SPECT/MRI INSERT Compatibility: Assessment, Solutions, and Design Guidelines

In spite of the growing interest in multimodal imaging, and in particular of the consolidation of PET/MRI, the development of simultaneous SPECT/MRI systems is still very limited. SPECT offers potential for multitracer imaging and wider diffusion with respect to PET, but, due to the collimator, poses more stringent challenges on space occupation and MRI compatibility. In this paper, we present a systematic analysis of mutual compatibility between standard 3 T magnetic resonance (MR) scanners and a preclinical SPECT insert composed of a static ring of ten silicon photomultipliers (SiPM)-based gamma cameras. On the MR image quality side, the most critical element is the SPECT tungsten multipinhole collimator, whose distortion of the static field is reduced below 1 ppm thanks to second order shimming. On the SPECT side, MR switching gradients are the most critical source of eddy currents and interferences. Thanks to a board mapping tool, developed for this purpose, the most sensitive tracks and electronic components have been identified and replaced. Shielding, heavy low-pass filtering and star layout of the SiPM high voltage tracks, together with limited use of metallic packages, meshed ground planes, and vias, enabled the preservation of the same energy resolution (14% for 99mTc) during MRI. The design guidelines and the experimental results here reported pave the way to MRI compatibility of the clinical version of this insert.