O.19 - A new approach to expand the field of view for MV cone beam computed tomography

Abstract

Introduction: The field of view inMVCBCT is limited by the size of the electronic portal imaging device. This severely hampers the use of these images for planning purposes. The author proposes a novel new non-isocentric image acquisitionmethodology that by-passes this limitation. The basic idea has been patented by Siemens. Unfortunately the hardware was not able to support a full implementation of the technique resulting in inferior image quality. The original idea will be presented in this paper. Materials and Methods: Images are acquired while the gantry rotates around the patient; at the same time the couch moves in a coordinated fashion to describe an ellipse. In this way the edge of the patient’s body is included in every frame. Radio-opaque markers are placed on the patient to assist in correcting for any inaccuracies in the couch movement as well as patient motion during the acquisition. Results: A careful examination of the geometry of the images reveals that, shifting the center of the images and using a novel differential magnification algorithm, the set can be converted to the equivalent isocentric case. Since the position of each marker should fall on a simple sinus curve, small positional adjustments can be made to correct for the errors in the couch movement and even patient motion. The reconstruction of the image set can then proceed as normal and no complicated algorithms have to be invented. Conclusion: This approach can be implemented on a number of different linear accelerators andwould increase the FOV to the point where the images can be used for planning on virtually any body part.