Frequency-combined extended 3D reconstruction for multiple circular cone-beam CT scans

In circular cone-beam CT a single circle scan often does not cover the complete z-range of interest. If this is the case, two or more circle scans are acquired. The standard combination of the separate reconstructions has two disadvantages: 1) The reconstructable volume is smaller than possible, thus dose remains unused and the noise level is higher than necessary. 2) The cone-beam artifacts are increased at the edges of the partial volumes which have a large distance to the midplanes. To overcome these disadvantages we developed a method that simultaneously reconstructs all circle scans and thereby is able to reconstruct larger segments from each circle scan on the one hand and that reduces cone-beam artifacts by combining the segments in frequency domain on the other hand. The proposed method was evaluated using a simulation study as well as measured data from different flat-panel cone-beam CT scanners, as for example the Varian OBI scanner. In the example geometry we used the maximal reconstructable z-range can be increased by 25% and our approach additionally leads to a noise reduction of about 40% in the overlap region. Regarding the cone-beam artifacts, we were able to reduce the artifact level to a value as low as achievable by more complex algorithms that perform a voxel-wise weighted backprojection to favor voxels seen under small cone-angles. This paper demonstrates that the proposed algorithm is able to significantly improve the reconstruction of sequence scans while the reconstruction time is kept equivalent to the standard approach. We further demonstrate that the method can be used in clinical practice.