3D Ultrasound--CT Alignment of the Liver using Combined Landmark--Intensity Registration

Purpose
An important issue in computer-assisted surgery
of the liver is a fast and reliable transfer of preoperative
resection plans to the intraoperative situation. One problem
is to match the planning data, derived from preoperative
CT or MR images, with 3D ultrasound images of the liver,
acquired during surgery. As the liver deforms significantly
in the intraoperative situation non-rigid registration is neces-
sary. This is a particularly challenging task because pre- and
intraoperative image data stem from different modalities and
ultrasound images are generally very noisy.

Methods
One way to overcome these problems is to
incorporate prior knowledge into the registration process.
Conclusion The proposed algorithm offers the possibility to
incorporate additional a priori knowledge—in terms of few
landmarks—provided by a human expert into a non-rigid
registration process.
We propose a method of combining anatomical landmark
information with a fast non-parametric intensity registration
approach. Mathematically, this leads to a constrained optimi-
zation problem. As distance measure we use the normalized
gradient field which allows for multimodal image registra-
tion.

Results
A qualitative and quantitative validation on clinical
liver data sets of three different patients has been perfor-
med. We used the distance of dense corresponding points on
vessel center lines for quantitative validation. The combined
landmark and intensity approach improves the mean and per-
centage of point distances above 3 mm compared to rigid and
thin-plate spline registration based only on landmarks.

Conclusion
The proposed algorithm offers the possibility to
incorporate additional a priori knowledge—in terms of few
landmarks—provided by a human expert into a non-rigid
registration process.