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 (3.8x10-4). 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

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