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22-Sept-2003 1
Towards Clinically-relevant Standardization of Image Quality
• Ehsan Samei, Duke University• Alan Rowberg, University of Washington• Ellie Avraham, Eastman Kodak Company• Craig Cornelius, Eastman Kodak Company
22-Sept-2003 2
Objectives• Describe current medical image quality
and consistency performance efforts• Identify limitations in existing standards• Outline 3 specific proposals:
– Add new image quality factors to standards– Update DICOM IQ performance services– Research technical-clinical connection
22-Sept-2003 3
DICOM Image Consistency Efforts• Three DICOM initiatives (1998-99):
– Grayscale Standard Display Function (GSDF)– Presentation LUT (P-LUT)– Grayscale Softcopy Presentation State (GSPS)
• And in actual practice…– Are widely & effectively implemented– Via IHE Consistent Presentation of Images
solution:• Promoted conformance testing• Demonstrated inter-vendor consistency
22-Sept-2003 4
DICOM GSDF: Barten Curve
0.1
1
10
100
1000
0 200 400 600
J1 J2
L1
~3 cd/m2
L2
~150 cd/m2
Number of perceptual levels J1 = J2
Absolute luminance incrementL1 << L2
22-Sept-2003 5
AAPM Task Group 18 Efforts
• Team of academic, clinical, and industry contributors
• “Assessment of Display Performance for Medical Imaging Systems” (2002):– Practical guidelines for qualitative & quantitative
display assessment– Includes all key aspects of display performance– Defines IQ test patterns and procedures– Recommends specific IQ acceptance criteria
22-Sept-2003 6
Standardization gaps
• DICOM GSDF:– Pros:
• Mathematical definition based on Human Visual System model
– Limitations:• Tonescale consistency
only: no other IQ factors• No acceptance criteria /
conformance procedures• Only for grayscale images• No Display Device
Services (Capabilities)
• AAPM TG18 report:– Pros:
• Provides Professional recommendations
• Covers all key display performance aspects
• Acceptance Criteria• Quantitative measures
– Limitations:• Only guidelines• Not a “standard”
22-Sept-2003 7
The technical-clinical gap:
• The connection between quantifiable IQ metrics & clinical performance is unknown:– Luminance: deviations from GSDF, number of
gray levels displayed– Spatial: resolution, noise, geometric distortion – Chromaticity variations– Environmental: ambient light, glare, reflection,– …
• Amount of acceptable variation is unknown
22-Sept-2003 8
3 proposals to bridge the gaps:
1. Extend DICOM standard beyond luminance response
2. Add and update DICOM Service Classes for image quality / performance
3. Promote research on clinical - technical image quality relationship
22-Sept-2003 9
1. Display Image Quality (DIQ) Initiative
• Add measurable and quantifiable elements of AAPM display performance procedures
• Include testing methodologies & defined limits for clinical / diagnostic performance
• Quantify visual performance using– Simple test images– Specific observer protocols– Relative acceptance indicators
• For both softcopy and hardcopy presentation
22-Sept-2003 10
DIQ Softcopy Examples
• Quantify % deviation from GSDF curve• Define criteria for min & max luminance• Define visual luminance evaluation• Evaluate specific image quality factors:
– Ambient light limits: specular and diffuse– Spatial resolution with TG18-QC/CX– Check geometric distortion with TG18-QC
22-Sept-2003 11
Contrast response comparison
0.001
0.010
0.100
0 100 200 300 400 500 600 700
JND index
dL/L
for a
JN
D
1.1cd/m2
237cd/m2
Non-standardized display contrast
GSDF contrast+/- 10%
22-Sept-2003 12
TG18-MP: bit-depth / continuous grayscale
TG18-CT:contrast / luminance response
AAPM TG18 Patterns
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The comprehensive TG18-QC test pattern for evaluation of key display characteristics:
• Resolution
•Luminance
•Geometric distortion
Comprehensive TG18-QC
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Other DIQ Extensions• Hardcopy quality metrics:
– Media & printer quality (e.g., visible coating variations, distortions, artifacts)
– GSDF compliance, # of JNDs theory vs. actual– Printable matrix size– Spatial frequency response fidelity
• Color extensions:– Standardize for grayscale areas of color images– Add descriptions for color image characterization– Color display and print device calibration
22-Sept-2003 15
2. DICOM Service Extensions
• Add Display Performance Service Class– Query image quality / performance information– Control / configure manageable settings– Include new IQ factors, e.g., MTF, ambient, …
• Extend Printer Configuration Retrieval Service Class– Include access to additional IQ factors
Note: Measurables include both human-evaluated and automatically-measured values
22-Sept-2003 16
Use cases: Standardizing Output• Printing application
– Retrieves matrix size and MTF of film printer– Determines type of magnification, if any, to be applied
to the image for smallest artifact
• Display Performance Service Class User – Requests the luminance characteristic curve from its
workstation’s display system – Determines if the display is standardized– If needed, computes an internal image tonescale
correction, producing GSDF standardized result
22-Sept-2003 17
0.00
50.00
100.00
150.00
200.00
250.00
300.00
0 100 200 300 400 500 600 700Digital Value
Lum
inan
ce
Target GSDFActual device
Desiredoutput
Original Input443
Modified Input345
Real device standardized performance
22-Sept-2003 18
Use Case: Quality Control Management
• Centralized management application:– Queries devices for calibration date, luminance
characteristics, ambient light settings, etc.– Records the results in a central database– Creates maintenance lists for displays and printers
• Reporting application uses database for:– Regulatory and management reports– Stability and lifetime statistics on displays to
support replacement schedules and budgets
22-Sept-2003 19
Request
Film
Printer Film
Printer
Diagnostic display Diagnostic display Diagnostic display
Web - based
displays Modality Capture console
Clinical display Reflective Hardcopy
Web - based displays
console Clinical display Clinical display
ResponseCharacteristic curve, MTF, other performance measures
Summary data
Administrator’s
Administrator’s
Characteristic curve, MTF, other performance measures
Summary data
Network-wide Quality Control
Regu
latory
Repo
rtsPerformance
& Lifetime
Statistics
22-Sept-2003 20
More use cases…
• Consultation:– Maximize perceptual similarity to ensure “What I see
is what you will get!”– Display performance information gives confidence
• Capture Consoles:– “The technologist sees what the doctor will get.”
– Reduce errors, retakes, miscommunication
– Extend quality control program to consoles
22-Sept-2003 22
3. Clinical Significant of IQ Measures
GSDF vs. Monitor gamma differences
-20
0
20
40
60
80
100
120
0 50 100 150 200 250 300
digital value
Lum
inan
ce c
d/m
^2
Hmm…
Physical metrics
We assume there is a
connection…
What deviations matter, and how
much?Clinical performance
22-Sept-2003 23
Rendered with DICOM GSDF Not rendered with DICOM GSDF
Clinical impact: unknown
22-Sept-2003 24
Goals of proposed research:
• Determine the clinical consequence of variations in image quality metrics (e.g., GSDF conformance, MTF, noise, …)
• Define what constitutes image quality from a diagnostic perspective
• Incorporate results into new joint standards that will utilize standardized test patterns, procedures, and clinical use cases
22-Sept-2003 25
Suggested research approach
• Sample research:1. Obtain images from 3
radiographic modalities2. Present images,
simulating nonstandard display behavior
3. Run observer performance experiments at major professional meetings and events
4. Analyze by ROC methods
• Form inter-society committee to: 1. Design specific research
projects2. Obtains & review data sets3. Solicit & encourage active
participation by researchers: radiologists, scientists, …
4. Arrange reporting of results5. Define recommendations to
standards and professional groups
22-Sept-2003 26
Conclusions
• Existing standards are insufficient to assure consistent, high quality medical image output.
• Steps are proposed to further the reach and impact of DICOM toward quality medicine.
• New directions will provide benefits for PACS users, administrators, vendors, and patients.
22-Sept-2003 27
Contact Information• Ehsan Samei, Duke University
• Alan Rowberg, M.D., University of Washington, [email protected]
• Ellie Avraham, Eastman Kodak Company,[email protected]
• Craig Cornelius, Eastman Kodak Company,[email protected]
• AAPM Task Group 18 web site: http://deckard.duhs.duke.edu/tg18
