Phantom-based assessment of a modified 4D-MRI protocol for abdominal tumor motion tracking: Effects of sequence parameters and modality comparison.

purposeTo investigate the effects of targeted modifications to a clinical 4D-MR pulse sequence on displacement accuracy for abdominal tumor motion tracking, and to compare its performance with established 4D-MR and 4D-CT protocols using a motion phantom driven by clinically representative respiratory waveforms.

methodsA commercially available MRI-compatible motion phantom was used to simulate abdominal tumor motion driven by both sinusoidal and patient-derived respiratory waveforms. Imaging was performed on a 1.5T MRI scanner using a 3D radial stack-of-stars sequence for 4D-MR imaging and a clinical CT scanner. Four acquisition parameters, slice thickness, acquisition orientation, number of radial views, and number of respiratory bins, were systematically varied. Displacement measurements were performed using line intensity profiles extracted from coronal slices and analyzed based on full-width-at-half-maximum calculations. Comparisons were made against both ground-truth programmed displacements and clinical 4D-CT measurements. Additional displacement measurement comparisons between a coronal 10-bin 4D-MR protocol and an axial 5-bin 4D-MR and 4D-CT scans were evaluated on both sinusoidal and patient-derived respiratory traces. Organ displacement measurements between the MR protocols were compared on compression-belt patients as a preliminary study of the in vivo comparison.

resultsThe modified 4D-MR protocol, incorporating ten respiratory bins and coronal acquisition, significantly improved displacement accuracy relative to the clinical axial MR protocol. For sinusoidal waveforms, displacement differences between coronal MR and CT were comparable (p > 0.05), being consistently within 0.5 mm of each other, while axial MR underestimated displacement by more than 1 mm across amplitudes and breathing periods (p < 0.001). Linear mixed-effects modeling of patient-derived respiratory waveforms showed that the coronal MR protocol outperformed the axial MR protocol by 1.70 mm for regular and 3.10 mm for irregular patterns (p < 0.001), with no significant differences observed between coronal MR and CT.

conclusionsA modified 4D-MRI protocol incorporating optimized acquisition parameters demonstrated displacement accuracy equivalent to 4D-CT in a 1D motion phantom, including under patient-derived respiratory conditions. These results support the clinical feasibility of 4D-MR as a radiation-free alternative for motion-resolved imaging in select cases, with further validation in multi-directional motion and in vivo settings recommended.