Cone-Beam Computed Tomography (CT) for a Megavoltage Linear Accelerator (LINAC) Using an Electronic Portal Imaging Device (EPID) and the Algebraic Reconstruction Technique (ART)

This study investigates the feasibility and applicability of cone beam CT for a megavoltage therapeutic LINAC. A Rando head phantom was irradiated using the 15 MV beam of a Varian 2100C LINAC at MSKCC for gantry angles from -102° to 102° with a 2° increment. The projection image for each gantry angle was obtained using a Varian Mark II EPID. Reference images without phantom were also collected at different angles. Pixel readings of each image were converted to dose rate using the EPID calibration curve. The ray sum (sum of linear attenuation coefficients along the ray from the source to a pixel) is calculated as the negative logarithm of the ratio of dose rate of that pixel to that of the corresponding pixel in the reference image. The ray sums were then used for volumetric reconstruction using ART. ART is an iterative method that solves a system of linear equations by iteratively updating the volume to reduce the errors between the measured and calculated ray sums. Our results indicate that reasonably good tomographic images can be obtained using projections at every 8°, even after only one iteration (typically, 2-3 iterations are used.) The image quality depends on the number of projections and the number of iterations. The reconstruction can be achieved within a reasonable time (~2 hours for pure software and ~5 min with the help of graphics hardware). We thus conclude that cone beam CT for megavoltage therapeutic LINAC has a potential to obtain useful tomographic images.