Extended-field-of-view three-dimensional transesophageal echocardiography using image-based X-ray probe tracking.

The use of ultrasound imaging for guidance of cardiac interventional procedures is limited by the small field of view of the ultrasound volume. A larger view can be created by image-based registration of several partially overlapping volumes, but automatic registration is likely to fail unless the registration is initialized close to the volumes' correct alignment. In this article, we use X-ray images to track a transesophageal ultrasound probe and thereby provide initial position information for the registration of the ultrasound volumes. The tracking is possible using multiple X-rays or just a single X-ray for each probe position. We test the method in a phantom experiment and find that with at least 50% overlap, 88% of volume pairs are correctly registered when tracked using three X-rays and 86% when using single X-rays. Excluding failed registrations with errors greater than 10 mm, the average registration accuracy is 2.92 mm between ultrasound volumes and 4.75 mm for locating an ultrasound volume in X-ray space. We conclude that the accuracy and robustness of the registrations are sufficient to provide useful images for interventional guidance.