RSNA 2002 – Role of Imaging Physicist/Scientist in Quality Control – Course #132

Overview

Physicists’ Quality Control for MR Equipment

• MRI Standards • MRI Accreditation Program • MRI Quality Control Program – Definitions – Physicist’s Responsibilities – Technologist’s Daily & Weekly QC Tasks – Other Tests for the Physicist

Geoffrey D. Clarke, Ph.D. University of Texas Health Science Center at San Antonio Radiological Sciences Division

ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI Equipment 1999 (Res. 19)

Standards & Accreditation • ACR Standard for Performing and Interpreting MRI (Res. 16-2000)

Qualifications Qualified Medical Physicist ••certified certified in diagnostic radiological physics ••continuing continuing education in MRI physics Qualified MR Scientist ••graduate graduate degree in physical science ••3 3 years documented clinical MR experience ••continuing continuing education in MRI physics

• ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI Equipment (Res. 19-1999) • ACR Magnetic Resonance Imaging Accreditation Program

ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI Equipment 1999 (Res. 19)

ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI Equipment 1999 (Res. 19)

Responsibilities Acceptance Testing Annual Performance Evaluation Establish Continuous Quality Control Program • set action criteria • review QC program records during annual survey Provide written survey reports on timely basis

Continuous QC Program Performed 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

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ACR Standard for Diagnostic Medical Physics Performance Monitoring of MRI 1999 (Res. 19) Equipment 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 Program Features • 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 Program

ACR MRI Accreditation Program

MRI Survey Agreement

Quality Control Section

• 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

• 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:

ACR Magnetic Resonance Imaging Quality Control Manual 2001

ACR Definition of Quality Assurance

G.D. Clarke (UTHSCSA)

– Documented by qualified service engineer – Repairs documented and records maintained by the MR site

All 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.

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Quality Assurance Policy Manual

ACR Definition of Quality Control

• 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

Part of quality assurance: a series of distinct technical procedures that ensure the production of high-quality diagnostic images.

Radiologist’s Responsibilities

Responsibilities of the Qualified Medical Physicist or MRI Scientist

1. Acceptance testing 2. Establishment of baseline performance 3. Detection of changes in equipment performance 4. Verification that equipment performance faults have been corrected.

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.

• Write Purchase Specifications • Perform Acceptance Testing – Baseline Measurements

• Determine Action Limits • Set up Daily/Weekly QC Tests • MRI equipment performance review

Technologist Responsibilities

MRI QC Technologist’s Responsibilities

• Designated QC Technologist(s) • QC Notebook

• Daily magnetic resonance image quality control procedures • Weekly quality control of hard copy and soft copy Images • Routine visual inspection of equipment

G.D. Clarke (UTHSCSA)

– 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

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Documenting the Quality Control Program

Technologist’s Tests • • • • • • • • •

Center frequency Table positioning Setup & Scanning Geometric accuracy High contrast resolution Low contrast detectability Artifact analysis Film quality control Visual Checklist

Daily Daily Daily Daily Daily Daily Daily Weekly Weekly

• 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

*ACR MRI QC Manual, Table 1 (p.25)

Magnetic Field Homogeneity

Annual Survey Tests • • • •

Ideal Homogeneity Good Homogeneity Poor Homogeneity

Magnetic Field Homogeneity Slice Position Accuracy Slice Thickness Accuracy Radio Frequency Coil Checks

FWHM FWHM

– Volume coils FWHM

1. Signal-to-noise ratio 2. Percent integral uniformity 3. Percent signal ghosting

ωoo

Denotes Denotes aa totally uniform uniform magnetic magnetic field. All signal signal is is at at resonant resonant frequency, ωo. o.

– Surface Coils SNR Tests

• •

Inter-Slice RF Interference Soft Copy Displays (monitors)

Magnetic Field Homogeneity

ωoo

Fourier Fourier transform transform of signal produces produces a Lorentzian Lorentzian peak in well-shimmed magnet magnet

Magnet Magnet field field homogeneity homogeneity can can be be characterized characterized using FWHM of resonance of peak peak

Magnetic Field Homogeneity

Phase images from GRE sequences with 10ms difference in TE’s

Phase and Unwrapped Phase Images

G.D. Clarke (UTHSCSA)

ωoo

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• 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.

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Slice Position Accuracy

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

SLICE #1

Slice Spacing

SLICE #11

Crossed wedges should be of equal length if position and spacing are accurate

MRI Slice Thickness

Slice Position Accuracy

• Signal ramps have a slope of 10:1

• Causes of poor performance:

• Signal from ramp is 10 x slice thickness

• Operator error • Table positioning shift • Miscalibrated gradients • High Bo inhomogeneities

• Two ramps are used to compensate for inplane rotation of the phantom • Phantom does not compensate for tilting backwards or swaying left-right

Slice Thickness

Slice Thickness Measurement

4. Lower display to one/half the average. 5. Measure lengths of top and bottom ramps and calculate slice thickness.

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.

G.D. Clarke (UTHSCSA)

Slice Thickness = 0.2 ×

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(top × bottom) (top + bottom)

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Slice Thickness • ACR-MRAP: slice thickness measured should be + 0.7 mm of prescribed value –

MRI Equipment Performance Evaluation

Site & Equipment Data

Site: _____________________________ MRAP Number: ____________________

Date: ________ Serial Number: ___________

Equipment: MRI System Manuafacturer: _________________ Model : ________ Processor Manufacturer:

_________________ Model: _________

PACS Manufacturer:

_________________ Model: _________

ACR MRAP Phantom Number used: _________

1. Magnetic Field Homogeneity

Bo Homogeneity

+ 14% error on 5mm slice, may be too generous

Method Used (check one): Spectral Peak ___ Phase Difference ___ Other (describe) __________________________ Measured Homogeneity:

Diameter of Spherical Volume (cm)

• Corrective actions:

Homogeneity (ppm)

________ ________ ________

– 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

Slice Position Accuracy Slice Thickness Accuracy

_______ _______ _______

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

______

Bottom

______

(fwhm in mm)

Calculated slice Thickness (mm) ______

Duplicate these forms so they will be available for repeated use.

Uniformity Pattern

Bird-Cage Head Coil

Birdcage Coil High Field

RF coils produce uniformity patterns characteristic of their design.

Volume RF Coil Measurements

Solenoid Coil Low Field

Image Intensity Uniformity • Performance criteria: PIU ≥ 90%  (high − low)  percent integral uniformity = 100 ×  1 −   (high + low) 

• Measurement Considerations: ACR Phantom Slice #7

G.D. Clarke (UTHSCSA)

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• 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

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Phased- Array Coils

Image Intensity Uniformity

•The signal is viewed from more “angles” •Using four channels does not produce √4 * S/N

• Causes of failure: • Poor phantom centering in head coil (usually AP) • Ghosting • Motion or vibration • Mechanical failure in head coil

Abdomen-pelvis phased-array

Torso Phased Array

Cervical-Cranial Phased Array

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 Data Maximum Signal-toNoise

Maximum signal SD of Background Signal

4. RF Coil Performance Evaluation A. VOLUME RF COIL RF Coil Description: __________________________ Date: ____________

Slice Cross-Talk Measurements

Phantom Description: ___________________________________________

Volume Coil

Pulse Sequence: degrees

Type: ____

FOV: _____ cm2

Data Recorded

TR: _____

TE: ______

flip angle _____

Matrix: ___________ BW: _________kHz ; NSA ___ Slice thickness ______mm; spacing _______ mm TX attenuation (or gain) __________

1. Position 5mm slices on the uniform volume

Data Collected: Mean Signal

Maximum Signal

Minimum Signal

Background Signal

Calculated Values: Uniformity SNR Ghosting

Noise Standard Deviation

Ghost Signal

2. Repeat measurements decreasing the slice gap :

Calculated Values: Signal-to-Noise Ratio

Percent Image Uniformity

Percent Signal Ghosting

B. RF SURFACE COIL RF Coil Description: __________________________ Date: ____________ Phantom Description: ___________________________________________ Pulse Sequence:

Surface Coil Signal-to-Noise

Slice thickness ______mm; spacing _______ mm

Maximum Signal

Noise Standard Deviation

Image uniformity distribution OK?

________

Image ghosting OK?

________

Maximum Signal-toNoise Ratio

Window width ________

1

2

3

4

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.

TX attenuation (or gain) __________

HARD COPY IMAGE:

ACR MRI QC QC Manual, Manual, pg. pg. 125 125

Type: ____ TR: _____ TE: ______ FOV: _____ cm2 Matrix: ___________ BW: _________kHz ; NSA ___

-

Series # No. Slices

4. Plot the SNR vs. percentage slice gap Window level _______

Several copies of this page may be required to report on all RF coils.

G.D. Clarke (UTHSCSA)

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Signal-to-Noise vs. Inter-slice Gap

Soft Copy Displays • Requires precision luminance meter • Four tests

Signal-to-Noise Ratio (percent of Maximum)

100%

SE, TE = 20 FE, TE =8 TFE, TE =4

90%

– Maximum and minimum luminance – Luminance uniformity – Resolution – Spatial Accuracy

80%

70% 0%

25%

50%

75%

100%

Percentage Slice Gap

5. Interslice RF Interference Phantom Description: ___________________________________________ Pulse Sequence:

2

Type: ____ TR: _____ TE: ______ FOV: _____ cm

MRI Equipment Evaluation Summary

Matrix: ___________ BW: _________kHz ; NSA ___

S eries Number

Slice Gap (mm)

Site ___________________ Report Date: __________ System MRAP #_____________ Survey Date: __________ MRI System Manufacturer ___________ Model: __________ Physicist/MRI Scientist: ____________________ Signature: ________________________________

100 %

S ignalto-Noise Ratio M e a sure d S NR

RF Slice Interference

Number of slices______

1 2 3 4

90 %

Equipment Evaluation Tests 80 %

70 %

Summary Sheet

0% 25% 50% 75% 100% Inte r-slice Ga p (pe rce nt of slice thickne ss)

6. Soft Copy Displays Monitor Description: __________________________________________

Soft Copy Displays

-2

Maximum Luminance: ________________________ Cd m . -2

Minimum Luminance: _________________________ Cd m . Luminance Uniformity:

(pg. 131)

Average of values obtained in four corners of screen: ______ Cd m-2. Luminance measured in center of screen:

______ Cd m-2.

Percent difference: ________ % -

Review of Routine QC Program

|(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)

_________

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

Pass / Fail _________ _________ _________ _________ _________ _________ _________ _________ _________

Medical Physicist’s or MRI Scientist’s Recommendations for Quality Improvement:

______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________

_________ _________

7) Geometric Accuracy Measurements: (daily)

_________

8) Spatial Resolution Measurements: (daily)

_________

9) Low Contrast Detectability: (daily)

_________

10) Film Quality Control (weekly)

_________

Visual Checklist: (weekly)

_________

ACR ACR MRI MRI QC Manual, pg. 127

MRI QC Program Summary

Successful MRI QC Program

• 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

1. Technologists run QC scans on a daily basis 2. If exceed action criteria – repeat QC procedure 3. Physicist reviews QC data annually 4. Record data - report problems to service 5. Have service record problems and solutions in a service log

• 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

G.D. Clarke (UTHSCSA)

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