Automated Registration of Ultrasound with CT Images: Application to Computer Assisted Prostate Radiotherapy and Orthopedics Gelu Ionescu, PhD., St´ ephane Lavall´ ee, PhD., Jacques Demongeot, MD., PhD. TIMC Laboratory, University Joseph Fourier, Grenoble France gelu.ionescu@imag.fr Abstract. Ultrasound (US) imaging could potentially play a major role in the field of Computer Assisted Surgery (CAS). For doctors and sur- geons to make full use of CAS tools in planning and executing surgical operations, they also need user-friendly automatic software based on fast, precise and reliable registration algorithms. The main goal of this paper is to take advantage of the segmentation/registration duality to extract the relevant information from US images. This information will allow the precise and automatic registration of anatomical structures contained in the pre-operative model and of intra-operative data contained in US im- ages. The result of registration will be further used to guide a computer- assisted intervention such as orthopedics or radiotherapy. 1 Introduction Among the various imaging techniques available, 2D echography is becoming more and more important, both for the diagnostic purposes and registration in CAS applications. In this paper, we will consider the latter case, with the specific instance of Computer Assisted Orthopedics Applications [1,2] and pa- tient positioning in prostate radiotherapy [3]. In spite of the problems inherent to US imaging such as low signal-to-noise ratio and the fact that the images depend on the angle and the texture of the target anatomy, this modality has become very popular among practitioners mainly because of its safety, low cost and non-invasiveness. Since external radiotherapy makes use of the properties of X - rays to de- stroy live tissue, the precise localization of the tumor is extremely important in order to avoid the destruction of neighboring healthy tissues. As described in details in [4], in the context of isocentric technique applied to radiotherapy, the procedure for repositioning must ensure that the center of the tumor localized by the practitioner in the pre-operative model, coincides with the center of ro- tation of the radiation device. For the intra-operative data acquisition, we are interested particularly in the transpubic way to realize the US examination of the prostate. The goal of the registration is to realize an automatic orientation of the external X - ray beams according to a predetermined dosimetry plan. The results presented here were obtained on patients. In orthopedic applications, a C. Taylor, A. Colchester (Eds.): MICCAI’99, LNCS 1679, pp. 768–778, 1999. c  Springer-Verlag Berlin Heidelberg 1999