A hybrid method for registration of interventional CT and ultrasound images

Abstract

An experimental workstation is presented that allows the overlay of a live ultrasound image with the corresponding slice from preoperative CT on a single screen. Also a three-dimensional view of the organ shapes segmented from the CT image is provided, in which localized instruments and the life ultrasound image from the also localized transducer are placed. For the registration of preoperative CT images of an abdominal phantom with live ultrasound images, external registration is performed with an optical position measurement system. In addition, image-based methods for rigid registration are tested and compared to manual registration. In a novel approach, the segmentation of organ shapes from the CT image and the estimated transducer position are used to automatically preprocess the CT slice to appear more similar to an ultrasound image before a standard intensity-based similarity measure is applied. The method masks-out the bones in the slice and the shadowed region behind them. According to the estimated angle of intersection on the bone surface also the echo of the proximal bone surface is simulated during registration. This masking and preprocessing step proves to significantly improve the registration accuracy in experiments reaching an accuracy of 3.4 mm.