Selective X-Ray Reconstruction and Registration for Pose Estimation in 6 Degrees of Freedom

Particle beams in radiological cancer treatment provide high accuracy in dose delivery. Thus approaches from image-guided radiotherapy (IGRT) are used to overcome accuracy limitations caused by the patient misalignment in the treatment device. By comparing stereoscopic X-ray images of the patient in treatment position to a reference Computed Tomography (CT) scan, a correction of the initial patient set-up can be computed. Automatic registration of the X-ray images with digital reconstructed radiographs (DDRs) from the CT and back-projection of the transformation gives a pose correction in 5 degrees of freedom (DOF). To obtain a 6 DOF correction, DDRs have to be generated for a large amount of hypothetical alignments to find the optimal match the X-ray images. To accelerate this time consuming process and to reduce the disturbing influence of image contents that do not match correctly, we automatically exclude regions that may not improve the resulting pose correction from the rendering as well as from the matching process. Therefore these regions are identified in the X-ray images and transferred into the plane of the respective DDR. We then perform the radon transform for DDR generation only for a subset of possible pixel values and exclude the missing information from the registration process. As a result of this approach, the time needed for a full automatic pose correction computation in 6 DOF is decreased by means of 4 and more and additionally misregistrations caused by unsuitable image contents can be avoided.