Task 2.5, 2D-3D Registration and Reconstruction, was initiated in Year 3 in response to clinician and industrial interest in the use of C-arms in surgery. The technical goal is to reconstruct a patient-specific 3D model of anatomy, during therapy, from 2D x-ray projections. The core research involves computed reconstruction, 2D-3D registration, and statistical atlases. The clinical opportunities are numerous, including pelvic surgery and mega-voltage radiosurgery. Three investigators (Wells, Taylor, Prince) have partnered in collaborative research on mutual information methods for 2D-3D registration, tetrahedral statistical atlases, and cone-beam reconstruction algorithms. Within the last year, we used preliminary results for papers, for a (pending) NIH R21 grant, and in an invention disclosure (with industrial partner Siemens Corporation). In addition, algorithm developments arising from core ERC funding are enabling several types of systems which are being developed in collaboration with various partners. With Siemens Corporation, we are working on the integration of prior CT data with partial information obtain from C-arms. With NIH funding we will be working with clinicians in Orthopedic Surgery on a C-arm system for visualization during total hip replacement and hip fracture reduction surgeries. And, we are collaborating with clinicians in Radiation Oncology on research on both fractionated stereotactic spinal radiosurgery and external beam radiation therapy for soft tissue localization. Next year, we will continue to develop and refine our core methods, improving their performance within the specific targeted applications, while working on enhancements that will generalize the methods for more extensive application.