A pilot study of respiratory motion characterization in the abdomen using a fast volumetric 4D‐MRI for MR‐guided radiotherapy

ABSTRACT Purpose This pilot study explored the use of fast volumetric four‐dimensional magnetic resonance imaging (4D‐MRI) for motion characterization in the abdomen, ultimately aiming for the translation to future MR‐guided radiotherapy (MRgRT) applications. Methods Nine healthy subjects and four patients underwent shallow free‐breathing abdominal scans on a 1.5 T MR simulator using a fast volumetric 4D‐MRI sequence with a temporal resolution up to three frames per second. The reference left/right liver and gross target volume (GTV) were segmented on the first frame, and the following consecutive dynamic images were nonlinearly registered to the reference to create the dynamic binary masks. Respiratory probabilistic volumes (rPPVj,i%), which correspond to the probability of being occupied by a volume of interest (VOI), were generated by summating the dynamic binary masks. Subject‐specific rPPV144,0% and rPPV144,5% of the GTV were quantitatively compared to the 4D computed tomography (CT)‐determined internal target volume (ITV) using the dice similarity coefficient (DSC), volumetric difference, and Hausdorff distance (HD). Results The results showed slightly smaller rPPV144,0% and rPPV144,5% but larger DSC and small HD compared to the 4D‐CT‐determined ITV, which preliminarily demonstrates the feasibility and capability of characterizing abdominal respiratory motion using the volumetric 4D‐MRI‐derived rPPV approach. Conclusion The potential usefulness of fast volumetric 4D‐MRI for personalized motion management in future MRgRT is proposed.