Projection-based deformable registration for tomographic imaging in ion beam therapy

The purpose of the study is to investigate a projection-based deformable registration method between the treatment planning CT and in-room ion-based tomographic imaging (iCT). The rationale of the study is to make use of iCT projections for treatment geometry verification and treatment planning CT image update. The hypothesis to be investigated is the possibility to register the treatment planning CT and the iCT images in the domain of projections, thus postponing iCT image reconstruction to registration. The iCT-CT image fusion in projection domain is expected to join the high spatial resolution of treatment planning CT images with the accurate tissue stopping power information provided by the ion beam projections. The strategy is tested on the anthropomorphic 4D CT NURBS-based cardiac-torso (NCAT) phantom, featuring the respiration induced organ motion. Specific features of the iCT imaging are also mimicked by applying piecewise linear gray level modification and ion (proton) scattering. Different imaging angular ranges (180°-135°-90°-45°) and number of projection views (180-135-90) are simulated. In order to assess the algorithm performance on measured carbon iCT data of the Alderson head phantom, the phantom CT image is rigidly transformed. The strategy suggests potential advantages for those in-beam/in-room imaging techniques where a complete 360° angle acquisition could be geometrically hampered but also when a limited number of projection angles are desirable for reduced dose or acquisition time.