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Respiratory motion management (RMM) is recommended for tumours subjected to excursions of more than 5 mm. Prolonged breath-holds (PBH) of >5 minutes can be achieved by mechanical ventilation induced hypocapnia after preoxygenation. During these prolonged breath-holds, the lungs gradually deflate causing the right diaphragm to drift cranially by approximately 3.0 mm/min. Here, we studied the correlation between the diaphragm drift and abdominal organ motion using deformable image registration (DIR) during prolonged breath-holds. Furthermore, we studied the correlation between the right diaphragm top and abdominal organ motion, to investigate its applicability as respiratory motion surrogate.

Seven volunteers performed a total of eleven PBHs from end-inspiration (four volunteers, two PBHs each; three volunteers, one PBH each). 3D cine-MRIs of the upper abdomen were acquired with individual 3D image acquisition duration of 14 s. Seven images (the first, then at 25%, 50% and 75% of the PBH duration and the last three) were registered using groupwise DIR resulting in a Deformation Vector Field (DVF) from the first to every other image (example DVF in Figure 1A). Regions-of-interest (ROI) were defined by delineating the right and left diaphragm, the right diaphragm top (RDT), liver, spleen, and both kidneys (Figure 2B and 2C) on the first image. The distribution of voxel displacements from the first to all other images for each ROI was computed for the 3-dimensional vector magnitude of the motion resulting in a Motion Volume Histogram (MVH; Figure 1B). Modal displacements were extracted from these MVHs, serving as a measure for 3D organ displacement. Correlations between organ displacements were calculated for each individual PBH for pairs of organs using a linear regression model (Figure 1C). Median R² , slope values and relative inter-quartile ranges (IQR), as percentages of the median were determined over the eleven PBHs. For each set of organs compared, a Wilcoxon signed rank test was used to determine if the slope was significantly different from 1. 

Median PBH duration was 315s (IQR 165s). Abdominal organ motion showed to be highly correlated to RDT drift within individual PBHs (R²  >0.98) (Figure 2A). Corresponding IQR values were low (all <5%) displaying small variation over all PBHs. Median values for the slope were not significantly different from 1, providing no indication that the diaphragm moves significantly more than the abdominal organs. High IQRs were found for the slope for numerous organ comparisons (>45% for 8 of the 11 organ sets) indicating that the relation between RDT motion and organ motion varies greatly between the different PBHs. 

Abdominal organ motion during a single PBH is highly correlated to the diaphragm and right diaphragm top. The high IQR values of the slope suggest that the relation between RDT motion and abdominal organ motion varies between subjects and within subjects.