Hybrid helical image reconstruction in volumetric CT using ray-wise weighted cone beam filtered backprojection algorithm to significantly improve dose efficiency

In clinical applications, CT images are acquired in both helical and axial scans. In general, a helical scan can provide better image quality and faster patient throughput, and thus is performed more frequently in 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. Hence, due to image location indention, the dose efficiency of helical scanning is usually not as good as that of axial scan in which no image location indention occurs. With increasing detector dimension along z-direction, the dose efficiency difference between helical and axial scans is becoming more significant, particularly at relatively small helical pitches. To match the dose efficiency of helical scan with axial scan as much as possible, a hybrid helical CB-FBP algorithm is presented to reconstruct helical images beyond the conventional indented image zone. The hybrid algorithm is actually a combination of the ray-wise 3D weighted CB-FBP algorithms that are previously proposed for helical and axial CB image reconstructions, in which the ray-wise 3D weighting becomes dependent on both helical pitch and image plane location. Phantom study shows that the conventional indented image zone in helical scan can be extended substantially by using the presented algorithm. Consequently, the dose efficiency in volumetric CT in helical scan can be improved significantly, especially in clinical applications where the helical pitch is relatively low. With increasing dose discrepancy between helical and axial scans, the presented algorithm will become more attractive in clinical applications.