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This article describes a compact single-photon emission computed tomography (SPECT) system that consists of a high-resolution detector combined with a lofthole collimator. The detector is based on a NaI(Tl) scintillator, a position sensitive photomultiplier (PSPMT), dedicated read-out electronics that digitize all PSPMT anodes and finally, a maximum likelihood algorithm for position estimation. The collimator has a new pinhole geometry, called the lofthole. Our choice of magnification (1.06 for the mouse setup, 0.63 for the rat setup) results in a small system with a footprint of 45 cm × 25 cm. Design and measurements of both the detector and the SPECT system are shown. Detector measurements with a beam source have been done to investigate the spatial and energy resolution of the detector. Two SPECT setups have been made, one that fits rat-size phantoms and one that fits mouse-size phantoms. On both setups we have done measurements of a Derenzo phantom and a uniformity phantom. The results show that the detector resolution is 1.1 mm and energy resolution is 9.3% in the center of the detector. With the tomographic rat setup we are able to distinguish the 2.4 mm hot rods in a Derenzo phantom. The mouse setup allows us to distinguish the 1.6 mm rods. This demonstrates the SPECT capabilities of this compact prototype scanner.

Today, new single photon emission computed tomography (SPECT) reconstruction techniques rely on accurate Monte Carlo (MC) simulations to optimize reconstructed images. However, existing MC scintillation camera models which usually include an accurate description of the collimator and crystal, lack correct implementation of the gamma camera's back compartments. In the case of dual isotope simultaneous acquisition (DISA), where backscattered photons from the highest energy isotope are detected in the imaging energy window of the second isotope, this approximation may induce simulation errors. Here, we investigate the influence of backscatter compartment modelling on the simulation accuracy of high-energy isotopes. Three models of a scintillation camera were simulated: a simple model (SM), composed only of a collimator and a NaI(Tl) crystal; an intermediate model (IM), adding a simplified description of the backscatter compartments to the previous model and a complete model (CM), accurately simulating the materials and geometries of the camera. The camera models were evaluated with point sources ((67)Ga, (99m)Tc, (111)In, (123)I, (131)I and (18)F) in air without a collimator, in air with a collimator and in water with a collimator. In the latter case, sensitivities and point-spread functions (PSFs) simulated in the photopeak window with the IM and CM are close to the measured values (error below 10.5%). In the backscatter energy window, however, the IM and CM overestimate the FWHM of the detected PSF by 52% and 23%, respectively, while the SM underestimates it by 34%. The backscatter peak fluence is also overestimated by 20% and 10% with the IM and CM, respectively, whereas it is underestimated by 60% with the SM. The results show that an accurate description of the backscatter compartments is required for SPECT simulations of high-energy isotopes (above 300 keV) when the backscatter energy window is of interest.

The usage of pinholes is very important in preclinical micro-SPECT. Pinholes can magnify the object onto the detector, resulting in better system resolutions than the detector resolution. The loss in sensitivity is usually countered by adding more pinholes, each projecting onto a specific part of the detector. As a result, gamma rays have an oblique incidence to the detector. This causes displacement and increased uncertainty in the position of the interaction of the gamma ray in the detector, also known as parallax errors or depth-of-interaction (DOI) errors. This in turn has a large influence on image reconstruction algorithms using ray tracers as a forward projector model, as the end-point of each ray on the detector has to be accurately known. In this work, we used GATE to simulate the FLEX Triumph-I system (Gamma Medica-Ideas, Northridge, CA), a CZT-based multi-pinhole micro-SPECT system. This system uses 5 mm thick CZT pixels, with 1.5 mm pixel pitch. The simulated information was then used to enhance the image resolution by accurately modeling the DOI. Two hundred point sources were simulated and rebinned to use the DOI information. This data was then used in a GPU-based iterative reconstruction algorithm taking the simulated DOI into account. The average displacement was then determined for all point sources, and the FWHM was calculated in three dimensions, by fitting the point sources with 3D Gaussians. We show that the displacement is reduced by 83% on average. We also show a 15% resolution gain when only 5 DOI levels are used.

In preclinical Single Photon Emission Tomography (SPECT), absolute quantification is interesting, expressed in percentage of injected radioactive dose per gram of tissue. This allows for accurate evaluation of disease progression and precise follow-up studies without the need for sacrificing animals. Accurate modeling of image degrading effects is currently under development for isotopes different from Tc-99m. The aim of this work is to develop absolute micro-SPECT quantification for three different isotopes: Tc-99m, In-111 and I-125. This selection of isotopes covers a wide range of energies, is pre-clinically relevant and allows us to optimally validate the algorithms used for image reconstruction. Furthermore, we will mix these isotopes with additional iodine-based CT contrast agent, to mimic contrast-enhanced SPECT/CT protocols. For each isotope, both a calibration phantom and three 1-ml vials were scanned on the CZT-based FLEX Triumph-I scanner (GM-I). The calibration phantom allows the conversion of reconstructed voxel counts to MBq/ml. The 3-vial phantom consists of 3 different concentrations of radioactivity. Two vials contain iodine-based CT contrast agent to significantly increase the attenuation. All datasets were reconstructed using a GPU-based reconstruction algorithm, which includes resolution recovery, pinhole penetration, geometrical sensitivity correction, scatter correction and attenuation correction. We show good quantification for Tc-99m and In-111. The absolute quantification of I-125 is suboptimal, due to insufficient scatter correction. No influence can be seen when iodine-based CT contrast agent is used together with I-125.

A new brain SPECT system based on multipinhole collimation has been designed and the properties have been studied with Monte Carlo simulations. Results show the system has better resolution and acceptable sensitivity compared to commercial SPECT systems.

Hadron therapy offers some advantages over conventional therapy. However, small errors in particle range may significantly increase dose delivered to healthy tissue or produce incomplete coverage of the target volume. Consequently, developing a method to verify the delivered dose and the Bragg peak location during treatment is crucial. Recent investigations have shown that the prompt gamma photons produced during the patient irradiation with hadron beams are in good correlation with the dose delivered to the patient and therefore, they can be used for in-beam range monitoring. Slit collimated gamma camera has been proposed as a system for prompt gamma imaging. Image reconstruction can potentially improve the accuracy of that system. The purpose of this work is to study and implement a reconstruction process for in-beam range monitoring using slit-collimated gamma camera. Obtained results will guide the design of the slit-collimated camera. The results indicate that the range monitoring is possible with slit camera using ML-EM reconstruction. The average deviation obtained using reconstructed profiles is 2.06 mm.