CBCT image reconstruction of a moving target with an on-board imaging system for radiation therapy

To accomplish accurate irradiation to a target, image information about the target is very useful in radiotherapy. A kilo-voltage cone-beam computed tomography (kV-CBCT) system mounted on a linear accelerator can verify the accuracy of the set-up position and the size and location of the target on each treatment day. However, the gantry of the CBCT system is heavy and so requires a long data acquisition time. As a result, image distortion occurs in the case of a moving target. In this study, we proposed a method to reconstruct a target less affected by any motion of the target, and evaluated the proposed method with a moving phantom. In our method, CBCT images of the moving phantom were acquired, and the target position in two directions was detected with a template matching method using these projection images. And we selected data acquisition angles of the projection images in which the target was located in predefined regions in the projection images, and made a sinogram with the projection image of selected angles. A less blurred image was reconstructed with a filtered-backprojection method from the sinogram. The results of experiments showed that the blurring caused by the movement of the phantom was reduced with our proposed method.