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

ACRMagnetic

ResonanceImaging QualityControl Manual2001

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

Volume RF Coil MeasurementsVolume RF Coil Measurements

ACR PhantomSlice #7

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

Surface RF Coil Measurements

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