Voxel-based reconstruction combined with motion detection for slow rotating 4D CBCT

Flat panel detector (FPD) cone-beam computed tomography (CBCT) systems, such as C-arm CT scanners or onboard imaging systems, rotate far slower than a typical motion cycle of the heart or lung. Therefore 4D CBCT is more complicated with flat panel detectors than with clinical CT. Recent approaches for 4D imaging from FPD CBCT either use multiple scans over the same angular range or a single slow rotation, and they perform a motion phase-dependent weighting of complete projections (e.g. using a respiratory monitor). This leads to a relatively high temporal resolution but also to high image noise and in the second approach to artifacts due to sparse angular sampling. Our proposed method also uses the data from several scans without causing streak artifacts. But instead of weighting the whole projections the weighting is applied to the motion affected areas in the projection only. These regions are automatically estimated using motion detection techniques between consecutive projections. Thus, projection data are used in our algorithm that are completely ignored in conventional approaches. To evaluate our method simulated data of a breathing thorax phantom and measurements acquired with the OBI scanner (Varian Medical Systems, Palo Alto, CA) were reconstructed. Image quality was compared with the projection-weighting approach for whole projections and standard reconstructions without phase-correlation.