Overview of an ultrasound to CT or MR registration system for use in thermal ablation of liver metastases

We present a method to register a pre-operative 3D CT or MR volume to a set of intra-operative tracked ultrasound images. Our aim is to allow the transfer of information from pre-operative modalities to ultrasound images to aid needle placement during thermal ablation of liver metastases. Spatial correspondence between ultrasound slices is achieved by tracking the probe using a Polaris optical tracking system. Registration is achieved using a modified version of the iterative closest point algorithm. The features on which the registration is based are a combination of the liver vessels and the liver surface. Problems arising due to liver motion have been reduced by automatically selecting ultrasound slices acquired at maximum exhalation. Registrations have been carried out between MR and ultrasound images acquired from a volunteer. Registration evaluation was performed using three methods. Initially simulated data was used, then a consistency measure was used to compare two independent registration results and finally the registration results were compared to manually performed registrations. These preliminary results show an overall mean target registration error of 10mm which is sufficiently accurate for treatment of large lesions (>50mm in diameter) in our intended clinical application.