Author: Michael W. Kissick, Chung Tin Lo, Jamie R McClelland, Niek Schreuder, Tracy Underwood 👨🔬
Affiliation: Hawkes Institute, University College London, Leo Cancer Care 🌍
Purpose: The emerging field of upright radiotherapy raises the question of how respiratory motion differs between upright and supine treatment postures. Therefore, we have developed a versatile pipeline for analysing respiratory motion from 2D lung cine MRIs.
Methods: The pipeline processes a series of cine MRIs capturing a single sagittal slice through the lung over multiple breathing cycles. The first end-exhale image is used as the reference image, and the lung is segmented in this image. A 2D diffeomorphic DIR algorithm (implemented in NiftyReg) is used to register the reference image to every other image in the series, providing both the forward and inverse transformations between the reference image and all others.
These transformations are used to: propagate the segmentations to all images, enabling the lung area to be calculated for each time point; track the motion of multiple points on the diaphragm, chest wall, and internal lung structures; and quantify the local expansion/contraction of each lung pixel at each time point from the Jacobian determinant.
Analysing the magnitude and reproducibility of the different measurements, and the relationships between them, provides a detailed characterisation of the breathing motion.
Results: A proof of principle experiment was conducted using two series of cine MRIs, one acquired in supine position and the other upright, from one healthy participant on a FONAR multi-position 0.6 T MRI scanner. The pipeline successfully characterised the motion from both series, identifying several potentially clinically relevant ways in which the motion differed between supine and upright postures.
Conclusion: Our pipeline enables a detailed and clinically relevant analysis of breathing motion from cine MRIs with minimal manual user input. This can enable in-depth and large-scale studies of how the motion varies between individuals and between different body positions.