Physics testing of image detectors
Parameters to test
G
Features and Weaknesses of
Phantoms for CR/DR System Testing
•
Spatial resolution
•
Contrast resolution
•
Uniformity/geometric distortion
•
Dose response/signal dynamic range
•
Noise
Added filtration
Experiments/testing methods
G
Donald Peck, PhD
Henry Ford Hospital
•
Direct measurements
•
Phantoms/image tools
• Qualitative
• Quantitative
detector
Phantom/image Tool
2
Phantom Types
G
Attenuation test tools
Attenuation
G
Metals
• Aluminum, Copper, etc.
G
High Contrast/Spatial Resolution
G
Plastics/Composite Materials
• Lucite, Tissue Equivalent, etc.
G
Low Contrast/Contrast Sensitivity
G
Anthropomorphic
G
Water
3
Detectors - CR Scatter Sensitivity
Digital detectors can be significantly more sensitive to scatter
radiation than traditional phosphor screens
•
G
Modifying beam quality
•
Scatter needs to considered when testing systems
Se
Detector
1
Absorption Efficiency
Attenuator positioning
1
0.8
Storage
Phosphor
0.6
0.6
0.4
0.4
Rare
Earth
Phosphor
0.2
0.2
Pelvic Phantom
Scatter
0
0
20
40
60
Energy (keV)
G
Position attenuators far from
detector to minimize scatter
contribution in measurement
Simulating patient attenuation
•
Pelvic Phantom
Primary
0.8
Relative Intensity
G
4
Position close to detector in
same location as patient
0
80
100
120
Correct Positioning
Incorrect
Positioning
5
6
•1
Attenuator Construction
G
Attenuation test tools
Attenuator “purity” may not
be acceptable for the
measurement
•
G
Measurement of
mammography HVL requires
attenuators that are at
least 99.9% Aluminum
G
G
G
Easy to use
Placement of attenuator needs to be considered based
on the test
Purity or Uniformity of material may not be adequate for
some tests
Tissue equivalent materials
may not be uniform
7
High Contrast/Spatial Resolution Test tools
G
Line pair patterns
G
Mesh patterns
G
Edge phantoms
8
Line pair patterns
9
Aliasing and Moiré Effect
10
Line pair patterns
Grid
Detector Pitch
11
12
•2
Moiré Effect
MTF/DQE Measurement
IEC 62220-01 (2003)
G
•
Method for determining Detective
Quantum Efficiency (DQE) of
digital imaging systems
•
Defines specifications for a test
device required to make these
measurements
13
G
G
MTF/DQE Measurement Issues
Requires Pre-processed image values
that are “linear” with exposure
G
•
Only issue at high kVp
•
Important if comparing to other
MTF/DQE measurements
Determination of edge response
•
Need to bin pixel data along edge
•
Phantom positioning critical for
consistent results
G
Line Spread Function
•
Variations in method used may produce
different results
•
Important to standardize if comparing
to other MTF/DQE measurements
Distance
Edge Response Function
Fluorescent
Radiation
Noise Power Spectrum (NPS)
determination
•
Need to remove effects of trends
associate with heel effect, etc.
•
Variations in method used may
produce different results
•
Important to standardize if
comparing to other MTF/DQE
measurements
LSF
Edge Spread Function
Smoothing/fitting of edge response
curves to allow utilization of Fourier
Analysis
ESF
G
Fluorescent radiation
12000
10000
Image Value
MTF/DQE Measurement Issues
14
8000
6000
4000
2000
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Image Position
Distance
15
MTF Measurements
High Contrast/Spatial Resolution Test tools
G
Quantitative results
G
Good indication of changes
G
G
CsI indirect detection
1
• MTF Determination
0.9
RQA5, Ver XQ/i
RQA5, Hor XQ/i
• Requires development of software to do the calculations
RQA5, Ver DR-1000
0.8
RQA5, Hor DR-1000
0.7
0.6
0.6
MTF
0.7
0.5
0.4
0.3
0.3
0.2
0.2
0.1
• Task Group No. 162 “Research Software for 2D Image”
• Valid for determining if changes have occurred over time if
performed “consistently”
0.5
0.4
• Requires standardization of methods used if comparisons
between systems or results from different physicists are
compared
0.1
0
0
2
4
6
8
Spatial frequency (cycles/mm)
10
Edge Phantoms
• Objective
Selenium direct detection
1
0.9
0.8
Line pair patterns
• Subjective
Subtleties in the measurement can make comparisons between
measurements by different tests inaccurate
MTF
G
16
0
0
2
4
6
8
10
Spatial frequency (cycles/mm)
17
18
•3
Low Contrast/Contrast Sensitivity test tools
Contrast threshold detection index (TCDD)
G
Contains objects of varying
size and attenuation
G
TCDD gives an indication of the
lowest contrast detectable (CT) as
a function of the detail size (the
square root of the detail Area, A)
and can be quoted in terms of the
threshold detection index (HT)
Requires observers to
determine which objects are
visible
G
•
HT(A) = 1/[CT * A½]
Subjective
G
High value for HT(A) indicates
good visibility
19
IPEM Criteria (example)
Institute of Physics and Engineering in Medicine (IPEM)
G
G
20
Goals:
•
Improving standards in clinical practice
•
Providing advice on scientific and
engineering issues in healthcare to other
healthcare professionals, government
and the public.
G
Most results are subjective !
Develops Reports and other publications
to achieve these goals
•
Owns several journals:
• Physics in Medicine and Biology
• Physiological Measurement
• Medical Engineering and Physics
•
Report 91 Recommended Standards for
the Routine Performance Testing of
Diagnostic X-Ray Imaging Systems
• Specifies the use of phantoms
throughout the testing procedures
21
Original Equipment Manufacturer (OEM) Products
ACR Radiography/Fluoroscopy Accreditation Phantom
G
G
Modules included
•
Chest
•
General
•
Fluoroscopy
G
G
Automated Image Quality Control Tool
• Reproducible quantitative results
Phantom image
•
22
• May detect sub-visible changes in image quality
performance to initiate timely preventive maintenance
Radiography Chest/Abdomen
ACR Discontinued
Radiography/Fluoroscopy
Accreditation Program in 2005
• Highly automated procedure
• Most provide data reporting in spreadsheet format
23
•4
Test Phantom for Kodak (i.e. CareStream)
DIRECTVIEW Total Quality Tool
KODAK User Interface
G
Uniformity
G
Noise
Spatial frequency
response (MTF)
G
Exposure linearity
G
Pixel size accuracy and
aspect ratio
G
Phantom image
artifacts
G
G
Laser Beam Function
G
Residual signal erase
24 x 30 cm
35 x 43 cm
18 x 24 cm
26
*Images provided by Eastman Kodak Company
Kodak Temporal Test Results
Kodak Test Result Details
MTF (slow scan)
29.5%
27
Test Limits
G
Pre-set by OEM
G
Basis for limit may not be justified in OEM literature
G
28
DIN 6868-58 (2001) and 6868-13 (2002)
G
Acceptance testing and constancy
checks of projection radiography
systems with digital image receptors
•
German standard for testing of
Storage Phosphor systems using a
specially designed phantom to
measure image quality parameters
•
Can purchase a phantom that will
meet the requirements of this
standard from several vendors
If system fails a test, Service Engineer may not be
educated how to correct problem
29
30
•5
Anthropomorphic phantoms
G
Shape “mimicking”
G
Anatomically Accurate
Shape “mimicking”
31
Shape “mimicking”
32
Anatomically Accurate
33
34
European Protocol for QC of … mammography
Addendum on Digital Mammography
Image Processing
G
G
G
35
It is acknowledged that at present it
is not possible to get unprocessed
images from some systems.
unprocessed
The image processing may introduce
artifacts on phantom images and may
be different from image processing
for mammograms due to histogram or
local texture based processing
techniques.
Therefore care needs to be taken in
interpretation of these processed
images.
processed
36
•6