Monte Carlo Simulation of Diverging Collimator Geometries for Ring SPECT/MR

The potential of a diverging fan beam collimator for use in a multimodal SPECT-MR system has been investigated. Collimation was designed for use with a stationary ring of gamma camera modules each comprised of a 32 × 32 pixel CZT detector. The collimators provide a desired field of view (FOV) of 25.0 cm at the center of the bore. Eleven collimator designs were compared, yielding between 13 to 23 modules per ring. Each design was evaluated using reconstructed resolution and sensitivity metrics. The designs were simulated with the Monte Carlo software, GEANT4 Application for the Tomographic Emission (GATE) and tomographic reconstruction was performed with a maximum-likelihood expectation maximization (ML-EM) algorithm in MATLAB. The results showed that a practical SPECT/MR design using 18 detectors per ring with a 3.83 cm length collimator gave equivalent tomographic resolution to that of a clinical SPECT/CT system but with 7.0 times greater detection sensitivity compared to the conventional rotating dual-head camera. Resolution across the reconstructed 25 cm x 25 cm FOV did show slight non-uniformity, with resolution improving around the periphery of the FOV as much as two-fold. A smearing artifact was seen in the corners of the FOV likely due to undersampling within those regions. A reconstructed hot-rod resolution phantom matched the previous results, giving similar resolution performance. However, the simulation also showed that the system suffers from aliasing effects when reconstructing features of 7.9 mm or less. To further investigate how the design choices affected the tomographic resolution, parameters for collimator hole size, detector pixel size, and number of projection angles were explored. Both the reduction of hole size and pixel size each allowed for improved resolvability down to 7.9 mm and 6.4 mm respectively. Increasing the number of projection angles was found to remove smearing artifacts from the image, however it did not significantly change the resolution. The resolution is therefore believed to be limited by the 2.46 mm pixel size and associated pixel matched collimator. These are promising results that show that a diverging fan beam collimator could be a viable choice for a SPECT/MR system.