A Parallel-Cone Collimator for High-Energy SPECT

In SPECT using high-energy photon-emitting isotopes, such as 131I, parallel-hole collimators with thick septa are required to limit septal penetration, at the cost of sensitivity and resolution. This study investigated a parallel-hole collimator with cone-shaped holes, which was designed to limit collimator penetration while preserving resolution and sensitivity. The objective was to demonstrate that a singleslice prototype of the parallel-cone (PC) collimator was capable of improving the image quality of high-energy SPECT. Methods: The image quality of the PC collimator was quantitatively compared with that of clinically used low-energy high-resolution (LEHR; for 99mTc) and high-energy general-purpose (HEGP; for 131I and 18F) parallel-hole collimators. First, Monte Carlo simulations of single and double point sources were performed to assess sensitivity and resolution by comparing point-spread functions (PSFs). Second, a prototype PC collimator was used in an experimental phantom study to assess and compare contrast recovery coefficients and image noise. Results: Monte Carlo simulations showed reduced broadening of the PSF due to collimator penetration for the PC collimator as compared with the HEGP collimator (e.g., 0.9 vs. 1.4 cm in full width at half maximum for 131I). Simulated double point sources placed 2 cm apart were separately detectable for the PC collimator, whereas this was not the case for 131I and 18F at distances from the collimator face of 10 cm or more for the HEGP collimator. The sensitivity, measured over the simulated profiles as the total amount of counts per decay, was found to be higher for the LEHR and HEGP collimators than for the PC collimator (e.g., 3.1 x 10-5 vs. 2.9 x 10-5 counts per decay for 131I). However, at equal noise level, phantom measurements showed that contrast recovery coefficients were similar for the PC and LEHR collimators for 99mTc but that the PC collimator significantly improved the contrast recovery coefficients as compared with the HEGP collimator for 131I and 18F. Conclusion: High-energy SPECT imaging with a single-slice prototype of the proposed PC collimator has shown the potential for significantly improved image quality in comparison with standard parallel-hole collimators