Application of asymmetric cone-beam CT in radiotherapy

In many implementations of cone-beam CT in radiotherapy for target positioning, it is not uncommon that the maximum allowable field of view (FOV) cannot cover the patient because of the limited size of the flat-panel detector. In this situation, the measurements will contain truncated projections, leading to significant artifacts in reconstructed images. Asymmetric cone-beam configurations can be used for increasing the FOV size by displacing the detector panel to one side. From the data acquired with such an asymmetric configuration, the well-known algorithm developed by Feldkamp, Davis, and Kress (FDK) can be modified to reconstruct images. With increasing detector asymmetry, however, the modified FDK algorithm may produce significant aliasing artifacts. In this work, we develop a novel algorithm for image reconstruction in asymmetric cone-beam CT, which can generate images with improved numerical properties and allow for large detector asymmetry. We have employed the asymmetric configuration and the developed algorithm in a cone-beam CT system in radiotherapy to increase the FOV size. Preliminary phantom studies have been conducted to validate the asymmetric configuration and the proposed reconstruction algorithm.