Objective:
We would like to improve the image reconstructions for both signal-to-noise ratio (snr) and spatial resolution characteristics for the small animal positron emission tomograph yap-pet, built at the department of physics of ferrara university. the three-dimensional (3d) filtered backprojection (fbp) algorithm, usually used for image reconstruction, has a limited angle restriction due to the tomograph geometry, which causes a serious loss in sensitivity.
Methods:
We implemented a 3d iterative reconstruction program using the symmetry and sparse properties of the 'probability matrix', which correlates the emission from each voxel to the detector within a coincidence tube. a fraction only of matrix elements are calculated before the reconstruction and stored on disk: this allows us to avoid on-line computation. a depth dependent function differentiates the voxels in a coincidence tube. three experimental phantoms with no background were reconstructed by using the program, in comparison with traditionally used fbp.
Results:
The adopted method allowed us to reduce the computation time significantly. furthermore, the simple depth dependent function improved the spatial resolution. with 64 x 64 x 20 voxels of 0.625 x 0.625 x 2.0 mm(3) in the field of view, the computation time was less than 4 min per iteration on a sparc ultra 450 workstation, and less than 6 min per iteration on a mac-ppc g3 300 mhz: the spatial resolution measured with a 0.8 mm diameter 18f-fdg filled capillary reconstructed in this way was 2.0 mm fwhm. by decreasing the voxel size to 0.3125 x 0.3125 x 2.0 mm(3) per voxel the transaxial fwhm was 1.7 mm with a computation time of 15 min per iteration on a sparc ultra 450. by using all the acquired data, the snr improves from 1.3 to 6.0 in the worst measured case, a pair of 0.8mm diameter 18f-fdg filled capillaries, which are 2.5 mm apart each other.
Conclusion:
The adoption of iterative reconstruction allowed us to overcome the loss in sensitivity of previously used fbp: this improved the snr. the studies of symmetry and sparse properties avoided a severe increase of the reconstruction time and of storing space on disk. this fast em algorithm is now routinely used for the image reconstruction with the yap-pet tomograph.