Speaker
Description
Objectives: Upright radiotherapy offers potential therapeutic and financial advantages, yet posture-dependent differences in intrafractional respiratory motion remain poorly understood. Characterising these variations is essential for safe upright treatment planning and dose delivery[1]. We hypothesised that posture significantly alters breathing-induced motion, which can be quantified using AI-based diaphragm apex tracking on supine and upright cine 2D MRI.
Methods: Fourteen healthy volunteers underwent torso cine 2D MRI imaging (EKNZ 2023-00764) on a 0.5T open system (HASTE sequence; TR/TE: 750/44ms). Continuous images were acquired in alternating sagittal and coronal planes centred on the liver apex, in both supine and upright positions. Each plane was acquired three times per position (152s each; total session duration 793–1557s). Following a day-one training session, day-two data were analysed. The diaphragm apex was manually identified on the first frame of each sequence and then tracked using CoTracker3 [2] after contrast-limited adaptive histogram equalisation. Tracking accuracy was visually verified for all images, and major errors were manually corrected. Superior-inferior motion was extracted, normalised to the median position, and evaluated. Supine versus upright motion was compared using mean peak-to-peak amplitudes across subjects, with significance assessed via a Wilcoxon signed-rank test.
Results: CoTracker3 provided accurate and robust tracking, including during most large-amplitude motion. We observed considerable inter- and intra-subject respiratory variability. Mean peak-to-peak breathing amplitude was 9.63mm (SD=4.38mm) in supine and 6.20mm (SD=2.05mm) in the upright position. Thirteen of fourteen volunteers showed reduced upright breathing motion compared to supine (reduction range: 2.13-70.51%), while one showed a 14.03% increase. Across the cohort, respiratory motion was significantly lower in the upright position, with mean and median reductions of 35.57% and 30.50%, respectively (p<0.001).
Outlook & Discussion: Respiration motion, measured via the diaphragm apex, differed significantly between supine and upright positioning. Over 50% of the cohort demonstrated an upright motion reduction of more than 27%, though individual variations highlight the need for personalised assessment. These preliminary findings support further investigation using larger cohorts and longitudinal measurements to establish the reproducibility of posture-aware motion.