MR QA Flashcards
Health Technology Management
- Procurement of appropriate equipment, technology, and maintenance agreements
- Acceptance testing and clinical user acceptance of what was agreed to be purchased
- Routine QC testing; annual performance testing
- Review of QC, service logs, repairs, and maintenance agreements
- Appropriate transport or disposal of equipment
- In the best interests of the patients and staff, independent of vendors and manufacturers
AHPRA - Responsibilities of Medical
Radiation Practitioners
- Equipment issues will usually be encountered first by radiographers or radiation therapist
- Day-to-day quality control and maintenance of imaging systems
- Following vendor specific instructions of operation and use of equipment
- Identifying and arranging equipment repairs when there is an equipment fault
- Clinical decision making whether equipment is still appropriate for use on patients or stop until equipment issue is resolved
Safety and Quality Management
- Equipment issues can be minor or major
- Medical physicists, biomedical engineers, and imaging technicians support clinics in resolving equipment issues with vendors or to make improvements
- Established pathway to escalate and notify TGA
- Investigate equipment faults or failures, patient and staff safety, and other clinical sites
- Continuous improvement of quality and safety
- Keep documented evidence
- Equipment history: acceptance, reports, servicing, repairs, testing
- Requires multi-disciplinary teamwork and multi-site collaboration
MRI General System Checks
- Table docking and movement, RF coil integrity and connections, temperature, and laser operations
- Transmitter (central frequency) and gain calibration
MRI Bore Lasers and Table Position Accuracy + test method
- MRI bore lasers are set to drive a certain table position to isocentre
- The rest of the QA tests and phantom scans depend on the accuracy of the MRI bore lasers and table position
Distance Accuracy Test Method:
* Place an object with a known distance on the table
* Drive the table a known distance with the MRI bore laser on and compare object dimensions with the change in displayed table position
MRI Bore Lasers and Isocentre
- Use the MRI bore laser to setup a phantom and drive the center of the phantom to isocentre
- Compare the displayed z position in the image localiser
Slice Position Accuracy
- Localiser is a low-resolution image used to set the scan region for a given protocol
- The scan region is set by selecting the central slice position
- The system adjusts the selected slice position relative to isocentre and table position
what is geometric Accuracy
Geometric accuracy is a measure of the difference in spatial localisation between an object and the corresponding MR image of the object.
Geometric Accuracy Test
- Typical phantoms have either a uniform grid or hole pattern
- Scan phantom of known geometry and dimensions spanning the volume of the FOV
- Scanning protocol: SE without distortion correction will test spatial linearity and gradient calibration of system
- Percent geometric distortion is calculated between the actual dimensions of the phantom and the measured dimensions in the final MR image
Geometric Accuracy Test equation + tolerance criteria
geometric distortion (%GD) = 100 x (actual phantom dimensions - measured phantom dimensions on image / measured phantom dimensions on image)
tolerance criteria with phantom
- less than 2% for treatment planning
- less than or equal to 2mm
Slice Thickness Accuracy + test method and tolerance
- Depends on RF excitation bandwidth and the gradient field amplitude
- Affects spatial resolution, SNR, and minimal slice gaps
Test Method:
* Phantom with crossed-ramps (NEMA; ACR)
* Tolerance criteria for 5 mm slices or more: ≤ 10% (NEMA); -/+ 1 mm
High Contrast Spatial Resolution Accuracy + test method
- Distinguish between two nearby objects with minimal noise
- Mainly depends on acquisition matrix size (pixel size)
- Other factors include image processing, and display resolution
Test Method:
* Phantom with high contrast objects of varying size or MTF method
* Tolerance criteria: distinguish between objects that are at least one pixel width in size, and separated by one pixel width
Percent Signal Ghosting - artefact
- Appears as low-intensity signals from the object superimposed at a different location
- Occurs in the phase encoding direction
- Due to hardware, pulse sequence or coil design
- Can also be caused by motion
Percent Signal Ghosting test Method + tolerance
Test Method
* Ghosting is easier to detect in low-level signal areas (ie. background)
* Homogeneous phantom or insert
* Depends on pulse sequence
Tolerance Criteria
* PSG compares the ratio of the background signal, ghosting signal, and mean phantom signal
* PSG should be ≤ 1%
Magnetic Field Homogeneity Test
- Measures the variation in magnetic field over a diameter spherical volume (DSV)
- Depends on MRI hardware, coil design, ferromagnetic structures close to the magnet
- Optimised by magnet shimming
Test Method
* Spherical phantom, homogeneous solution
* Changing bandwidth, or uniformity maps
Magnetic Field Homogeneity Test - bandwidth difference + tolerance criteria
- Acquire slice using 2 different bandwidths
- Compare change in diameter or distance
- Evaluate for each orientation
Tolerance Criteria
* Depends on: the dimensions and features of the phantom used; scan protocol
What information is needed from the MRI images?
- Anatomical regions of interest
- Biomarkers for pathology
- Required patient positioning
- Scan setup and coil configuration
How will the MRI information be used?
- MRI-only vs CT/ MRI co-registration
- Different requirements and tolerance levels for different applications
- MR images supplementary to CT can use CT images as a standard comparison
What are the major sources of error or uncertainty in the imaging/treatment chain?
- Patient positioning
- Geometric distortion
- RF coil performance
- Laser alignment
What sources of error or uncertainty can be reduced, and which ones can’t be changed?
- Treatment position
- RF coils and coil holder size
- Field of View
RF Coil Test (PIU,PSG, SNR) + test method and tolerance criteria
RF Coil Checks
* Vendor specific coil tests for routine maintenance
* Coil and protocol specific tests (application based)
Test Method
* Vendor instructions, depends on MRI coil design and subsystem
* Clinical testing depends on clinical setup
Tolerance Criteria
* Vendor specific or benchmark performance for site or system model
* Depends on protocol and procedure
RF Coil - SNR
- SNR affects contrast resolution and signal localisation
- Lower SNR could result from RF coil failure or external sources of RF noise
- Hard flat-top couch and coil holders reduces body deformation, but also reduces SNR due to coils being further away from the body
Accuracy of External Lasers and MRI Bore Lasers test + tolerance
- Patient is set-up on MRI table based on external laser position
- Vendor specific phantoms, or a phantom with known dimensions
- Patient is positioned at MRI isocentre based on lasers and relative table position
Laser Accuracy Tolerance
* Defined by laser tolerance used for treatment setup < 2 mm
Geometric Distortion for RT Planning
- Geometric distortion increases further away from isocentre
- Degree of distortion can depend on scan protocol
- Can affect target delineation depending on application
- Vendors provide correction algorithms to minimise system related distortion based on their coil design
- Scan should cover FOV, or at least extend beyond ROI
- Images should be acquired in RT setup and coil configuration with distortion algorithms on
- Phantom with grid lines makes it easier to visibly inspect differences in spatial localisation
- Phantom with hole pattern makes it easier to quantify and analyse degree of distortion and correct if needed
Geometric Distortion for RT Planning Test Method
- Comparison with CT images
- CT and MRI compatible phantom for distortion analysis/ correction
- Compare CT and MRI images of patient with image analysis software
Artefacts
- Patient related distortion: spatial and signal localisation
- Can affect delineation of organs of interest for accurate treatment planning
- Intrinsic: Magnetic susceptibility, chemical shift, tissue boundaries (mm)
- Extrinsic: Metallic or conductive implants (cm)
- Patient-motion: breathing, movement
- Scans are repeated with optimised scan protocols if necessary to minimise artefacts
purpose of MR QA
- ensures safe and effective use of medical imaging technology
- provides a clear pathway to resolve equipment issues
- prevent avoidable equipment failures and risks to patients and staff
