Quantitative analysis of the image quality in megavoltage cone-beam computed tomography

Objective: To quantitatively analyze the image quality of megavoltage cone-beam CT (MVCBCT) under different scanning conditions to provide reference in clinical applications. Methods: Siemens ONCOR linear accelerator with MVCBCT was used to scan the phantom under different conditions. The image quality was evaluated in terms of image noise, uniformity, spatial resolution, contrast resolution, the number of Monitor Units(MUs) used in imaging, and the size of the reconstruction matrix. The comparison of the image quality between MVCBCT and conventional simulator CT was also analyzed. Results: The image noise was decreased with the increase of the number of MUs. The uniformity index showed that the system uniformity was weakly dependent on MU numbers or the size of the reconstruction matrix. Except for the images with 5 MUs, all other images had the spatial resolution of 0.4 1p/mm with a reconstruction matrix of 256 x 256. Better low contrast resolution was achieved by using more MUs. For typical pelvis and head-and-neck patients, the imaging dose at the center was 0.8 cGy/MU and 0.7 cGy/MU, respectively, and the maximum dose was about 1.2 cGy/MU. For typical abdomen patients, the image maximum dose and center dose was 1.3 cGy/MU and 0.7 cGy/MU, respectively. Conclusions: The image quality of MVCBCT is inferior to the conventional kilo-voltage CT. However, with the optimization of the parameters in imaging, we can achieve sufficient image contrast in the bone, air and some soft-tissue structures with low imaging dose to patients. Such images can be used for IGRT. (authors)