Characterisation of inter- and intra-subject variation of internal-external respiratory motion correspondence

It is well known that respiratory motion affects image acquisition and also external beam radiotherapy (EBRT) treatment planning and delivery. However often the existing approaches for respiratory motion management are based on a generic view of respiratory motion such as the general movement of organ, tissue or fiducials. This paper thus aims to present a more in depth analysis of respiratory motion based on 4D MRI for further integration into motion correction in image acquisition or image based EBRT. The focus is on the correspondence of internal and external respiratory motion across multiple volunteers, with the aim of possible use in deducing internal motion from a markerless external surrogate, such as that captured from distance cameras that are currently becoming more available. The phenomenon of hysteresis is evident even after dimensionality reduction of the high dimensional motion data, in line with previous observations in literature. With a simple manipulation of the dimensionality, it is demonstrated how hysteresis can be handled and taken advantage of in motion compensation approaches.