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Physicists’ Quality Control for MR Equipment
Geoffrey D. Clarke, Ph.D.University of Texas Health Science Center at
San AntonioRadiological Sciences Division
Overview
• MRI Standards• MRI Accreditation Program• MRI Quality Control Program
– Definitions – Physicist’s Responsibilities– Technologist’s Daily & Weekly QC Tasks– Other Tests for the Physicist
Standards & Accreditation
• ACR Standard for Performing and Interpreting MRI (Res. 16-2000)
• ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI Equipment (Res. 19-1999)
• ACR Magnetic Resonance ImagingAccreditation Program
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment 1999 (Res. 19)
Qualifications Qualified Medical Physicist
•certified in diagnostic radiological physics •continuing education in MRI physics
Qualified MR Scientist•graduate degree in physical science•3 years documented clinical MR experience•continuing education in MRI physics
Qualifications Qualifications Qualified Medical PhysicistQualified Medical Physicist
••certified in diagnostic radiological physics certified in diagnostic radiological physics ••continuing education in MRI physicscontinuing education in MRI physics
Qualified MR Scientist••graduate degree in physical sciencegraduate degree in physical science••3 years documented clinical MR experience3 years documented clinical MR experience••continuing education in MRI physicscontinuing education in MRI physics
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment 1999 (Res. 19)
Responsibilities Acceptance Testing Annual Performance EvaluationEstablish Continuous Quality Control Program
• set action criteria • review QC program records during annual
surveyProvide written survey reports on timely basis
Responsibilities Responsibilities Acceptance Testing Acceptance Testing Annual Performance EvaluationEstablish Continuous Quality Control Program
•• set action criteria set action criteria •• review QC program records during annual review QC program records during annual
surveysurveyProvide written survey reports on timely basis
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment 1999 (Res. 19)
Continuous QC ProgramPerformed by Technologist:
•RF calibration for the head coil•Magnetic field gradient calibration•Image SNR & uniformity for the head coil•Processor sensitometry•Physical and mechanical inspection •Hard copy fidelity
Continuous QC ProgramContinuous QC ProgramPerformed by Technologist:
••RF calibration for the head coilRF calibration for the head coil••Magnetic field gradient calibrationMagnetic field gradient calibration••Image SNR & uniformity for the head coilImage SNR & uniformity for the head coil••Processor Processor sensitometrysensitometry••Physical and mechanical inspection Physical and mechanical inspection ••Hard copy fidelityHard copy fidelity
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment
ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI
Equipment 1999 (Res. 19)
Physicist’s Annual Equipment Survey:All of the routine QC tests plus:– Phase stability– Magnetic field homogeneity– Calibration of all radiofrequency coils – Image signal-to-noise ratio & uniformity for
all coils– Inter-slice RF interference– Artifact evaluation
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
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
ACR Definition of Quality AssuranceACR Definition of Quality AssuranceAll of the management practices instituted to
ensure that: 1. every imaging procedure is necessary and
appropriate to the clinical problem at hand,2. the images generated contain information
critical to the solution of that problem,3. the recorded information is correctly interpreted
and made available in a timely fashion to the patient's physician, and
4. the examination results in the lowest possible risk, cost, and inconvenience to the patient.
Quality Assurance Policy Manual• responsibilities and procedures for QC
testing• records of the most recent QC tests• a description of the orientation program
procedures for use and maintenance of equipment;
• MRI techniques to be used• precautions to protect the patient • proper maintenance of records, including
records of testing, equipment service and QA meetings
• procedures for cleaning and disinfection
ACR Definition of Quality ControlACR Definition of Quality ControlPart of quality assurance: a series of
distinct technical procedures that ensure the production of high-quality diagnostic images.
1. Acceptance testing 2. Establishment of baseline performance3. Detection of changes in equipment
performance 4. Verification that equipment performance faults
have been corrected.
Radiologist’s ResponsibilitiesEleven 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.
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
MRI QC Technologist’s Responsibilities
• Daily magnetic resonance image quality control procedures
• Weekly quality control of hard copy and soft copy Images
• Routine visual inspection of equipment
Technologist Responsibilities
• Designated QC Technologist(s)• QC Notebook
– QC policies and procedures – data forms where QC procedure results are
recorded – notes on QC problems and corrective actions
• Review QC Data with QA Committee • Only Use Alternative Phantoms & Procedures
when documented by physicist• Follow Established Action Limits
Technologist’s Tests• Center frequency• Table positioning• Setup & Scanning• Geometric accuracy• High contrast resolution• Low contrast detectability• Artifact analysis• Film quality control• Visual Checklist
DailyDailyDailyDailyDailyDailyDailyWeeklyWeekly
DailyDailyDailyDailyDailyDailyDailyWeeklyWeekly
*ACR MRI QC Manual, Table 1 (p.25)*ACR MRI QC Manual, Table 1 (p.25)
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
Annual Survey Tests• Magnetic Field Homogeneity• Slice Position Accuracy• Slice Thickness Accuracy• Radio Frequency Coil Checks
– Volume coils1. Signal-to-noise ratio2. Percent integral uniformity3. Percent signal ghosting
– Surface Coils SNR Tests• Inter-Slice RF Interference• Soft Copy Displays (monitors)
Magnetic Field HomogeneityMagnetic Field Homogeneity
ωoωoDenotes a totally uniform magnetic
field.All signal is at
resonant frequency, ωo.
Denotes a totally uniform magnetic
field.All signal is at
resonant frequency, ωo.
ωoωo ωoωo
Magnet firld homogenity can be characterized using FWHM of resonance
peak
Magnet firld homogenity can be characterized using FWHM of resonance
peak
FWHMFWHM
FWHMFWHM
Fourier transform of signal produces aLorentzian peak in
well-shimmed magnet
Fourier transform of signal produces aLorentzian peak in
well-shimmed magnet
Ideal HomogeneityIdeal Homogeneity Good HomogeneityGood Homogeneity Poor HomogeneityPoor Homogeneity
Magnetic Field Homogeneity
Phase images from GRE sequences with 10ms difference in TE’s
Phase and Unwrapped Phase Images
Magnetic Field Homogeneity
• Overall, the phase mapping technique provides the best mechanism for evaluating field homogeneity.
• Phase-maps in several planes can be obtained to determine the spherical harmonic coefficients and allows a means of “shimming” the magnet.
• 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 PositionSlice Position Slice SpacingSlice Spacing
SLICE #1SLICE #1SLICE #1 SLICE #11SLICE #11SLICE #11
Crossed wedges should be of equal lengthif position and spacing are accurate
Crossed wedges should be of equal lengthCrossed wedges should be of equal lengthif 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 Thickness4. Lower display to
one/half the average.
5. 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 EvaluationSite: _____________________________ 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
BoHomogeneity
Slice PositionAccuracy -
Slice ThicknessAccuracy
Bird-Cage 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
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 lowhigh 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
TorsoPhased Array
Cervical-Cranial Phased Array
Volume Coil Data
Ghost SignalMean SignalBackground Signal
Percent Signal Ghosting
Mean SignalSD of Background Signal
Signal-to-Noise
Max SignalMin Signal
% Image Uniformity
Surface Coil DataMaximum signalSD of Background Signal
Maximum 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.
Volume CoilData Recorded
Calculated Values:Uniformity
SNRGhosting
Surface Coil
Signal-to-Noise
-
ACR MRI QC Manual, pg. 125ACR MRI QC Manual, pg. 125
Slice Cross-Talk Measurements1. Position 5mm slices on the uniform volume 2. Repeat measurements decreasing the slice
gap :Series # 1 2 3 4No. Slices 11 11 11 11
Slice Gap (mm) min 0.5 1.0 53. 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
Sign
al-to
-Noi
se R
atio
(per
cent
of M
axim
um)
Soft Copy Displays
• Requires precision luminance meter
• Four tests– Maximum and minimum
luminance– Luminance uniformity– Resolution– Spatial Accuracy
5. Interslice RF Interference Phantom Description: ___________________________________________ Pulse Sequence: Type: ____ TR: _____ TE: ______ FOV: _____ cm2
Matrix: ___________ BW: _________kHz ; NSA ___ Number of slices______
6. Soft Copy Displays
Monitor Description: __________________________________________ Maximum Luminance: ________________________ Cd m-2. Minimum Luminance: _________________________ Cd m-2. Luminance Uniformity: Average of values obtained in four corners of screen: ______ Cd m-2. Luminance measured in center of screen: ______ Cd m-2. Percent difference: ________ %
|(Center – Average Corners)/(Center) x 100% < 30%|
7. Evaluation of Site’s Technologist QC Program 4) Set up and positioning accuracy: (daily) _________ 5) Center Frequency: (daily) _________ 6) Transmitter Attenuation or Gain: (daily) _________ 7) Geometric Accuracy Measurements: (daily) _________ 8) Spatial Resolution Measurements: (daily) _________ 9) Low Contrast Detectability: (daily) _________ 10) Film Quality Control (weekly) _________ Visual Checklist: (weekly) _________
Series Slice Signal-Number Gap to-Noise
(mm) Ratio
1
2
Mea
sure
d S
NR
100%
90%
80%
70%0% 25% 50% 75% 100%
Inte r-slice Gap (percent of slice thickness)
3
4
Series Slice Signal-Number Gap to-Noise
(mm) Ratio
1
2
Mea
sure
d S
NR
100%
90%
80%
70%0% 25% 50% 75% 100%
Inte r-slice Gap (percent of slice thickness)
3
4
RF Slice Interference
Soft CopyDisplays
-
ACR MRI QC Manual, pg. 127ACR MRI QC Manual, pg. 127
Review of Routine QC
Program
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)
MRI QC Program Summary• MRI Physicist
– runs baseline tests of system performance– sets action criteria for routine ACR phantom tests– performs quarterly calibration checks with
appropriate phantoms– reviews QC program
• Technologist – performs daily tests to assess image quality using
ACR phantom– Weekly checks of hard copy output
• All measurements made, problems discovered, and actions required to resolve the problems are recorded for review
Successful MRI QC ProgramSuccessful MRI QC Program1. Technologists run QC scans on a
daily basis2. If exceed action criteria – repeat QC
procedure3. Physicist reviews QC data annually4. Record data - report problems to
service 5. Have service record problems and
solutions in a service log
1. Technologists run QC scans on a daily basis
2. If exceed action criteria – repeat QC procedure
3. Physicist reviews QC data annually4. Record data - report problems to
service 5. Have service record problems and
solutions in a service log
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