Simulating Dynamic Ultrasound Using MR-derived Motion Models to Assess Respiratory Synchronisation for Image-Guided Liver Interventions

Tracked intra-operative ultrasound can be registered to real-time synthetic ultrasound derived from a motion model to align pre-operative images with a patient's anatomy during an intervention. Furthermore, synchronisation of the motion model with the patient's breathing can be achieved by comparing diaphragm motion obtained from the tracked ultrasound, with that obtained from the synthetic ultrasound. The purpose of this study was to assess the effects of spatial misalignment between the tracked and synthetic ultrasound images on synchronisation accuracy. Deformable image registration of 4-D volunteer MR data was used to build realistic subject-specific liver motion models. Displacements predicted by the motion model were applied to acoustic parameter maps obtained from segmented breath-hold MR volumes, and dynamic B-mode ultrasound images were simulated using a fast ultrasound propagation method. To prevent synchronisation errors due to breathing variations between motion model acquisition and interventional ultrasound imaging from influencing the results, we simulated both the synthetic and the tracked ultrasound using a single motion model. Spatial misalignments of up to ±2 cm between the tracked and synthetic ultrasound resulted in a maximum motion model breathing phase error of approx. 3%, indicating that respiratory synchronisation of a motion model using tracked ultrasound is relatively insensitive to spatial misalignments.