  For augmented fluoroscopy during cardiac catheter ablation procedures, a
preoperatively acquired 3-D model of the left atrium of the patient can be
registered to X-ray images. Therefore the 3D-model is matched with the contrast
agent based appearance of the left atrium. Commonly, only small amounts of
contrast agent (CA) are used to locate the left atrium. This is why we focus on
robust registration methods that work also if the structure of interest is only
partially contrasted. In particular, we propose two similarity measures for
CA-based registration: The first similarity measure, explicit apparent edges,
focuses on edges of the patient anatomy made visible by contrast agent and can
be computed quickly on the GPU. The second novel similarity measure computes a
contrast agent distribution estimate (CADE) inside the 3-D model and rates its
consistency with the CA seen in biplane fluoroscopic images. As the CADE
computation involves a reconstruction of CA in 3-D using the CA within the
fluoroscopic images, it is slower. Using a combination of both methods, our
evaluation on 11 well-contrasted clinical datasets yielded an error of
7.9+/-6.3 mm over all frames. For 10 datasets with little CA, we obtained an
error of 8.8+/-6.7 mm. Our new methods outperform a registration based on the
projected shadow significantly (p<0.05).
