| Ultrasound-based navigation is an increasingly important technique in diagnostic and interventional medical procedures. The goals of these procedures are to image tissue targets within the body and, in interventions, to manipulate them. The navigation task is to produce coordinated motions of the imaging transducer and instruments to achieve these goals. While ultrasound imaging possesses the advantage of safety, portability, and high temporal resolution in comparison to other medical imaging modalities, a variety of challenges arise in its application to image-based navigation. Two primary challenges are due to ultrasound's limited field of view and to the distortions produced in imaging metal instruments. A limited field of view leads to disorientation, in which the clinician is unable to mentally orient the current image within the anatomy and is also unsure how to achieve desired instrument motions. The second challenge of distortions produced in imaging metal instruments makes it difficult for the clinician to know the instrument's precise location with respect to the tissue.; This dissertation provides solutions to each of these challenges. First, a system for overcoming the limited field of view is presented. The ultrasound transducer motion is registered to a 3-dimensional model of the patient's anatomy containing only features easily identifiable in the ultrasound image. This serves as a roadmap for navigation that is robust to large tissue deformations. The system is validated for laparoscopic ultrasound-based pancreatic cancer staging using the abdominal arteries for the anatomical roadmap. Second, a family of passive markers for improving instrument localization is described. Markers are attached to the cylindrical shafts of existing instruments and encode the instrument coordinate frame. They appear clearly despite the instrument distortions and require only simple image processing for measurement. Methods for minimizing measurement error are addressed theoretically and validated experimentally. Applications are presented in the context of beating heart intra-cardiac surgery, including graphical instrument overlays, real-time instrument tracking, and automatic instrument navigation. |
