Design and performance of a compact and stationary microSPECT system

Purpose: Over the last ten years, there has been an extensive growth in the development of microSPECT imagers. Most of the systems are based on the combination of conventional, relatively large gamma cameras with poor intrinsic spatial resolution and multipinhole collimators working in large magnification mode. Spatial resolutions range from 0.58 to 0.76 mm while peak sensitivities vary from 0.06% to 0.4%. While pushing the limits of performance is of major importance, the authors believe that there is a need for smaller and less complex systems that bring along a reduced cost. While low footprint and low-cost systems can make microSPECT available to more researchers, the ease of operation and calibration and low maintenance cost are additional factors that can facilitate the use of microSPECT in molecular imaging. In this paper, the authors simulate the performance of a microSPECT imager that combines high space-bandwidth detectors and pinholes with truncated projection, resulting in a small and stationary system. Methods: A system optimization algorithm is used to determine the optimal SPECT systems, given our high resolutions detectors and a fixed field-of-view. These optimal system geometries are then used to simulate a Defrise disk phantom and a hot rod phantom. Finally, a MOBY mouse phantom, with realistic concentrations of Tc99m-tetrofosmin is simulated. Results: Results show that the authors can successfully reconstruct a Defrise disk phantom of 24 mm in diameter without any rotating system components or translation of the object. Reconstructed spatial resolution is approximately 800 mu m while the peak sensitivity is 0.23%. Finally, the simulation of the MOBY mouse phantom shows that the authors can accurately reconstruct mouse images. Conclusions: These results show that pinholes with truncated projections can be used in small magnification or minification mode to obtain a compact and stationary microSPECT system. The authors showed that they can reach state-of-the-art system performance and can successfully reconstruct images with realistic noise levels in a preclinical context. Such a system can be useful for dynamic SPECT imaging. 2013 American Association of Physicists in Medicine

Inclusion of quasi-vertex views in a brain-dedicated multi-pinhole SPECT system for improved imaging performance

With brain-dedicated multi-detector systems employing pinhole apertures the usage of detectors facing the top of the patient\u27s head (i.e., quasi-vertex views) can provide the advantage of additional viewing from close to the brain for improved detector coverage. In this paper, we report the results of simulation and reconstruction studies to investigate the impact of the quasi-vertex views on the imaging performance of AdaptiSPECT-C, a brain-dedicated stationary SPECT system under development. In this design, both primary and scatter photons from regions located inferior to the brain can contribute to SPECT projections acquired by the quasi-vertex views, and thus degrade AdaptiSPECT-C imaging performance. In this work, we determined the proportion, origin, and nature (i.e., primary, scatter, and multiple-scatter) of counts emitted from structures within the head and throughout the body contributing to projections from the different AdaptiSPECT-C detector rings, as well as from a true vertex view detector. We simulated phantoms used to assess different aspects of image quality (i.e., uniform sphere and Derenzo), as well as anthropomorphic phantoms with multiple count levels emulating clinical(123)I activity distributions (i.e., DaTscan and perfusion). We determined that attenuation and scatter in the patient\u27s body greatly diminish the probability of the photons emitted outside the volume of interest reaching to detectors and being recorded within the 15% photopeak energy window. In addition, we demonstrated that the inclusion of the residual of such counts in the system acquisition does not degrade visual interpretation or quantitative analysis. The addition of the quasi-vertex detectors increases volumetric sensitivity, angular sampling, and spatial resolution leading to significant enhancement in image quality, especially in the striato-thalamic and superior regions of the brain. Besides, the use of quasi-vertex detectors improves the recovery of clinically relevant metrics such as the striatal binding ratio and mean activity in selected cerebral structures

Development and Initial Evaluation of an MR Compatible Preclinical SPECT Insert for Simultaneous SPECT/MR Imaging

Multi-modality medical imaging systems have become increasingly important in research and clinical applications of biomedical imaging. Two complementary imaging modalities that have not yet been fully integrated into a multimodality system are Single Photon Emission Computed Tomography (SPECT) and Magnetic Resonance Imaging (MRI). To this end, our team has developed an MR compatible SPECT insert for simultaneous preclinical SPECT/MR imaging. The SPECT insert’s detector is composed of five rings Cadmium Zinc Telluride (CZT) detector modules and an interchangeable cylindrical multi-pinhole (MPH) collimator. This dissertation discusses several new and significant contributions made towards the development of our SPECT insert. We developed methods to determine optimized design parameters for MPH collimators for the SPECT insert. These methods were used to design two MPH collimators with different imaging resolutions. Simulation results demonstrated that both collimators can be used to obtain artifact-free SPECT images with the designed resolutions. We then developed novel techniques to fabricate the collimators using MR compatible materials. Without proper system calibration and data correction, SPECT images reconstructed from data acquired with our insert exhibit poor image quality. We developed a novel energy calibration method to identify the photopeak of the gamma photons from a Tc-99m source at all 24,320 detector pixels simultaneously and a two-stage detector uniformity correction method to identify and correct for non-uniformities and malfunctioning pixels in the detector modules. Additionally, a method was developed to correct for the drift of electron-hole pairs within the detector modules due to the Lorentz force when operating the SPECT insert inside a magnetic field. After applying the system calibration and correction methods to the acquired data, reconstructed SPECT images showed significant improvement in terms of resolution, uniformity, contrast, and artifact reduction. Finally the SPECT insert was evaluated experimentally as a standalone SPECT system and as an insert inside an MRI system for simultaneous SPECT/MR imaging through phantom and small animal studies. The experimental results demonstrated that the SPECT insert met design specifications. Most importantly, results demonstrate that the insert can be used to obtain high quality SPECT images during simultaneous SPECT/MR image acquisition

SPECT System Design Optimisation for a Simultaneous SPECT/MRI Clinical Scanner

The aim of this project was to optimize the design of a Single Photon Emission Computed Tomography (SPECT) insert based on high-resolution detectors and a high-sensitivity collimator, for a Magnetic Resonance Imaging (MRI) scanner, in order to perform simultaneous human brain SPECT/MRI and improve radionuclide-based therapies for glioma patients. The radionuclides of interest are 99mTc, 111In and 123I. Specific emphasis was given to the collimator and overall system design, data simulation and performance assessment, which would feed directly into the European-funded INSERT project. The SPECT insert was to consist of a stationary system with SiPM-based photodetectors, insensitive to magnetic fields. Regarding the design, a number of system and collimator geometries were evaluated considering the restricted space in the MRI bore and the limited angular sampling. High sensitivity was prioritised over high spatial resolution, because of the clinical application. Gamma shielding design was also addressed. Analytical calculations of system sensitivity and resolution, in addition to Monte Carlo simulations, were performed to compare various slit-slat and pinhole collimator designs. A new collimator design was proposed: multi-mini-slit slit-slat (MSS) collimator. The MSS has multiple mini-slits, some of which are shared between adjacent detectors, and they are embedded in the slat component, allowing for longer slats in comparison to a standard slit-slat collimator. The MSS design demonstrated to have the best overall performance, and the final system design consisted of a partial ring with 20 detectors. A framework for geometrical calibration of the system was developed and assessed, utilising a single prototype detector equipped with a prototype collimator. This framework takes advantage of the specific collimator design to estimate geometrical parameters from independent measurements of calibration phantoms. Experimental evaluation with tomographic acquisition of phantoms demonstrated the applicability of the new collimation concept, confirming the superiority of the MSS design over equivalent pinhole collimation