Multi-resolution extended-volume model for iterative reconstruction in cone beam CT

Cone beam computed tomography (CBCT) often has a truncated acquired field of view (FOV) due to the limited detector size, leading to image reconstruction from truncated projection data. CBCT reconstructions using an image volume that just encloses the acquired FOV exhibit reconstruction artifacts due to attenuation in the tissues outside the image volume. On the other hand, extending the high-resolution voxel volume far enough beyond the FOV to fully enclose the imaged body often leads to a significant increase of the computational complexity in model based iterative reconstruction techniques. We propose a multi-resolution reconstruction model that eliminates the out-of-FOV reconstruction artifacts and enables accurate recovery of Hounsfield Unit (HU) values within the FOV. Approach: We propose a multi-resolution extended reconstruction volume (MR-ERV) approach that extends the image volume beyond the FOV using separate extension volumes with coarser voxel representation, leading to appropriate modeling of the observed rays outside the FOV without significant increase of the computational complexity. Furthermore, we demonstrate that by augmenting the model with a simple projection extrapolation yields a further reduction of the out-of-FOV artifacts. In this study, the model is evaluated with model based iterative reconstruction minimization using high-resolution 3D CBCT data. The optimization problems considered are non-negativity constrained least-squares estimation, with and without regularization. The optimization is performed using a Primal-Dual Hybrid Gradient (PDHG) algorithm. Results: The proposed MR-ERV model effectively removes out-of-FOV reconstruction artifacts and it also achieves accurate HU values within the FOV when the volume extension fully encloses the imaged body in the transaxial direction. Significance: The MR-ERV model provides a platform for computationally efficient and accurate model based iterative reconstruction of CBCT data.