Room: Karl Dean Ballroom B1
Purpose: MR-IGRT systems provide online visualization of tumor and surrounding tissue, thus holding great potential to improve respiratory motion management for abdominal cancer. Although cine MRI is available on the MR-linac, it only obtains 2D motion information and lacks volumetric information for a more comprehensive motion evaluation. In addition, a prospective internal navigator-controlled 4D imaging is available, but requires lengthy image acquisition times. The aim of this work was to demonstrate the feasibility of retrospective 4D-MRI with an image-based respiratory phase sorting on a pre-clinical MR-linac.
Methods: Multi-slice 2D cine of an MRI-compatible motion phantom and one healthy volunteer under an IRB-approved study(PA14-1002) were acquired. A sinusoidal waveform(amplitude:40mm;period:3second) was input to control the moving part of the phantom in superior-inferior(SI) direction. Images of the healthy volunteer were acquired under free-breathing condition. An in-house developed Matlab program was designed to extract body area(BA) of a selected region of interest, as an internal surrogate for respiratory motion. Respiratory phases were then calculated, and 2D images were sorted into 6 respiratory phases to generate 4D-MRI. For evaluation, motion of a selected point of interest(POI) were captured on 4D-MRI. The motion of the phantom was compared with the sinusoidal waveform. The average absolute amplitude error(AAAE) over all phase-bins was calculated. Motion amplitudes were also measured similarly for the healthy volunteer.
Results: 4D-MRI acquired with an MR-linac clearly demonstrated respiratory motion. AAAE of a POI for the phantom in SI direction was 3.3Â±2.5mm. Motion amplitudes measured on 4D-MRI of the healthy volunteer were 6.8mm and 16.1mm, in anterior-posterior and SI directions, respectively.
Conclusion: A 4D-MRI technique was successfully implemented on an MR-linac. Image-based retrospective sorting was employed with BA as the respiratory motion surrogate. The proposed 4D-MRI is a promising tool to improve the visualization and delineation of mobile tumors for image-guided radiotherapy on MR-linac.