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

Development of a Practical Calibration Procedure for a Clinical SPECT/MRI System Using a Single INSERT Prototype Detector and Multimini Slit-Slat Collimator

In the context of the INSERT project, we have been developing a clinical single photon emission computed tomography (SPECT) insert for a magnetic resonance imaging (MRI) system, in order to perform simultaneous SPECT/MRI of the human brain. This system will consist of 20 CsI:Tl scintillation detectors, 5-cm wide and 10-cm long, with a 72-channel silicon photomultiplier (SiPM) readout per detector, and a multimini slit-slat (MSS) collimator set up in a stationary partial ring. Additionally the system has a custom-built transmit/receive MR coil to ensure compatibility with the SPECT system. Due to the novel design of the system/collimator, existing geometric calibration methods are not suitable. Therefore we propose a novel and practical calibration procedure that consists of a set of specific independent measurements to determine the geometric parameters of the collimator. This procedure was developed utilizing a prototype system that consists of a reduced-size single detector with a 36-channel SiPM-based readout and a single MSS collimator module. Validation was performed by reconstructing different imaging phantoms, using a rotating stage to simulate a tomographic acquisition. Regarding uniformity, the coefficient of variation (COV) for the cylinder phantom reconstructed with correct calibration parameters is 6.7%, whereas the COV using incorrect parameters is 9.4%. The quality of the phantom reconstructions provide evidence of the applicability of the proposed method to the calibration of the prototype system. This procedure can be easily adapted for the final INSERT system

Development of a Practical Calibration Procedure for a Clinical SPECT/MRI System Using a Single INSERT Prototype Detector and Multimini Slit-Slat Collimator

In the context of the INSERT project, we have been developing a clinical single photon emission computed tomography (SPECT) insert for a magnetic resonance imaging (MRI) system, in order to perform simultaneous SPECT/MRI of the human brain. This system will consist of 20 CsI:Tl scintillation detectors, 5-cm wide and 10-cm long, with a 72-channel silicon photomultiplier (SiPM) readout per detector, and a multimini slit-slat (MSS) collimator set up in a stationary partial ring. Additionally the system has a custom-built transmit/receive MR coil to ensure compatibility with the SPECT system. Due to the novel design of the system/collimator, existing geometric calibration methods are not suitable. Therefore we propose a novel and practical calibration procedure that consists of a set of specific independent measurements to determine the geometric parameters of the collimator. This procedure was developed utilizing a prototype system that consists of a reduced-size single detector with a 36-channel SiPM-based readout and a single MSS collimator module. Validation was performed by reconstructing different imaging phantoms, using a rotating stage to simulate a tomographic acquisition. Regarding uniformity, the coefficient of variation (COV) for the cylinder phantom reconstructed with correct calibration parameters is 6.7%, whereas the COV using incorrect parameters is 9.4%. The quality of the phantom reconstructions provide evidence of the applicability of the proposed method to the calibration of the prototype system. This procedure can be easily adapted for the final INSERT system

A SiPM-based detection module for SPECT/MRI systems

In the present work we present the development of a Silicon PhotoMultiplier (SiPM)-based detection module for hybrid SPECT/MRI instruments. The module is designed for preclinical SPECT systems for mouse and rat brain imaging, but can also be exploited for clinical SPECT brain scanners. The gamma-ray detection module is designed on the well established Anger architecture, with a continuous 5 cm × 5 cm CsI:Tl scintillator read by an array of SiPMs (RGB-HD with 25 μm SPAD cells) from Fondazione Bruno Kessler. The current signals are conveyed to a 36-channel ASIC realized in 0.35 μm CMOS technology and digitized by an external data acquisition system. An operative temperature of 0◦C is mandatory to reduce the dark count rate of the SiPM array and to enhance the final performance of the detector in terms of energy and spatial resolution. For this purpose, an MRI-compatible heat sink is realized with a plastic material (Coolpolymer D5506) with a glycol-water mixture as cooling fluid. Gamma-ray measurements with Co-57 (122 keV) have provided an energy resolution better than 14% and an average intrinsic spatial resolution below 1.0 mm

Development of a SiPM-based Anger camera for INSERT, a new multi-modality SPECT/MRI system for preclinical and clinical imaging

A new multi-modality imaging tool is under development in the framework of the INSERT (INtegrated SPECT/MRI for Enhanced Stratification in Radio-chemoTherapy) project, supported by the European Community. A custom SPECT apparatus, used as an insert for commercially available MRI systems, could enhance the treatment of brain tumors (primarily glioma) by offering more effective and earlier diagnosis with potentially better outcome in survival. In this work we briefly describe the architecture of the gamma-ray photo-detection module with focus on the Silicon PhotoMultiplier (SiPM) detectors and their performance with low-energy gamma-rays (57Co - 122 keV). The camera has a 50.40 mm × 51.70 mm photo-detection area, comprising of 144 SiPMs with 36 readout channels, each one composed of four short-circuited SiPMs for an 8 mm × 8 mm active area. We have developed an Anger camera to characterize and qualify the array when coupled to a CsI:Tl scintillator (8 mm thick). We have measured an intrinsic spatial resolution better than 1.35 mm and an energy resolution below 14% using a 57Co source, when the tile is cooled down to -17°C to reduce the impact of the dark count rate on the measurements