Improved Resolution for PET Volume Imaging through Three-Dimensional Iterative Reconstruction

It has been demonstrated that in two-dimensional iterative reconstruction, a resolution model can improve image resolution while controlling noise. With the lower noise levels of three-dimensional PET volume imaging, these iterative reconstruction advantages may be extended to three dimensions to further improve the reconstructed image resolution. We have implemented three-dimensional versions of iterative filtered backprojection (IFBP) and the maximum likelihood by expectation maximization (ML-EM) reconstruction algorithms and applied them to three-dimensional PET volume datasets. The results were compared to images obtained using the standard three-dimensional reprojection reconstruction (3DRP) algorithm. For IFBP with 15 iterations and no regularization compared to 3DRP, both using a ramp filter, the transaxial resolution improved 52%, and the axial resolution improved 39%. With a strong regularization, the transaxial and axial resolution improvements were reduced to 6% and 5%, respectively. If a Hanning roll-off is applied to the ramp filter in the transaxial direction, the transaxial resolution for IFBP without regularization improved 35% compared to 3DRP; with regularization the improvement dropped to 19%. The axial resolution for IFBP and 3DRP was unaffected by this transaxial smoothing in the reconstruction filter. With the same Hanning roll-off, the noise for IFBP without regularization increased by a factor of 6 compared to 3DRP; with regularization the noise was increased only by a factor of 3. Compared to IFBP, the three-dimensional ML-EM reconstruction produced similar resolution improvements with a much smaller increase in noise and slower convergence. Resolution improvements from both IFBP and ML-EM reconstructions are visually apparent in three-dimensional FDG brain images and result in increased activation signals in a three-dimensional [15O]water functional activation study. Our results demonstrate that resolution improvement is possible for IFBP and ML-EM compared to 3DRP with or without noise increase.