Minimization of over-ranging in helical volumetric CT via hybrid cone beam image reconstruction--benefits in dose efficiency

Diagnostic computed tomography (CT) images are usually acquired in both helical and axial scans in the clinical applications using cone beam volumetric CT. In addition to faster patient throughput, a helical scan in volumetric CT can provide better image quality because of the satisfaction of data sufficiency condition, and thus has been performed far more frequently so far in the clinic. However, the first and last images in a helical scan are usually prescribed at the locations that are half helical turn indented from the starting and ending points of the scan. Due to such an indention, the dose efficiency of helical scan deteriorates with increasing detector dimension along z direction. To improve the dose efficiency of helical scan in volumetric CT, a hybrid helical cone beam filtered backprojection (CB-FBP) algorithm is presented here to reconstruct helical images beyond the conventional indented image zone. The hybrid algorithm is a combination of the ray-wise three-dimensional (3D) weighted CB-FBP algorithms that are recently proposed for helical and axial CB image reconstructions. Through the hybridization, the ray-wise 3D weighting becomes dependent on both helical pitch and image slice location. Phantom study shows that the conventional indented image zone in helical scan can be extended substantially by using the hybrid algorithm. Consequently, the dose efficiency of volumetric CT in helical scan can be improved significantly. It is believed that, with increasing detector dimension along z direction in cone beam volumetric CT, the hybrid algorithm will become more attractive in clinical applications.