  The emergence of real-time 3D ultrasound (US) makes it possible to consider
image-based tracking of subcutaneous soft tissue targets for computer guided
diagnosis and therapy. We propose a 3D transrectal US based tracking system for
precise prostate biopsy sample localisation. The aim is to improve sample
distribution, to enable targeting of unsampled regions for repeated biopsies,
and to make post-interventional quality controls possible. Since the patient is
not immobilized, since the prostate is mobile and due to the fact that probe
movements are only constrained by the rectum during biopsy acquisition, the
tracking system must be able to estimate rigid transformations that are beyond
the capture range of common image similarity measures. We propose a fast and
robust multi-resolution attribute-vector registration approach that combines
global and local optimization methods to solve this problem. Global
optimization is performed on a probe movement model that reduces the
dimensionality of the search space and thus renders optimization efficient. The
method was tested on 237 prostate volumes acquired from 14 different patients
for 3D to 3D and 3D to orthogonal 2D slices registration. The 3D-3D version of
the algorithm converged correctly in 96.7% of all cases in 6.5s with an
accuracy of 1.41mm (r.m.s.) and 3.84mm (max). The 3D to slices method yielded a
success rate of 88.9% in 2.3s with an accuracy of 1.37mm (r.m.s.) and 4.3mm
(max).
