Portal image registration using the phase correlation method

The precise patient positioning is a very important step in radiotherapy. The positioning of the patient for treatment with a LINAC is usually performed by aligning a radiograph to a reference image and the obtained alignment parameters are used to move the patient to the required position. To automate the positioning process, a fast and accurate image registration technique is usually required. In this work we investigate the performance of the phase correlation method (PCM) as a registration tool for Electronic Portal Imaging Device (EPID) images. We propose the usage of the PCM method for fast and accurate rigid registration of misaligned EPID images. To illustrate the concept, EPID images are obtained on a Siemens PRIMUS Linear Accelerator by irradiating two different phantoms - a standardised contrast and spatial resolution phantom (”Las Vegas“ phantom) and a realistic phantom of a human pelvis area. Each of the phantoms is placed at the patient table and several images at different table positions are acquired. The position of the table at each image acquisition provides information about the true displacements between the obtained images with a millimeter-level precision. In order to evaluate the accuracy of the PCM registration, the obtained translational parameters using the method are compared with the true displacements known from the table positions. The obtained results show that the method provides very accurate shift estimation with a millimeter-level precision. The method can find potential applications in the calibration and quality assurance procedures of this type of linear accelerator systems. The authors anticipate that the PCM can find application for patient positioning not only by registration of EPID images, but with images from other projection-based imaging modalities as well.