Novel collimation for simultaneous SPECT/MRI

The aim of this work is to develop an optimal collimator design for a stationary, compact SPECT insert for an MRI system, intended to perform simultaneous brain SPECT/MRI in humans. The SPECT insert will consist of a single ring of 25 SiPM-based detectors, insensitive to magnetic fields (5 by 10cm, 0.8mm intrinsic resolution). Compactness is needed due to limited space inside the MRI bore. Here we introduce the concept of an interior slit in the slat component of a slit-slat collimator, providing the possibility of having longer slats extending beyond the slit collimator. We also explore the use of mini-slit arrays to obtain improved angular sampling of the object - we call this an MSS collimator.We compared various multi-pinhole and multi-slit slit-slat configurations, using analytical calculations of sensitivity for a target resolution of 10mm FWHM and simulations with digital phantoms. All slit-slat configurations provided higher sensitivity when compared to the corresponding pinhole designs. The highest sensitivity was obtained for the 2-slit configuration (3.8×10-4). Simulations with a uniform phantom showed reduced sampling artefacts for both the MSS and the corresponding multi-pinhole configuration, in comparison to the other geometries. With a Derenzo phantom, better reconstructed uniformity was observed for the same configurations, with slightly better resolution for the pinhole configuration. With a Defrise phantom, better axial resolution was observed for the slit-slat as compared to the pinhole collimators, and also a more uniform axial coverage. The proposed MSS design demonstrates good reconstructed uniformity and sensitivity, and less sampling artefacts when compared to other collimator configurations, and is therefore the design of choice for the SPECT/MRI insert.