TU‐EE‐A3‐03: Performance of a Bench‐Top, Megavoltage CT (MVCT) Scanner Using Cadmium Tungstate‐Photodiodes

Abstract

Purpose: To evaluate the imaging performance of a prototype fan‐beam megavoltage CT (MVCT) scanner in and 6 MV beams. Method and Materials: The 80‐element detector is fabricated by tiling 8‐element (element size 0.275 × 0.8 × 1 cm3) and photodiode arrays and arranging them on an arc (radius = 110 cm). A precision rotary stage and its control are added to create a third generation CT scanner. The attenuation of and 6 MV beams was measured as a function of solid water thickness, and fit to a second order polynomial to correct for spectral hardening artifacts. A calibration procedure was established to remove ring artifacts caused by the distinctly asymmetric line spread functions at the ends of 8‐element blocks. The low contrast resolution (LCR) as a function of dose and object size, the signal to noise ratio (SNR) as a function of dose, and the linearity of CT numbers with density were quantified. Results: Throwing away one‐ninth of collected projection angles to reduce the dose per image adversely affects the resolution in 6 MV images; however, 15 mm targets at 1.5% level are still visible at 7 cGy. The low contrast target of 1.5% at 6 mm diameter is visible in images at 2cGy. The LCR in the objects stays approximately constant with the dose reduced from 17 to 2 cGy. In general, the contrast decreases as the target diameter decreases. The SNR2 obtained from a uniform phantom increases linearly with dose (R2=0.9977). CT numbers as a function of the density show a linear trend (R2= 0.9923). Conclusion: The prototype detector performance is satisfactory to achieve the ultimate goal of this project of creating a focused 2‐D MV detector with high detective quantum efficiency such that reasonable LCR at low dose can be obtained in MVCT.