<0.2 mR
• Underexposed: repeat
0.30.3-0.2 mR
• Underexposed: QC exception
OD
OD
1.41
1.41
1.38
1.38
1.45
1.45
0.07
0.07
NA
NA
NA
NA
NA
NA
NA
NA
• 300 - 600
1.01.0-0.3 mR
• Underexposed: QC review
SS
SS (1mR)
(1mR)
0.83
187.00
154.79
0.83
187.00
154.79
0.91
108.00
98.71
0.91
108.00
98.71
0.85
124.00
105.85
0.85
124.00
105.85
Maximum
Maximum Difference:
Difference: 56.08
56.08
OD
OD
1.40
1.40
1.38
1.38
1.40
1.40
0.02
0.02
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
• 150 - 300
1.31.3-1.0 mR
• Acceptable range
• 75 -150
1.31.3-2.7 mR
• Overexposed: QC review
• 50 - 74
4.04.0-2.7 mR
• Overexposed: QC exception
• <50
>4.0 mR
• Overexposed: repeat
m
mR-IP
R-IP
NA
NA
180.00
180.00
160.00
160.00
120.00
120.00
100.00
100.00
140.00
140.00
120.00
120.00
Response
sponse
Re
Response
Response
80.00
80.00
100.00
100.00
80.00
80.00
60.00
60.00
40.00
40.00
60.00
60.00
40.00
40.00
20.00
20.00
0.00
0.00
50
50
• >1000
• 600 – 1000
SS
SS (1mR)
(1mR)
0.91
121.00
110.59
0.91
121.00
110.59
0.91
108.00
98.71
0.91
108.00
98.71
0.98
115.00
113.04
0.98
115.00
113.04
Maximum
Difference:
14.33
Maximum Difference: 14.33
m
mR-IP
R-IP
Filtration
Filtration Dependency
Dependency
kVp
kVp
Indication
SS(1mR)
(1mR)
70
70
90
90
kVp
kVp
110
110
130
130
20.00
20.00
0.00
0.00
none
none
SS(1mR)
(1mR)
11 Al/0.5
Al/0.5 Cu
Cu
Filtrati
Filtration
on
1Al/2.5Cu
1Al/2.5Cu
Sensitivity number, S
• Imaging plate (HR vs. ST) differences
• Examination specific histogram shapes
– S number varies with examination type
• EDR mode effects
– Automatic (determines S1 and S2 values on histogram)
– SemiSemi-automatic (average value within ROI)
– Fixed (system acts like screenscreen-film detector)
• X-ray beam spectrum effects
What S value is appropriate?
• Determined by examination
– Adult exams (CXR, abdomen, etc)
– Extremities (ST plates)
– Pediatrics
UCDMC targets
150 – 300
75 – 150
300 – 600
• CR’s variable speed should be used to advantage
• Anatomical information can be lost with too high or too
low exposure
– S number varies with beam hardness (calibration required)
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
9
Adult portable chest calculated exposures
First half, 1994, 4572 exams
38.3%
600
53.9%
7.8%
Adult portable chest calculated exposures
Second half, 1994, 4661 exams
Target
exposure
range
600
500
3.4%
Q3
300
Q4
Target
exposure
range
Low
High
Low
Incident Exposure
Other
12%
“Exposure Creep”
180
180
160
160
140
140
120
120
100
100
80
80
60
60
40
40
20
20
0
0
<50
System speed (S #)
Incident Exposure
April 1 - 17, 1996
Adult Portable Chest
100
<50
100
200
300
0
400
100
0
200
200
100
300
200
400
400
Q2
500
300
#exams
Q1
System speed (S #)
Grid technique
without a grid
>>1
100
000
6600
00-6699
99
5500
00554
99
4400
00-4444
99
3355
0-3
74
7
4
3300
00-3322
44
2255
002277
44
2200
00-222
1155 244
00-1177
44
1100
0-1
2244
5500
--77
44
Number of examinations
73.5%
500
400
500
#exams
23.1%
Sensitivity number
Radiation Dose for CR
• Variable Speed Detector
• Optimal dose for typical adult chest image is 2X higher
than 400 speed screen/film
– Lower absorption efficiency
– Quantum and electronic noise
– Readout inefficiencies of latent image
• AntiAnti-scatter grids necessary for most procedures
Wrong exam
5%
High
Repeated
Examinations with CR
Motion
6%
Positioning
46%
Reprinting
9%
Underexposure
10%
Overexposure
12%
Total # repeats = 1043 from
Willis, RSNA 1996
AEC adjustment procedures
• A 200200-speed equivalent exposure is desirable
• Empirically determine AEC setting(s) with simple uniform
phantoms
• UCDMC technique: use “fixed” mode (S=200) and sensitivity test
menu; adjust AEC response according to changes in film optical
density
• Verify settings with semisemi-auto mode
• Verify patient exposure “S” number; recheck often
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
10
Date:
7/10/98
Date:
7/10/98
Medical
Medical Physicist:
Physicist: Anthony
Anthony Seibert,
Seibert, Ph.D.
Ph.D.
Location:
Location:
System
System Identification:
Identification:
UCDMC,
UCDMC, ACC,
ACC, 33
CR
CR unit
unit 33
Problem areas
UC Davis Medical Center
CR
CR Reader
Reader and
and Screens
Screens
Inspection Results Summary
• FilmFilm-based performance measurements
Acceptab
Acceptable
le
1.
1. Physical
Physical Inspection
Inspection -- Inventory
Inventory
2.
2. Imaging
Imaging Plate
Plate Uniformity
Uniformity and
and Dark
Dark Noise
Noise
3.
3. Signal
Signal Response:
Response: Linearity
Linearity and
and Slope
Slope
4.
4. Signal
Signal Response:
Response: Calibration
Calibration and
and Beam
Beam Quality
Quality
5.
5. Laser
Laser Beam
Beam Function
Function
6.
6. High-Contrast
High-Contrast Resolution
Resolution
7.
7. Noise/Low-Contrast
Noise/Low-Contrast Response
Response
8.
8. Distortion
Distortion
9.
9. Erasure
Erasure Thoroughness
Thoroughness
10.
10. Anti-Aliasing
Anti-Aliasing
11.
11. Positioning
Positioning and
and Collimation
Collimation Errors
Errors
12.
12. Throughput
Throughput
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes*
Yes*
Yes
Yes
Yes
Yes
Yes
Yes
• Digital image analysis tools and evaluation methods
not readily available
• Low contrast resolution measurements
• Lack of a standardized QC phantom
Comments:
Comments:
Quality Control
Three levels of system performance quality control
1. Routine: Technologist level
- no radiation measurements
2. Full inspection: Physicist level
Periodic Quality Control
• Daily (technologist)
– Inspect CR system and status.
– Interfaces: PACS broker, ID terminal, QC workstation
- radiation measurements and nonnon-invasive adjustments
3. System adjustment: Vendor service level
– Erase image receptors (if status unknown).
- hardware and software maintenance
Periodic Quality Control
• Weekly / Biweekly (technologist)
– Calibrate review workstation monitors (SMPTE).
– Acquire QC phantom test images. Verify performance.
– Check filters / vents and clean as necessary.
Periodic Quality Control
• Quarterly (Technologist)
– Inspect cassettes. Clean with recommended agents.
– Review image retake rate and exposure trends.
– Update QC log. Review outout-ofof-tolerance issues.
– Clean screens with recommended agents.
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
11
Periodic Quality Control
• Annually (Physicist)
What is needed?
• Computer friendly phantoms
– Perform linearity / sensitivity / uniformity tests
– Inspect / evaluate image quality
• Objective quantitative analysis methods
• System performance tracking and database logs
– ReRe-establish baseline values (Acceptance Tests)
• Exposure monitoring tools and database tracking
– Review retakes, exposures, service records.
Fuji
Agfa
Line pair
phantoms
(contrast
transfer tests)
Lumisys
Home built
Diagonal bar
(laser jitter test)
40 line/cm grid
(visual aliasing)
Fiducial Markers
(distance accuracy)
Lead attenuator
(dynamic range)
Resolution
Bar Pattern
(qualitative)
Notches
(geometric
accuracy
tests)
Step wedge (signal, signal to
noise and linearity response tests)
Open area (scan
uniformity test)
Copper step wedge
(dynamic range, linearity, SNR)
Edge for
Presampled
MTF
Single exposure, qualitative
and quantitative
Additional Information / Help
• AAPM Task Group #10 document:
• Email: jaseibert@
jaseibert@ucdavis.
ucdavis.edu
• Dr. Ehsan Samei spreadsheets
– http://deckard
.mc.duke.edu
edu/~
/~samei
samei/downloads
/downloads
http://deckard.mc.duke.
• Vendor efforts for QC phantom development and
analysis
Summary
• CR is the mainstay for direct digital acquisition of
projection radiographs
• CR acceptance testing and QC are essential for
optimal operation
• A TEAM
approach is necessary
– Technologists, Radiologists, Physicists,
– Clinical Engineering, Information System Group
Computed Radiography: Acceptance Testing and Quality Control -- J.A. Seibert, Ph.D.
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