WE-E-201B-06: Obtaining Increased Field-Of-View in Cone Beam Computed Tomography Using the Standard Centered Detector Geometry

Abstract

Purpose: We present a novel image acquisition and reconstruction scheme for dramatically increasing the field‐of‐view (FOV) in cone beam computed tomography(CBCT) using a standard centered detector. The method enables CBCT datasets to be employed as a basis for treatment planning, since the entire extent of even very large patients can be imaged in a quantitative sense. Applications include planning for emergency cases based on CBCTimages, and at Third World facilities that lack multislice‐CT systems using megavoltage‐CBCT. Methods and Materials: The projection acquisition scheme proceeds as follows: 1) The patient couch is offset (in height and lateral position) from its intended position ( x 0) at the start of treatment by a distance related to the amount of additional FOV required. 2) Projections are acquired over a limited arc. 3) The couch is offset from x 0 by the same distance as in (1), but in the opposite direction 4) Projections are acquired over a second arc. Projections are corrected for scatter, beam‐hardening and non‐uniform beam profile, and then simultaneously reconstructed using an iterative algorithm. Evaluation: We image a pelvic anthropomorphic phantom using a Siemens ONCOR linac equipped with an amorphous‐silicon detector. Dose calculation is then performed on the resulting extended FOV megavoltage‐CBCT and a multislice‐CT of the same phantom. Results: The full cross section of the phantom is reconstructed with only minor artifacts. The dose study shows almost identical dose distributions for the reference and EFOV dataset: the central percentage depth‐dose profiles for a 10×10 cm2 field exhibits a maximum deviation of 0.8%, illustrating the insignificance of the artifacts from a dosimetric perspective. Conclusion: The method increases the FOV of an ONCOR linac by 67% to 45 cm. This exceeds the capabilities of current extended FOV implementations that are based on offset‐detector systems. The reconstructed volumes appear suitable for planning purposes.