Flat-panel cone-beam CT on a mobile isocentric C-arm for image-guided brachytherapy

Flat-panel imager (FPI) based cone-beam computed tomography (CBCT) is a strong candidate technology for intraoperative imaging in image-guided procedures such as brachytherapy. The soft-tissue imaging performance and potential navigational utility have been investigated using a computer-controlled benchtop system. These early results have driven the development of an isocentric C-arm for intraoperative FPI-CBCT, capable of collecting 94 projections over 180 degrees in 110 seconds. The C-arm system employs a large-area FPI (41 × 41 cm²) with 400 micron pixel pitch and Gd₂O₂S:Tb scintillator. Image acquisition, processing and reconstruction are orchestrated under a single Windows-based application. Reconstruction is performed by a modified Feldkamp algorithm implemented on a high-speed reconstruction engine (CBR-1500-2, Terarecon, Inc. San Mateo, CA). Non-idealities in the source and detector trajectories during orbital motion has been quantified and tested for stability. Cone-beam CT imaging performance was tested through both quantitative and qualitative methods. The system MTF was measured using a wire phantom and demonstrated frequency pass out to 0.6 mm^-1. Voxel noise was measured at 2.7% in a uniform 12 cm diameter water bath. Anatomical phantoms were employed for qualitative evaluation of the imaging performance. Images of an anesthesized rabbit demonstrated the capacity of the system to discern soft-tissue structures within a living subject while offering sub-millimeter spatial resolution. The dose delivered in each of the imaging procedures was estimated from in-air exposure measurements to be ∼ 0.1 cGy (center of 12 cm water cylinder). Imaging studies of an anthropomorphic prostate phantom were performed with and without radioactive seeds (∼80 Pd-103 sources). Soft-tissue imaging performance and seed detection appear to satisfy the imaging and navigation requirements for image-guided brachytherapy. These investigations advance the development and evaluation of such technology for image-guided surgical procedures, including brachytherapy, vertebroplasty and neurosurgery. The demonstrated soft-tissue visibility, excellent spatial resolution, low imaging dose, and convenient form factor make C-arm based cone-beam CT a powerful new technology for image-guidance applications.