A Learning-Based Patient Repositioning Method from Limited-Angle Projections

Abstract

This paper presents a novel patient repositioning method from limitedangle tomographic projections. It uses a machine learning strategy. Given a single planning CT image (3D) of a patient, one applies patient-specific training. Using the training results, the planning CT image, and the raw image projections collected at the treatment time, our method yields the difference between the patient’s treatmenttime postition and orientation and the planning-time position and orientation. In the training, one simulates credible treatment-time movements for the patient, and by regression it formulates a multiscale model that expresses the relationship giving the patient’s movements as a function of the corresponding changes in the tomographic projections. When the patient’s real-time projection images are acquired at treatment time, their differences from corresponding projections of the planning-time CT followed by applications of the calculated model allows the patient’s movements to be estimated. Using that estimation, the treatment-time 3D image can be estimated by transforming the planning CT image with the estimated movements,and from this, changes in the tomographic projections between those computed from the transformed CT and the real-time projection images can be calculated. The iterative, multiscale application of these steps converges to the repositioning movements. By this means, this method can overcome the deficiencies in limited-angle tomosynthesis and thus assist the clinician performing an image-guided treatment. We demonstrate the method’s success in capturing patients’ rigid motions with subvoxel accuracy with noise-added projection images of head and neck CTs.