Characterization of motion of the LINAC flat-panel detector
Radiation oncology is a common method for cancer treatment that uses ionizing radiation such as a megavoltage (MV) X-ray beam from a linear accelerator (linac) to destroy tumor tisue. The linac system incorporates a treatment head – from which the beam is delivered – and an electronic portal imaging device (EPID) – for imaging and dosimetry during treatment – mounted opposite to the treatment head on a gantry that can rotate 3600around a patient. The EPID, an MV detector panel, uses the exit
... l, uses the exit beam from the treatment head to construct a radiographic image. Due to the construction of the arm, the detector panel typically experiences sagging and other displacements due to gravity, which may affect the geometric accuracy of acquired images. In this thesis the displacement of the EPID of an Elekta Versa HD linac was quantified. An NDI Polaris Spectra optical tracking system was used in conjunction with 3D Slicer, a software designed for the visualization and analysis of medical images, to precisely track and measure the position of the detector panel at a number of gantry angles from 00 to +1800. Sagging was observed, which increased as the height of the detector panel increased with its gantry angle. At a gantry angle of 00, wherein the EPID is at the bottom, a downward displacement of 2.31 mm from the reference position was observed while at a gantry angle of 1800 this displacement was 19.51 mm. Longitudinal and lateral tilting were also observed which further contributed to displacement, though these tilts were found to be <10 for all gantry angles. Additionally, the displacements caused by the vibration of the detector panel after rotating into place were measured. These displacements were minor compared to the displacement caused by sagging, reaching a maximum ±0.5 mm and falling to within ±0.1 mm within 2.4 seconds. The data from this study will be used in future works to develop post-processing algorithms to compensate for this detector displacement to obtain more accurate images.