A recent visit by The Phantom Laboratory’s MR physicist Dr. Richard Mallozzi to the UCLA Department of Radiation Oncology helped kick off the use of the Magphan® RT on UCLA’s ViewRay MRIdian MR-guided radiotherapy machine to monitor geometric distortion, uniformity, and other key imaging parameters.
UCLA is an early adopter of MR guided radiotherapy. The use of MR guidance in radiotherapy has great benefits in the accurate real-time targeting of tumors with the improved soft tissue contrast of MR imaging. With improved imaging, margins around tumors and organs at risk can often be reduced to less than 3mm compared to 5mm common in conventional IGRT and IMRT treatments. Furthermore, a high proportion of the treatments on the MRIdian machine are abdominal and pelvic SBRTs with 3 to 5 fractions requiring great accuracy. With these needs for very precise imaging of tumors and organ position, it is critical that MR distortion and uniformity are accurately measured and controlled.
Dr. Mallozzi noted,
“While geometric distortion in modern MR scanners is minimized by the manufacturers, its stability is dependent upon a chain of conditions that should be carefully monitored and controlled.”
Dr. James Lamb, assistant professor and lead physicist for UCLA's ViewRay service, is responsible for implementing the QA procedures for the UCLA’s MRIdian system. He determined early on that daily measurement of MR distortion and uniformity were important as failures in components like coils could be difficult to detect through visual examination of images and could potentially lead to inferior treatments.
He found that existing QA phantoms for quantifying distortion either failed to cover a sufficient field of view to provide true 3D measurements or were so bulky that they precluded daily use by therapists working on the machine. Furthermore, existing analysis software was not automated so the therapists and physicists could not easily monitor MR image quality
When he learned that The Phantom Laboratory was developing a phantom and analysis to address these shortcomings he was immediately interested.
“We were looking for a comprehensive MR image check”, he commented.
The Magphan RT phantom’s modular design with all components weighing less than 12 kg makes it practical for a single person to safely set up and handle without special equipment. The design allows the addition of center modules to further extend the field of view or for future additional measurement tools.
The phantom with its attendant analysis service can provide accurate measurements for distortion and uniformity over a 35 x 27 x 21 cm.volume (extendible to 35 x 39 x 21 cm with the addition of a center module). The Magphan RT system can also provide modulation transfer and edge spread functions, slice thickness measurements, signal to noise ratios and positional accuracy measurements.
The analysis service from Image Owl is hosted online requiring no installation. A simple transfer of the MR DICOM image series starts the fully automated analysis and produces a comprehensive QA report with measurements compared against specifications that provide therapists and physicists a daily snapshot of imaging performance.
Dr. Lamb noted that besides distortion they plan to most closely monitor uniformity and noise as these directly impact targeting quality. In the longer term, they want to track a number of variables to study the stability and characteristics of the guidance MR. As UCLA implements MR planning separate from the ViewRay system, Dr. Lamb plans to use the Maghan RT to characterize and monitor distortion on those scanners as well.
Dr. Lamb says,
“ With MR guided radiotherapy becoming more mainstream the Magphan RT fills a significant gap in commercially available phantoms for comprehensive MR imaging QA”
The Magphan RT system will be on display at The Phantom Laboratory/ Image Owl booth #5009 during the AAPM meeting in Denver, CO, July 31 - August 3, 2017
Book an appointment to get a first-hand look at the phantom and analysis service.