D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
Educational Objectives
ACR MRI Accreditation: Medical
Physicist Role in the Application
Process
• Present an overview of the new modular ACR MRI
Accreditation program as it applies to phantom image
acquisition for accreditation application.
Donna M. Reeve, MS, DABR, DABMP
• Discuss the role of the medical physicist in the application
process.
Department of Imaging Physics
University of Texas M.D. Anderson Cancer Center
• Discuss phantom image quality failures, possible causes
and potential remedies.
• Provide advice for avoiding common pitfalls in the
phantom image submission process.
ACR MRI Accreditation Program
New Modular Program
Modular program launched October 20, 2008
6 modules (head, spine, body, MSK, MRA, cardiac)
Allows accreditation of whole body, dedicated
and certain specialty MRI systems
Added a second, small phantom for use in small
bore orthopedic MRI systems
•
•
•
•
Previous programs
„ Whole body
„ Cardiac
„ Head
„ Spine
„ Musculoskeletal
(MSK)
„ Body
„ Magnetic
Resonance
Angiography (MRA)
„ Cardiac
*Breast MR: to be included
with Breast Imaging
Accreditation programs
(Mammography, Stereotactic
Breast Biopsy, Breast Ultrasound)
ACR MRI Accreditation Program
ACR MRI Accreditation Program
• Unit based facility accreditation program: “For every
unit must apply for all modules routinely performed on
that unit for a facility to be accredited.”
accredited.”
• Additional guidance documents
• Allowances made for loaner units and “emergency use
of magnets”
magnets”
• Online application process
•
•
• Guidance documents available www.acr.org
• Longer time period to acquire phantom and
<10 exams in 30 day period, or
<50 exams in any 12 month period
clinical images (+/(+/- 1 month from phantom
exam)
• MRI scanners dedicated to Interventional MRI and
Radiation Therapy treatment planning are exempt.
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D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
Guidance documents
Clinical images – guidance
documents
www.acr.org
Describes criteria ACR
uses to evaluate clinical
images submitted for
each of the six modules:
Head
Spine
MSK
Body
MRA
Cardiac
ACR Technical Standards
Medical Physicist/MR Scientist
2004 ACR MRI Quality Control
Manual
Medical Physicist:
Board Certification in radiological physics or diagnostic radiological
radiological
physics (recommended)
Ref: ACR Accreditation Program Requirements, 2009
Radiologist’s section
Technologist’s section
Board Certification in appropriate subfield(s)
subfield(s) by the American Board of
Radiology (ABR), the Canadian College of Physics in Medicine, or for
MRI, by the American Board of Medical Physics (ABMP) in magnetic
resonance imaging physics.
Medical Physicist’s section
In the process of being revised.
Ref: ACR Technical Standard for Diagnostic Medical Physics
Performance Monitoring of MRI Equipment, 2006
Medical Physicist/MR Scientist
Medical Physicist / MR Scientist role
MR Scientist:
• Member of team: supervising physician, MRI
A qualified MR Scientist is an individual who has obtained a
graduate degree in a physical science involving nuclear MR
or MRI. Should have 3 years documented experience in a
clinical MRI environment.
technologist; involved in the accreditation process.
• Acquires or assists in acquisition of phantom images
• Performs phantom image evaluation to assess image
quality and determine if images will pass ACR.
• Evaluates spatial and temporal resolution of
submitted clinical images
Refs: ACR Accreditation Program Requirements, 2009;
ACR Technical Standard for Diagnostic Medical Physics
Performance Monitoring of MRI Equipment, 2006
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D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
ACR MR Accreditation Phantoms
Medical Physicist / MR Scientist role
Manufacturer: J.M Specialty Parts
San Diego, CA
• Scanner purchase specifications and MRI system
siting
• Acceptance
testing and scanner testing after
installation and after major upgrade or repair*
*
• Annual medical physicist performance evaluation*
evaluation*
• Establishment and oversight of equipment QC &
performance monitoring*
monitoring*
• Protocol development and optimization
$1050 “large phantom”
$780 “small phantom”
*topics covered by next two speakers
Guidance for Phantom tests
Guidance for Phantom tests
ACR (large) Phantom Analysis
#1
#5
#9
#7
#8
#10
#11
ACR (small)
Phantom
Analysis
Sag localizer) Geometric
accuracy
#1) Slice thickness and
position, geometric
accuracy, high contrast
resolution
#3) Geometric accuracy
#5) PIU, ghosting
#6-7) LCD
#1) Slice thickness and position, geometric accuracy, high contrast resolution
#5) Geometric accuracy
#7) Percent image uniformity, ghosting
#8-11) Low contrast object detectability, and slice position (in #11)
Slide courtesy of E.F. Jackson, PhD
1 sag 20 mm slice
7 axial 5mm slices
w/ 3mm gap
FOV 12 cm
192 x 152 matrix
From “Phantom Test Guidance for Small MRI Phantom for the ACR MRI Accreditation Program”
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D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
ACR Accreditation: Preparation
Common pitfalls – phantom images
• Service engineer check system prior to acquiring
Phantom image quality problems
phantom & clinical images.
• Physicist acquire and evaluate phantom images in
• Causes
advance of starting accreditation process.
• Identify and correct scanner and coil problems.
• ReRe-acquire phantom images and evaluate to ensure
images meet ACR limits.
• Potential remedies available to the
Medical Physicist
Phantom position
„
Large phantom - Slice Position
Scan 33-plane localizer prior to acquiring sagittal
series to optimize phantom position
• Axial rotation
• Sagittal tilt
• Coronal alignment
„
Poor phantom positioning (off isocenter)
isocenter)
Criteria:
<5mm
ƒ Geometric accuracy (gradient nonnon-linearity, B0
inhomogeneity)
• Low contrast detectability (LCD), high contrast resolution
• Phantom not centered in coil:
• PIU (non(non-uniform RF)
Slice 1
Small phantom - Slice Position
Slice 11
Phantom position
Criteria:
<5mm
Isocenter
Slice 1
4
Off isocenter 100 mm inferior
D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
Phantom position
Phantom position:
small bore orthopedic systems
„
„
Isocenter
Important to position phantom at center of bore
OffOff-isocenter:
isocenter: potential for distortion due to gradient nonnonlinearity and field inhomogeneity
Off isocenter 100 mm superior
Common pitfalls – clinical images
Low contrast detectability
• Poor image quality on Site T1 and Site T2 series due to
Factors that impact LCD score:
incorrect slice positions.
• Field strength
Solution:
• Slice position accuracy
• Scan 33-plane localizer to optimize phantom position
• Phantom position off isocenter
• Slices must intersect correct areas of the phantom.
• Large phantom: 5mm thick / 5mm gap Æ image spacing 10mm
• Small phantom: 5mm thick / 3mm gap Æ image spacing 8mm
• Coil, coil connection, whether coil is working
properly (passes QC test)
Reference: “Site Scanning Instructions for use of the MR Phantom for the MRI Accreditation
Program”, www.acr.org
Large phantom –
Low Contrast Detectability (LCD)
Slice 8:
1.4%
Slice 9:
2.5%
Slice 10:
3.6%
Slice 11:
5.1%
Small phantom –
Low Contrast Detectability (LCD)
Slice 6:
3.6%
Slice 7:
5.1%
Criteria:
Criteria:
at least 9 spokes (total)
≤1.5T - at least 9 spokes (total)
3T - at least 37 spokes (total)
Slice 7
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D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
Low field systems
SNR vs. Field strength
LCD score sensitive
to slice position
Lower SNR
„
„
Challenge: to pass low contrast detection test
Clinical protocols modified to increase SNR
ƒ Increase number of averages (impacts scan time)
ƒ Narrow bandwidth (impacts chemical shift, geometric
distortion)
ƒ Use of higher SNR pulse sequences
„
Phantom alignment and slice position in LCD
sections is important
1.5T
0.2T
Slice 11 – Low Contrast Detection section of large
ACR MRI phantom (ACR T1 series)
SNR vs. Field strength
Geometric accuracy
Factors that impact geometric accuracy:
• Phantom position off isocenter
• Gradient nonnon-linearity, B0 field inhomogeneity
• Gradient nonnon-linearity correction
• Gradient correction application is a user choice on some,
but not all scanners
• Receive bandwidth
3.0T
1.5T
Slice 8 – Low Contrast Detection section of large
ACR MRI phantom (8 channel array, ACR T1 series, SCIC)
Large phantom - Geometric Accuracy
Small phantom - Geometric Accuracy
Sag Loc
Slice 1
Slice 1
Slice 5
Criteria: 190 ± 2 mm
Slice 3
Criteria: 100 ± 2 mm
Criteria: 148 ± 2 mm
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Sag Loc
D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
Gradient nonlinearity correction
Correction ON
Correction OFF
190 mm
196 mm
Receive bandwidth: geometric accuracy
+/+/- 4 kHz = 31.25 Hz/pixel
+/+/- 16 kHz = 122.1 Hz/pixel
frequency
Isocenter
Isocenter
ACR Sag T1 – large ACR MRI phantom
ACR T1 series, Slice 5 – large ACR MRI phantom
Receive bandwidth: geometric accuracy
+/+/- 4 kHz = 31.25 Hz/pixel
Receive bandwidth: chemical shift
+/+/- 4 kHz
+/+/- 16 kHz = 122.1 Hz/pixel
+/+/- 16 kHz
frequency
ACR Axial T1 – large ACR MRI phantom
ACR Axial T1 – large ACR MRI phantom
Uniformity
Both phantoms Percent Image Uniformity
Factors that impact PIU score:
• Dielectric effect at 3T
• Use of 8 channel phased array coils
• Phantom position inside coil
• Gradient nonnon-linearity, B0 field inhomogeneity;
geometric distortion correction
PIU=[1-(max-min)/(max+min)] • 100 1.5T: Criteria: PIU≥ 87.5%
3.0T: Criteria: PIU≥ 82 %
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D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
3 Tesla systems – Dielectric effect
Phased array coils
• Small coil elements improve SNR
• Multiple elements for anatomical coverage
• Independent receivers – enables parallel imaging
Decreased uniformity:
Reduced
RF penetration with
increasing frequency
Composite image
Apply Surface Coil
Intensity
Correction
1.5T
3T
MRI Devices 8 channel HR Brain array:
Images acquired from individual channels
Impacts Uniformity measurement
8 channel head coils
Uniformity vs. Coil
“If your facility uses an eight channel head
coil, it is necessary to perform all phantom
scans using the surface coil intensity
correction option.”
option.”
1.5T Quad head coil
1.5T 8 channel array
1.5T 8 channel array
SCIC applied
ACR T1 series, Slice 7 – Uniform section of large
ACR MRI phantom
Reference: “ACR MRI Accreditation Program Testing Instructions”, rev 1-13-2009,
www.acr.org
Large phantom –
High Contrast Spatial Resolution
High contrast resolution
Factors that impact resolution score:
Must be able to resolve 1.0 mm
holes vertically and horizontally.
• Excessive phantom rotation
• Excessive ghosting due to motion or vibration
• Ensure that coil is stable
• Immobilize phantom in coil
25 cm FOV, 256 x 256 matrix
Pixels: 0.98 mm x 0.98 mm
• Excessive low pass filtration
• Filter application and degree of filtration is a user choice
on some, but not all scanners
1.1mm
8
1.0mm
0.9 mm
D.M.Reeve: AAPM 2009: Practical Medical
Physics - ACR MRI Accreditation Program
Small phantom –
High Contrast Spatial Resolution
0.9 mm
0.8 mm
0.7 mm
Low pass filter
FILTER ON
FILTER OFF
Must be able to resolve 0.8 mm
holes vertically and
horizontally.
12 cm FOV, 192 x 152 matrix
Pixels: 0.63 mm x 0.79 mm
Reduces truncation artifacts
May impact high contrast
resolution score
Prominent truncation artifacts
ACR T1 series, Slice 5 – large ACR MRI phantom
Summary
Common pitfalls – phantom images
• Presented a brief overview of the new modular ACR MRI
•
•
•
•
ACR Sagittal T1 image “localizer”
localizer” not included on CD
CD not readable by the reviewer
Not uncompressed DICOM format
Embedded viewer – ACR reviewer unable to evaluate
DICOM images
Accreditation program.
• Medical physicist involvement in the application process is
important as part of the accreditation team.
• Discussed phantom image quality problems, possible causes
Solution:
and potential remedies, with focus on solutions available to
the physicist.
• Review images on PC(s)
PC(s) separate from PACS and MRI
systems using (Osiris
(Osiris and/or KK-PACS) before submitting
to ACR.
• Verify correct series and scan parameters - view DICOM
header information.
• Provided advice for avoiding common pitfalls in the phantom
image submission process.
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