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Purpose: Interest in upright, gantry-less radiotherapy has grown, partly for the cost and accessibility benefits of rotating the patient rather than the treatment beam. However, data on anatomical and physiological differences between upright and supine posture remains scarce. As thoracic RT depends heavily on motion management, we investigated posture-induced differences in respiratory motion and their likely impact on thoracic treatment delivery.
Methods: We developed a pipeline to analyse respiratory motion in 10 healthy volunteers, imaged with 2D cine MR on a FONAT 0.6T multi-position scanner in both upright and supine postures, each acquisition spanning approximately 14 breathing cycles ($\approx$200 frames/50 seconds). Deformable image registration quantified motion of the chest wall, diaphragm, and an internal vessel. PCA extracted the principal direction of motion (PC1) and a hysteresis component (PC2). The Jacobian determinant was analysed to identify patterns of respiratory expansion and contraction across the cohort.
Results: Diaphragm motion (PC1) amplitude was significantly smaller upright than supine (p=0.008), with period also trending shorter (p=0.063). In contrast, chest wall amplitude was unaffected by posture, but its period was significantly longer upright (p=0.046). For internal vessel motion, both the mean and variability of PC1 amplitude were significantly lower upright (p=0.01, p=0.02), while PC2 (hysteresis) showed no significant postural difference. Jacobian determinant analysis of regional lung expansion and contraction showed considerable inter-individual variability, with no consistent postural trend across participants.
Outlook and Discussion: Diaphragmatic and internal vessel motion were significantly reduced and less variable upright, supporting more predictable respiratory motion in this posture. The opposing period trends in chest wall and diaphragm motion, however, indicate a posture-dependent shift in thoracoabdominal motion coordination that warrants further investigation. Future work should extend this single-slice pipeline to multi-slice or volumetric acquisition for true 3D, slice-matched comparisons in a larger cohort, and validate internal vessel motion as a tumour-motion surrogate in patients before these findings inform upright treatment planning.