Modeling the Varian On-Board Imager (OBI): Cone-beam CT (CBCT) operating modes

Cone-beam CT (CBCT) imaging is heavily being utilized in radiotherapy treatment as means of treating cancer patients. One of the platforms is the Varian On-Board Imager (oBI) where kilo-volts (kV) CBCT imaging is used. In this study, a model of the OBI is developed to simulate the two scan modes that are available, namely: Full-Fan (FF) and Half-Fan (HF) modes. By shifting the same set of 1024×768 detector panels laterally, a larger field-of-view (FOV) is achieved in the HF scan mode. Using a realistic XCAT phantom, the different FOVs are simulated using analytical Feldkamp-Davis-Kress (FDK) and iterative Simultaneous Algebraic Reconstruction Technique (SART) reconstruction methods. As suggested in literatures, ring and radiant artifacts can occur in the HF mode due to its geometry. To implement fast analytical method, an adequate weight factor needs to be applied on the projection data prior to reconstruction. The percentage normalized root mean squared error (NRMSE) value for FF and HF using analytical FDK reconstruction are 7.12% and 16.63% respectively. Using the iterative SART, the respective values are and 2.69% and 4.65%. The simulation model of the Varian OBI: CBCT operating mode is expected to encourage and enhance further studies on image guided radiation therapy (IGRT) during radiotherapy treatment delivery.