  This paper presents a method to register a preoperative CT volume to a sparse
set of intraoperative US slices. In the context of percutaneous renal puncture,
the aim is to transfer a planning information to an intraoperative coordinate
system. The spatial position of the US slices is measured by localizing a
calibrated probe. Our method consists in optimizing a rigid 6 degree of freedom
(DOF) transform by evaluating at each step the similarity between the set of US
images and the CT volume. The images have been preprocessed in order to
increase the relationship between CT and US pixels. Correlation Ratio turned
out to be the most accurate and appropriate similarity measure to be used in a
Powell-Brent minimization scheme. Results are compared to a standard rigid
point-to-point registration involving segmentation, and discussed.
