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Quality Control and the ACR’s MRI Accreditation Program

Quality Control and the ACR’s MRI Accreditation Program

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Quality Control and the ACR’s MRI Accreditation Program

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  1. 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

  2. Overview • ACR MRI Accreditation Program • MRI Quality Control Program • Definitions • Technologist’s Daily & Weekly QC Tasks • Physicist’s Responsibilities • Annual Tests for the Physicist

  3. 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

  4. 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

  5. ACR Phantom • Affordable ($730) • Results easily evaluated • 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

  6. Documents for Using Phantom

  7. 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

  8. ACR Magnetic Resonance Imaging Quality Control Manual 2001

  9. 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.

  10. 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

  11. 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

  12. Technologist QC • Daily (weekly) tests • Cheat sheets • Review data after ten days to establish baseline values and variability

  13. 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

  14. Central Frequency • Specific but not sensitive • Action Criteria: • Change in Hz from previous day > 2 * resonant frequency in MHz • Suggestive of: • Magnet drift • RF instability

  15. 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

  16. 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

  17. Geometric Accuracy • Measure distance along main axes of phantom • Compare with known values

  18. Potential Causes of Geometric Accuracy Failures • Phantom mispositioning • Gradient miscalibration • Bo inhomogeneity • Ferromagnetic objects in magnet • Poor magnet shimming • Gradient non-linearity • Inappropriate receiver bandwidth • Poor eddy current compensation • Combination of two or more of above

  19. Failure Due To: • Miscalibrated Gradients • Low Receiver Bandwidth •High Bo Inhomogeneities Geometric Accuracy - Axial BW = 7.4 kHz BW = 3.6 kHz Slice #1

  20. Gradient Correction • If gradients are inherently non-linear gradient correction may be applied

  21. Percent Geometric Distortion T1-Weighted, Central Slice 10.00% 8.00% 6.00% 4.00% Poor Gradient Calibration 2.00% 0.00% 1- 31- 30- 30- 29- 28- 29- 28- 28- 27- 27- 26- 25- 25- Oct Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Date of Measurement System #1 System #2 System #3

  22. Assessment of MR Image Quality • Performed by Technologist • Using ACR Phantom • High Contrast Resolution • Low Contrast Detectability

  23. RF Noise/Leaks/ Spikes Single frequency artifact shows up as zipper in middle of image.

  24. ACR MRI Phantom Resolution Insert Slice Thickness 1.1 mm 1.0 mm 0.90 mm Chem Shift Resolution Insert ZOOM = 2

  25. Spatial Resolution Matrix: Registration with Phantom PARTIAL VOLUME ARTIFACT Resolution Holes Image Matrix

  26. High Contrast Spatial Resolution • Evaluate conspicuity of holes arranged in two square arrays • Has to avoid partial-volume artifact… UL LR

  27. High Contrast Resolution • Specific but not sensitive • Action Criteria: • Any reduction in # of holes seen • Suggestive of: • Increased eddy currents • Poor gradient calibration • Poor Bo uniformity • Reduced stability of system

  28. Low Contrast Detectability 5.1% Contrast Detectability 3.7% Contrast Detectability Slice #10 Slice #12

  29. LCD and Signal-to-Noise 40 35 30 25 20 15 10 5 0 Total Number of Spokes 0 50 100 150 200 250 300 350 Signal-to-Noise Ratio

  30. Low 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

  31. 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

  32. Laser Film QC • Weekly: • View SMPTE pattern • Verify gray levels • 0/5% & 95/100% patches • Film 6 on 1 • 4 on 1 if necessary • Plot OD of • 10%, 40% & 90& patches • Observe film for artifacts SMPTE Test Pattern

  33. Action Limits

  34. 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

  35. If QC Test Fails • Common errors- • Check for magnetic objects in bore • Re-seat head coil • Reposition & landmark phantom • Make sure scan room doorsecurely closed • Repeat daily QC scan procedures • Record results again in QC notebook

  36. 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

  37. Annual Survey Tests • Magnetic Field Homogeneity • Slice Position Accuracy • Slice Thickness Accuracy • Radio Frequency Coil Checks • Volume coils • Signal-to-noise ratio • Percent integral uniformity • Percent signal ghosting • Surface Coils SNR Tests • Inter-Slice RF Interference • Soft Copy Displays (monitors)

  38. Magnetic Field Homogeneity Ideal Homogeneity Good Homogeneity Poor Homogeneity FWHM FWHM o o o Denotes a totally uniform magnetic field. All signal is at resonant frequency, o. Fourier transform of signal produces a Lorentzian peak in well-shimmed magnet Magnet firld homogenity can be characterized using FWHM of resonance peak

  39. 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.

  40. 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.

  41. 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

  42. Slice Position Accuracy Slice Spacing Slice Position SLICE #11 SLICE #1 Crossed wedges should be of equal length if position and spacing are accurate

  43. Slice Position Accuracy • Causes of poor performance: • Operator error • Table positioning shift • Miscalibrated gradients • High Bo inhomogeneities

  44. 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

  45. Slice Thickness Measurement • Use slice 1 of ACR T1. • Magnify by 2 to 4. Adjust window/level to see signal ramps. (Set window at minimum.) • Use rectangular ROI to measure mean of middle of each signal ramp. Take average.

  46. Slice Thickness • Lower display to one/half the average ramp signal. • Measure lengths of top and bottom ramps and calculate slice thickness.

  47. 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

  48. = =+ MRI Equipment Performance Evaluation Site & Equipment Data 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 Bo Homogeneity Method Used (check one): Spectral Peak ___ Phase Difference ___ Other (desc ribe) __________________________ Measured Homogeneity: Diameter of Spherical Homogeneity Volume (cm) (ppm) ________ _______ ________ _______ ________ _______ Slice Position Accuracy 2. Slice Position Accuracy From Sl ice Positionss #1 and #11 of the ACR Phantom: Wedge (mm) = - - =+ Slice Location #1 ________ Slice Location #11 ________ Slice Thickness Accuracy 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.

  49. Bird-Cage Type Head Coil RF coils produce uniformity patterns characteristic of their design.