Incorporating the Collimator Blurring Effect In Mlem Reconstruction of SPECT Imaging
Abstract
Purpose
A system matrix (SM) was designed to incorporate the distance dependent blurring in MLEM reconstruction for SPECT.
Methods
Two Tc-99m point sources of 0.5 mCi activity at center and edge of imaging field of view (FOV) were acquired at 64 angular samples in a 180 degrees rotation orbit for a total 32 million counts in a Siemens Symbia SPECT-CT scanner. The SM elements comprise of the probability that an emission from a pixel contributes detection in a projection line of response (LOR) based on pixel location relative to the LOR path (0: pixel on LOR, 1-4: number of additional LORs closest to the pixel). The MLEM reconstructed (10 iterations) point spread function was fitted to a Gaussian to estimate the resolution full width at half maximum (FWHM). The resolution was considered optimal when the change in FWHM with respect to the number of LORs was negligible. Difference of edge resolution in comparison to center resolution was also investigated.
Results
The resolution improved by increasing the number of LORs in the SM. The difference in resolution between each successive LOR numbers (1-0, 2-1, 3-2, 4-3) progressively decreased with lowest difference between LORs 3 and 4 (Center: 1% and Edge: 0%) indicating that resolution improvement plateaued at LOR number 3. When comparing the edge resolution of a LOR number to the center resolution of the next LOR number, lowest difference was obtained between 2 and 3 (-0.03) indicating that the optimal LOR for the edge was 3 and that for the center was 2. Reconstruction with a SM formed by linearly varying LOR numbers as a function of pixel location (edge 2, center 3) produced a uniform resolution across FOV (Center: 1.17, Edge: 1.11).
Conclusion
Utilizing a position dependent linearly varying SM produced optimal and uniform resolution across the imaging FOV.