Challenges in Acquiring Clinical Simultaneous SPECT-MRI on a PET-MRI Scanner

The INSERT is the world’s first clinical SPECTMRI brain imaging system based on scintillation detectors with a SiPM readout. Here we demonstrate its use within a clinical MRI environment for the first time. Using a standard transmit-receive head coil, and with an appropriate selection of a custom MRI sequence (GRE), we overcome mutual interference. The INSERT and its bulky 50 kg tungsten collimator introduce magnetic field inhomogeneity. Due to the specific MRI-compatible collimator design, inhomogeneity is compensated by shimming, leading to simultaneous acquisition. We process the SPECT data acquired alongside the MRI sequence to evaluate the SPECT system performance and the impact of the MRI. Finally, we present a set of simultaneous SPECT-MRI acquisitions, demonstrating multimodal imaging capabilities, albeit with a limited MRI sequence

A Statistical DOI Estimation Algorithm for a SiPM-Based Clinical SPECT Insert

A prototype clinical brain SPECT insert has been designed for use in simultaneous SPECT/MRI. The system utilises novel slit-slat collimators which, like pinhole collimators, suffers from parallax errors due to the large incident angle of photons. A statistical algorithm has been developed to determine the depth-of-interaction (DOI) with a view to improving image performance. The importance of DOI correction was demonstrated using Monte Carlo simulation. This simulation also indicated that 4 DOI layers (3×1.5 mm+3.5 mm) may be sufficient. The improvement in event localisation was demonstrated on a single detector before implementing the algorithm on the full clinical prototype where some limitations in event localisation in layers close to the readout plane were observed. Nevertheless DOI enabled the rejection of poorly localised events with improved resolution in reconstructed line sources

Challenges in Acquiring Clinical Simultaneous SPECT-MRI on a PET-MRI Scanner

The INSERT is the world's first clinical SPECT-MRI brain imaging system based on scintillation detectors with a silicon photomultiplier readout. Here, we demonstrate its use within a clinical MRI environment for the first time. Using a standard transmit-receive head coil, and with an appropriate selection of a custom MRI sequence (GRE), we overcome mutual interference. The INSERT and its bulky 50kg tungsten collimator introduce magnetic field inhomogeneity. Due to the specific MRI-compatible collimator design, inhomogeneity is compensated by shimming, leading to simultaneous acquisition. We process the SPECT data acquired alongside the MRI sequence to evaluate the SPECT system performance and the impact of the MRI. Finally, we present a set of simultaneous SPECT-MRI acquisitions, demonstrating multimodal imaging capabilities, albeit with a limited MRI sequence

First Simultaneous Acquisition of a Clinical SPECT-MRI Brain INSERT

The INSERT (INtegrated SPECT/MRI for Enhanced stratification of brain tumours in Radio-chemoTherapy) is currently the only MRI-compatible SPECT stationary system for clinical application, in particular for brain multimodal imaging with an inner bore of 28 cm and a field of view of 20 cm (transaxial) × 10 cm (axial). The intrinsic spatial resolution is 1 mm and the extrinsic one is 10 mm. This modular scanner fits in an unmodified MRI scanner and is a scale-up of a smaller preclinical version, whose mutual compatibility with MRI was extensively characterized. It is composed of 20 detection modules (with 8-mm thick CsI:Tl scintillators of 5 cm 10 cm area read by an array of SiPM) and a massive multi-mini×slit-slat collimator, realized in tungsten. Here we report the first demonstration of successful simultaneous SPECT/MRI acquisition of phantoms (hot rods) using a commercial transceiver coil in a Siemens Biograph mMR scanner

Impact of DOI in a Clinical SPECT/MRI System : A Simulation Study

A novel SPECT/MRI scanner has been modelled and tested here using Monte Carlo simulation software, SIMIND. The INSERT SPECT/MRI system faces challenges with event reconstruction due to photon depth of interaction. The novel SPECT system is subject to parallax errors due to its crystal size and slit aperture collimator. We present a simple measure of the DOI errors through SIMIND experiments; by modelling the DOI layers we are able to improve the reconstruction of projection data in the INSERT scanner. A set of capillary phantoms are simulated to explore the impact of DOI on the resolution of the scanner and establish corrections in the system's reconstruction