Intraoperative cone-beam CT for guidance of head and neck surgery: Assessment of dose and image quality using a C-arm prototype

Cone-beam computed tomography (CBCT) with a flat-panel detector represents a promising modality for intraoperative imaging in interventional procedures, demonstrating sub-mm three-dimensional (3D) spatial resolution and soft-tissue visibility. Measurements of patient dose and in-room exposure for CBCT-guided head and neck surgery are reported, and the 3D imaging performance as a function of dose and other acquisition/reconstruction parameters is investigated. Measurements were performed on a mobile isocentric C-arm (Siemens PowerMobil) modified in collaboration with Siemens Medical Solutions (Erlangen, Germany) to provide flat-panel CBCT. Imaging dose was measured in a custom-built cylindrical head phantom at four positions (isocenter, anterior, posterior, and lateral) as a function of kVp and C-arm trajectory ("tube-under" and "tube-over" half-rotation orbits). At , for example ("tube-under" orbit), the imaging dose was 0.059 (isocenter), 0.022 (anterior), 0.10 (posterior), and 0.056 (lateral) mGy/mAs, with scans at and typical for visualization of bony and soft-tissue structures, respectively. Dose to radiosensitive structures (viz., the eyes and thyroid) were considered in particular: significant dose sparing to the eyes (a factor of 5) was achieved using a "tube-under" (rather than "tube-over") half-rotation orbit; a thyroid shield ( Pb-equivalent) gave moderate reduction in thyroid dose due to x-ray scatter outside the primary field of view. In-room exposure was measured at positions around the operating table and up to from isocenter. A typical CBCT scan ( to isocenter) gave in-air exposure ranging from at from isocenter, to at from isocenter. Three-dimensional (3D) image quality was assessed in CBCT reconstructions of an anthropomorphic head phantom containing contrast-detail spheres and a natural human skeleton. The contrast-to-noise ratio (CNR) was evaluated across a broad range of dose . CNR increased as the square root of dose, with excellent visualization of bony and soft-tissue structures achieved at and , respectively. The prototype C-arm demonstrates CBCT image quality sufficient for guidance of head and neck procedures based on soft-tissue and bony anatomy at dose levels low enough for repeat intraoperative imaging, with total dose over the course of the procedure comparable to or less than the effective dose of a typical diagnostic CT of the head.