Mr Accreditation
66
Quality Control and the ACR’s MRI Accreditation Program Geoffrey D. Clarke, Ph.D. University of Texas Health Science Center at San Antonio Radiological Sciences Division
Transcript of Mr Accreditation
Quality Control and the ACR’s MRI Accreditation Program
Geoffrey D. Clarke, Ph.D.University of Texas Health Science Center
at San Antonio
Radiological Sciences Division
Overview
• ACR MRI Accreditation Program
• MRI Quality Control Program
– Definitions
– Technologist’s Daily & Weekly QC Tasks
– Physicist’s Responsibilities
– Annual Tests for the Physicist
ACR MRI Accreditation ProgramFeatures
• Evaluates effectiveness of quality control measures
• Will collect findings to further the development of quality control information
• Qualified Medical Physicist should be responsible for overseeing the equipment quality control program
ACR MRI Accreditation ProgramMRI Survey Agreement
• Official request for ACR Accreditation
• Site agrees to provide all documentation, including but not limited to quality control logs, films, records, or any necessary information requested by the survey team
• Agree to use the ACR MRI phantom
• Tests run in a short time
• Pulse sequences as compatible as possible with all commercial MRI scanners
• Specific measurements to account for :– Geometric Distortion
– Slice thickness & Position
– Factors Affecting Image SNR (resolution, bandwidth, ghosting)
– Image Uniformity
ACR PhantomACR Phantom• Affordable
($730)• Results
easily evaluated
• Affordable ($730)
• Results easily evaluated
Documents for Using Phantom
ACR MRI Accreditation ProgramQuality Control Section
• Refers to tests put forth in the ACR Standard for MRI
• All tests to be carried out in accordance with written procedures and methods
• Preventative maintenance:
– Documented by qualified service engineer
– Repairs documented and records maintained by the MR site
ACRMagnetic
ResonanceImaging QualityControl Manual2001
Radiologist’s Responsibilities Eleven specific responsibilities including: • To ensure that an effective quality control program
exists for all MRI• To select the technologist to be the primary quality
control technologist• To ensure that appropriate test equipment and
materials are available to perform the technologist's QC tests.
• To arrange staffing and scheduling so that quality control tests can be carried out.
• To select a qualified medical physicist or MRI scientist To ensure that records are properly maintained and updated in the MRI QC procedures manual.
MRI QC Technologist’s Responsibilities
• Weekly magnetic resonance image quality control procedures (daily recommended)
• Weekly quality control of hard copy and soft copy Images
• Routine visual inspection of equipment
Weekly Technologist’s Tests• Center frequency• Table positioning• Setup & Scanning
• Geometric accuracy• High contrast resolution• Low contrast detectability• Artifact analysis• Film quality control• Visual Checklist
Technologist QC
• Daily (weekly) tests
• Cheat sheets
• Review data after ten days to establish baseline values and variability
• Daily (weekly) tests
• Cheat sheets
• Review data after ten days to establish baseline values and variability
Measurement of Central Frequency
• Can be performed by technologist
• Record center frequency value on ACR phantom or manufacturer’s phantom
• Automated analysis and recording often available on modern MRI systems
Central FrequencyCentral Frequency
•Specific but not sensitive
•Action Criteria:Change in Hz from previous day > 2 * resonant frequency in MHz
• Suggestive of:Magnet driftRF instability
•Specific but not sensitive
•Action Criteria:Change in Hz from previous day > 2 * resonant frequency in MHz
• Suggestive of:Magnet driftRF instability
Transmitter Gain/Attenuation
• Reflects power required to optimize RF pulse:
– Depends on coil, phantom, pulse sequence, etc.
– Should remain constant over time if nothing in pulse sequence or hardware has changed
Terminology & Units Vary with Vendor• GE: displayed on screen (dB)• Philips: under system performance
parameters– rf_act_drivescale
• Siemens: “options” – “adjustments”– Frequency– Transmitter amplitude (temp) (V)
• Toshiba: “acquisition window”– Center frequency (MHz)– RF level
Geometric Accuracy
• Measure distance along main axes of phantom
• Compare with known values
Potential Causes of Geometric Accuracy Failures
1. Phantom mispositioning
2. Gradient miscalibration
3. Bo inhomogeneity
a. Ferromagnetic objects in magnet
b. Poor magnet shimming
4. Gradient non-linearity
5. Inappropriate receiver bandwidth
6. Poor eddy current compensation
7. Combination of two or more of above
Geometric Accuracy - Axial
Failure Due To: • Miscalibrated Gradients • Low Receiver Bandwidth
• High Bo Inhomogeneities
BW = 7.4 kHz BW = 3.6 kHz
Slice #1Slice #1
Gradient Correction
• If gradients are inherently non-linear gradient correction may be applied
Percent Geometric Distortion T1-Weighted, Central Slice
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
1-Oct
31-Oct
30-Nov
30-Dec
29-Jan
28-Feb
29-Mar
28-Apr
28-May
27-Jun
27-Jul
26-Aug
25-Sep
25-Oct
Date of Measurement
System #1 System #2 System #3
Poor Gradient Calibration
Poor Gradient Calibration
Assessment of MR Image Quality
• Performed by Technologist
• Using ACR Phantom
• High Contrast Resolution
• Low Contrast Detectability
RF Noise/Leaks/ Spikes
Single frequency
artifact shows up as
zipper in middle of image.
0.90 mm0.90 mm1.0 mm1.0 mm1.1 mm1.1 mm
ChemShiftChemShift
Slice ThicknessSlice Thickness
Resolution InsertResolution InsertResolution InsertResolution Insert
ACR MRI PhantomACR MRI PhantomResolution InsertResolution Insert
ZOOM = 2ZOOM = 2
Spatial Resolution Matrix: Spatial Resolution Matrix: Registration with PhantomRegistration with PhantomSpatial Resolution Matrix: Spatial Resolution Matrix: Registration with PhantomRegistration with Phantom
ImageMatrixImageMatrix
ResolutionResolutionHolesHolesResolutionResolutionHolesHoles
PARTIAL VOLUME ARTIFACT
High Contrast Spatial Resolution
• Evaluate conspicuity of holes arranged in two square arrays
• Has to avoid partial-volume artifact…
UL
LR
High Contrast ResolutionHigh Contrast Resolution
•Specific but not sensitive
•Action Criteria:Any reduction in # of holes seen
• Suggestive of:Increased eddy currentsPoor gradient calibration
Poor Bo uniformity
Reduced stability of system
•Specific but not sensitive
•Action Criteria:Any reduction in # of holes seen
• Suggestive of:Increased eddy currentsPoor gradient calibration
Poor Bo uniformity
Reduced stability of system
Slice #10Slice #10 Slice #12Slice #12
Low Contrast DetectabilityLow Contrast Detectability
5.1% Contrast Detectability5.1% Contrast Detectability3.7% Contrast Detectability3.7% Contrast Detectability
Low Contrast Detectability1.5 T System: T1-Weighted Scan
0.00
5.00
10.00
15.00
20.00
25.00
1-May 31-May 30-Jun 30-Jul 29-Aug 28-Sep 28-Oct 27-Nov 27-Dec 26-Jan 25-Feb 27-Mar 26-Apr
Date
Ho
le S
ize
Vis
ual
ized
(mm
)
1.4% contrast
2.4% contrast
3.7% contrast
5.1% contrast
LCD and Signal-to-NoiseT
ota
l Nu
mb
er o
f S
po
kes
To
tal N
um
ber
of
Sp
oke
sT
ota
l Nu
mb
er o
f S
po
kes
To
tal N
um
ber
of
Sp
oke
s
Signal-to-Noise RatioSignal-to-Noise RatioSignal-to-Noise RatioSignal-to-Noise Ratio0 50 100 150 200 250 300 3500 50 100 150 200 250 300 3500 50 100 150 200 250 300 3500 50 100 150 200 250 300 350
4040
3535
3030
2525
2020
1515
1010
55
00
4040
3535
3030
2525
2020
1515
1010
55
00
Low Contrast DetectabilityLow Contrast Detectability
•Sensitive but not specific•Action Criteria:
•Slice used dependent on Bo field strength•Sustained 5 row decrease in number of hole sets seen
• Suggestive of:•Reduced stability of system
•Sensitive but not specific•Action Criteria:
•Slice used dependent on Bo field strength•Sustained 5 row decrease in number of hole sets seen
• Suggestive of:•Reduced stability of system
ACR MRI StandardHardcopy Image QC Tests
• Sensitometric Measurements for Film Processors
• Hard Copy of SMPTE test pattern
• Similar to process used for mammography program
• Laser camera film less sensitive to temperature changes
Laser Film QCWeekly:Weekly:
View SMPTE patternView SMPTE pattern
Verify gray levelsVerify gray levels
• 0/5% & 95/100% 0/5% & 95/100% patchespatches
Film 6 on 1Film 6 on 1
• 4 on 1 if necessary4 on 1 if necessary
Plot OD of Plot OD of
• 10%, 40% & 90& 10%, 40% & 90& patchespatches
Observe film for Observe film for artifactsartifacts
Weekly:Weekly:
View SMPTE patternView SMPTE pattern
Verify gray levelsVerify gray levels
• 0/5% & 95/100% 0/5% & 95/100% patchespatches
Film 6 on 1Film 6 on 1
• 4 on 1 if necessary4 on 1 if necessary
Plot OD of Plot OD of
• 10%, 40% & 90& 10%, 40% & 90& patchespatches
Observe film for Observe film for artifactsartifacts
SMPTE Test PatternSMPTE Test Pattern
Action Limits
SMPTE patch
OD Control Limit
0 2.45 0.15
10% 2.10 0.15
40% 1.15 0.15
90% 0.30 0.08
Documenting the Quality Control Program
• Data form for daily equipment quality control – ACR MRI QC manual, pg. 64
• MRI Facility quality control visual checklist– ACR MRI QC manual, pg. 65
• Laser film printer control chart– ACR MRI QC manual, pg. 66
If QC Test Fails
• Common errors-– Check for magnetic objects in bore
– Re-seat head coil
– Reposition & landmark phantom
– Make sure scan room door securely closed
• Repeat daily QC scan procedures
• Record results again in QC notebook
Responsibilities of the Qualified Medical Physicist
or MRI Scientist • Write Purchase Specifications
• Perform Acceptance Testing – Baseline Measurements
• Determine Action Limits
• Set up Daily/Weekly QC Tests
• MRI equipment performance review
Annual Survey Tests• Magnetic Field Homogeneity
• Slice Position Accuracy
• Slice Thickness Accuracy
• Radio Frequency Coil Checks
– Volume coils
1. Signal-to-noise ratio
2. Percent integral uniformity
3. Percent signal ghosting
– Surface Coils SNR Tests
• Inter-Slice RF Interference
• Soft Copy Displays (monitors)
Magnetic Field HomogeneityMagnetic Field Homogeneity
oo oo oo
Fourier transform of signal produces a Lorentzian peak in
well-shimmed magnet
Fourier transform of signal produces a Lorentzian peak in
well-shimmed magnet
Magnet firld homogenity can be characterized using FWHM of resonance
peak
Magnet firld homogenity can be characterized using FWHM of resonance
peak
Denotes a totally uniform magnetic
field.All signal is at
resonant
frequency, o.
Denotes a totally uniform magnetic
field.All signal is at
resonant
frequency, o.
Ideal HomogeneityIdeal Homogeneity Good HomogeneityGood Homogeneity Poor HomogeneityPoor Homogeneity
FWHMFWHM
FWHMFWHM
Magnetic Field Homogeneity
Phase images from GRE sequences with 13.9 ms difference in TE’s
Phase and Unwrapped
Phase Images
John Pauley, Stanford Univ.John Pauley, Stanford Univ.
Magnetic Field Homogeneity
• Overall, the phase mapping technique provides the best mechanism for evaluating field homogeneity.
• For some system service personnel may have to provide use of phase-mapping acquisition and analysis tools.
• Filmed copy of vendor’s final homogeneity map and shim coefficients is useful for documentation and establishing a baseline.
Slice Position Accuracy
• Uses Crossed-Wedges as Reference for Positioning and Slice Spacing Accuracy
• MRAP pass criterion: magnitude of bar length difference 5 mm.
• The actual displacement is ½ of the measured difference
• ACR Accreditation criteria are very weak, physicist may want to hold manufacturer to a higher standard
Slice Position Accuracy
SLICE #1SLICE #1SLICE #1SLICE #1 SLICE #11SLICE #11SLICE #11SLICE #11
Slice PositionSlice Position Slice SpacingSlice Spacing
Crossed wedges should be of equal lengthCrossed wedges should be of equal length if position and spacing are accurateif position and spacing are accurate
Crossed wedges should be of equal lengthCrossed wedges should be of equal length if position and spacing are accurateif position and spacing are accurate
Slice Position Accuracy
• Causes of poor performance:
• Operator error
• Table positioning shift
• Miscalibrated gradients
• High Bo inhomogeneities
MRI Slice Thickness
• Signal ramps have a slope of 10:1
• Signal from ramp is 10 x slice thickness
• Two ramps are used to compensate for in-plane rotation of the phantom
• Phantom does not compensate for tilting backwards or swaying left-right
Slice Thickness Measurement
1. Use slice 1 of ACR T1.
2. Magnify by 2 to 4. Adjust window/level to see signal ramps. (Set window at minimum.)
3. Use rectangular ROI to measure mean of middle of each signal ramp. Take average.
Slice Thickness– Lower display to
one/half the average ramp signal.
– Measure lengths of top and bottom ramps and calculate slice thickness.
bottom top
bottom top 0.2 Thickness Slice
Slice Thickness
• ACR-MRAP: slice thickness measured should be + 0.7 mm of prescribed value
– + 14% error on 5mm slice, may be too generous
• Corrective actions:
– Check Axial Site Series Images
– Replace cables & connectors, look for other sources of distorted RF pulse shape in RF electronics
– Try switching RF coils
– Check gradient calibration
MRI Equipment Performance Evaluation Site: _____________________________ Date: ________ MRAP Number: ____________________ Serial Number: ___________ Equipment: MRI System Manuafacturer: _________________ Model : ________
Processor Manufacturer: _________________ Model: _________
PACS Manufacturer: _________________ Model: _________
ACR MRAP Phantom Number used: _________
1. Magnetic Field Homogeneity Method Used (check one): Spectral Peak ___ Phase Difference ___ Other (describe) __________________________ Measured Homogeneity: Diameter of Spherical Homogeneity Volume (cm) (ppm) ________ _______ ________ _______ ________ _______
2. Slice Position Accuracy From Slice Positionss #1 and #11 of the ACR Phantom: Wedge (mm)
Slice Location #1 ________
Slice Location #11 ________
3. Slice Thickness Accuracy From Slice Position #1 of the ACR Phantom: Slice Thickness Top ______ Calculated slice
(fwhm in mm) Thickness (mm) ______
Bottom ______
Duplicate these forms so they will be available for repeated use.
=+ = -
=+ =
-
Site & Equipment
Data
Bo
Homogeneity
Slice PositionAccuracy
Slice ThicknessAccuracy
Bird-Cage Type Head Coil
RF coils produce uniformity patterns characteristic of their design.RF coils produce uniformity patterns characteristic of their design.
Uniformity Pattern
Birdcage CoilHigh Field
Birdcage CoilHigh Field
Solenoid CoilLow Field
Solenoid CoilLow Field
ACR PhantomSlice #7
Volume RF Coil MeasurementsVolume RF Coil Measurements
Image Intensity Uniformity
• Performance criteria: PIU 90%
• Measurement Considerations:• Display may not show signal values• Display may not allow user to set signal
display level• There may not be a well-defined
high/low intensity level
percent integral uniformity = 100
1
( )
( )
high low
high low
Image Intensity Uniformity
• Causes of failure:
• Poor phantom centering in head coil (usually AP)
• Ghosting
• Motion or vibration
• Mechanical failure in head coil
Phased- Array Coils•The signal is viewed from more “angles”•Using four channels does not produce 4 * S/N
Abdomen-pelvisphased-array
Cervical-Cranial Phased Array
TorsoPhased Array
General Features of MRI Phantoms
– Nonsignal-producing container
– Proton density similar to water
– Shorten T1: NiCl & CuSO4
– Mimic Conductivity of tissues: NaCl
Agarose Agarose Gel Gel
Phantom Phantom with Insertswith Inserts
Uniform Uniform Spherical Spherical PhantomsPhantoms
Phantoms Developed by Users
Surface RF Coil Measurements
Volume Coil Data% Image Uniformity Max Signal
Min Signal
Signal-to-Noise Mean Signal
SD of Background Signal
Percent Signal Ghosting
Ghost Signal
Mean Signal
Background Signal
Surface Coil DataMaximum Signal-to-Noise
Maximum signal
SD of Background Signal
-
Volume CoilData Recorded
Calculated Values:Uniformity
SNRGhosting
Surface Coil
Signal-to-Noise
4. RF Coil Performance Evaluation
A. VOLUME RF COIL - RF Coil Description: __________________________ Date: ____________
Phantom Description: ___________________________________________
Pulse Sequence: Type: ____ TR: _____ TE: ______ flip angle _____ degrees
FOV: _____ cm2 Matrix: ___________ BW: _________kHz ; NSA ___
Slice thickness ______mm; spacing _______ mm
TX attenuation (or gain) __________ Data Collected:
Mean Signal
Maximum Signal
Minimum Signal
Background Signal
Noise Standard Deviation
Ghost Signal
Calculated Values:
Signal-to-Noise Ratio
Percent Image Uniformity
Percent Signal Ghosting
B. RF SURFACE COIL -
RF Coil Description: __________________________ Date: ____________
Phantom Description: ___________________________________________
Pulse Sequence: Type: ____ TR: _____ TE: ______ FOV: _____ cm2
Matrix: ___________ BW: _________kHz ; NSA ___
Slice thickness ______mm; spacing _______ mm
TX attenuation (or gain) __________
Maximum Signal
Noise Standard Deviation
Maximum Signal-to-Noise Ratio
Image uniformity distribution OK? ________
Image ghosting OK? ________
HARD COPY IMAGE: Window width ________ Window level _______
Several copies of this page may be required to report on all RF coils.
ACR MRI QC Manual, pg. 125ACR MRI QC Manual, pg. 125
Slice Cross-Talk Measurements
1. Position 5mm slices on the uniform volume
2. Repeat measurements decreasing the slice gap :
Series # 1 2 3 4
No. Slices 11 11 11 11
Slice Gap (mm) min 0.5 1.0 5
3. Measure the signal-to-noise ratio (SNR) for each of the image sets.
4. Plot the SNR vs. percentage slice gap
Signal-to-Noise vs. Inter-slice Gap
SE, TE = 20FE, TE =8TFE, TE =4
100%
90%
80%
70%0% 25% 50% 75% 100%
Percentage Slice Gap
Sig
na
l-to
-No
ise
Rat
io (
per
cen
t of M
axim
um
)
Soft Copy Displays
• Requires precision luminance meter
• Four tests– Maximum and minimum
luminance – Luminance uniformity – Resolution – Spatial Accuracy
-
RF Slice Interference
Soft CopyDisplays
Review of Routine QC
Program
5 . I n t e r s l i c e R F I n t e r f e r e n c e
P h a n t o m D e s c r i p t i o n : _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
P u l s e S e q u e n c e : T y p e : _ _ _ _ T R : _ _ _ _ _ T E : _ _ _ _ _ _ F O V : _ _ _ _ _ c m 2
M a t r i x : _ _ _ _ _ _ _ _ _ _ _ B W : _ _ _ _ _ _ _ _ _ k H z ; N S A _ _ _
N u m b e r o f s l i c e s _ _ _ _ _ _
6 . S o f t C o p y D i s p l a y s
M o n i t o r D e s c r i p t i o n : _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
M a x i m u m L u m i n a n c e : _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ C d m - 2 .
M i n i m u m L u m i n a n c e : _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ C d m - 2 .
L u m i n a n c e U n i f o r m i t y :
A v e r a g e o f v a l u e s o b t a i n e d i n f o u r c o r n e r s o f s c r e e n : _ _ _ _ _ _ C d m - 2 .
L u m i n a n c e m e a s u r e d i n c e n t e r o f s c r e e n : _ _ _ _ _ _ C d m - 2 .
P e r c e n t d i f f e r e n c e : _ _ _ _ _ _ _ _ %
| ( C e n t e r – A v e r a g e C o r n e r s ) / ( C e n t e r ) x 1 0 0 % < 3 0 % |
7 . E v a l u a t i o n o f S i t e ’ s T e c h n o l o g i s t Q C P r o g r a m
4 ) S e t u p a n d p o s i t i o n i n g a c c u r a c y : ( d a i l y ) _ _ _ _ _ _ _ _ _
5 ) C e n t e r F r e q u e n c y : ( d a i l y ) _ _ _ _ _ _ _ _ _
6 ) T r a n s m i t t e r A t t e n u a t i o n o r G a i n : ( d a i l y ) _ _ _ _ _ _ _ _ _
7 ) G e o m e t r i c A c c u r a c y M e a s u r e m e n t s : ( d a i l y ) _ _ _ _ _ _ _ _ _
8 ) S p a t i a l R e s o l u t i o n M e a s u r e m e n t s : ( d a i l y ) _ _ _ _ _ _ _ _ _
9 ) L o w C o n t r a s t D e t e c t a b i l i t y : ( d a i l y ) _ _ _ _ _ _ _ _ _
1 0 ) F i l m Q u a l i t y C o n t r o l ( w e e k l y ) _ _ _ _ _ _ _ _ _
V i s u a l C h e c k l i s t : ( w e e k l y ) _ _ _ _ _ _ _ _ _
Series Slice Signal-Number Gap to-Noise
(mm) Ratio
1
2
Measure
d SNR
100%
90%
80%
70%0% 25% 50% 75% 100%
Inter-slice Gap (percent of slice thickness)
3
4
Series Slice Signal-Number Gap to-Noise
(mm) Ratio
1
2
Measure
d SNR
100%
90%
80%
70%0% 25% 50% 75% 100%
Inter-slice Gap (percent of slice thickness)
3
4
ACR MRI QC Manual, pg. 127ACR MRI QC Manual, pg. 127
MRI Equipment Evaluation Summary
Site ___________________ Report Date: __________ System MRAP #_____________ Survey Date: __________
MRI System Manufacturer ___________ Model: __________ Physicist/MRI Scientist: ____________________ Signature: ________________________________
Equipment Evaluation Tests
Pass / Fail 1. Magnetic Field Homogeneity: _________ 2. Slice Position Accuracy _________
3. Slice Thickness Accuracy _________ 4. RF Coils’ Performance
a. Volume Coils’ Signal-to-Noise Ratio _________ b. Volume Coils’ Image Uniformity _________ c. Volume Coils’ Ghosting Ratios _________ d. Surface Coils’ Signal-to-Noise Ratio _________ 5. Inter-slice RF Interference _________ 6. Soft copy displays _________
Medical Physicist’s or MRI Scientist’s Recommendations for Quality Improvement:
______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
SummarySummarySheetSheet(pg. 131)(pg. 131)
Successful MRI QC ProgramSuccessful MRI QC Program
1.1. Technologists run QC scans on a Technologists run QC scans on a daily basisdaily basis
2.2. If exceed action criteria – repeat QC If exceed action criteria – repeat QC procedureprocedure
3.3. Physicist reviews QC data annuallyPhysicist reviews QC data annually
4.4. Record data - report problems to Record data - report problems to service service
5.5. Have service record problems and Have service record problems and solutions in a service logsolutions in a service log
1.1. Technologists run QC scans on a Technologists run QC scans on a daily basisdaily basis
2.2. If exceed action criteria – repeat QC If exceed action criteria – repeat QC procedureprocedure
3.3. Physicist reviews QC data annuallyPhysicist reviews QC data annually
4.4. Record data - report problems to Record data - report problems to service service
5.5. Have service record problems and Have service record problems and solutions in a service logsolutions in a service log
