Preclinical SPECT Research Articles

Preclinical SPECT and PET: Joint EANM and ESMI procedure guideline for implementing an efficient quality control programme.

The aim of this guideline is to provide recommendations for the implementation of an effective and efficient quality control (QC) programme for SPECT and PET systems in a preclinical imaging lab. These recommendations aim to strengthen the translational power of preclinical imaging results obtained using preclinical SPECT and PET. As for clinical imaging, reliability, reproducibility, and repeatability are essential when groups of animals are used in a longitudinal imaging experiment. The larger the variability of the imaging endpoint, the more animals are needed to be able to observe statistically significant differences between groups. Therefore, preclinical imaging requires quality control procedures to maintain reliability, reproducibility, and repeatability of imaging procedures, and to ensure the accuracy and precision of SPECT and PET quantification. While the Physics Committee of the European Association of Nuclear Medicine (EANM) has already published excellent procedure guidelines for Routine Quality Control Recommendations for Nuclear Medicine Instrumentationthat also includes procedures for small animal PET systems, and important steps have already been made concerning preclinical quality control aspects, this new guideline provides a review and update of these previous guidelines such that guidelines are also adapted to new technological developments.

P-153 - Performance assessment of quantitative preclinical SPECT imaging with 155Tb and 161Tb

P-153 - Performance assessment of quantitative preclinical SPECT imaging with 155Tb and 161Tb

NEMA NU 1-2018 performance characterization and Monte Carlo model validation of the Cubresa Spark SiPM-based preclinical SPECT scanner

BackgroundThe Cubresa Spark is a novel benchtop silicon-photomultiplier (SiPM)-based preclinical SPECT system. SiPMs in SPECT significantly improve resolution and reduce detector size compared to preclinical cameras with photomultiplier tubes requiring highly magnifying collimators. The NEMA NU 1 Standard for Performance Measurements of Gamma Cameras provides methods that can be readily applied or extended to characterize preclinical cameras with minor modifications. The primary objective of this study is to characterize the Spark according to the NEMA NU 1-2018 standard to gain insight into its nuclear medicine imaging capabilities. The secondary objective is to validate a GATE Monte Carlo simulation model of the Spark for use in preclinical SPECT studies.MethodsNEMA NU 1-2018 guidelines were applied to characterize the Spark’s intrinsic, system, and tomographic performance with single- and multi-pinhole collimators. Phantoms were fabricated according to NEMA specifications with deviations involving high-resolution modifications. GATE was utilized to model the detector head with the single-pinhole collimator, and NEMA measurements were employed to tune and validate the model. Single-pinhole and multi-pinhole SPECT data were reconstructed with the Software for Tomographic Image Reconstruction and HiSPECT, respectively.ResultsThe limiting intrinsic resolution was measured as 0.85 mm owing to a high-resolution SiPM array combined with a 3 mm-thick scintillation crystal. The average limiting tomographic resolution was 1.37 mm and 1.19 mm for the single- and multi-pinhole collimators, respectively, which have magnification factors near unity at the center of rotation. The maximum observed count rate was 15,400 cps, and planar sensitivities of 34 cps/MBq and 150 cps/MBq were measured at the center of rotation for the single- and multi-pinhole collimators, respectively. All simulated tests agreed well with measurement, where the most considerable deviations were below 7%.ConclusionsNEMA NU 1-2018 standards determined that a SiPM detector mitigates the need for highly magnifying pinhole collimators while preserving detailed information in projection images. Measured and simulated NEMA results were highly comparable with differences on the order of a few percent, confirming simulation accuracy and validating the GATE model. Of the collimators initially provided with the Spark, the multi-pinhole collimator offers high resolution and sensitivity for organ-specific imaging of small animals, and the single-pinhole collimator enables high-resolution whole-body imaging of small animals.

Efficient Monte-Carlo based system modelling for image reconstruction in preclinical pinhole SPECT

The use of multi-pinhole collimation has enabled ultra-high-resolution imaging of SPECT and PET tracers in small animals. Key for obtaining high-quality images is the use of statistical iterative image reconstruction with accurate energy-dependent photon transport modelling through collimator and detector. This can be incorporated in a system matrix that contains the probabilities that a photon emitted from a certain voxel is detected at a specific detector pixel. Here we introduce a fast Monte-Carlo based (FMC-based) matrix generation method for pinhole imaging that is easy to apply to various radionuclides. The method is based on accelerated point source simulations combined with model-based interpolation to straightforwardly change or combine photon energies of the radionuclide of interest. The proposed method was evaluated for a VECTor PET-SPECT system with (i) a HE-UHR-M collimator and (ii) an EXIRAD-3D 3D autoradiography collimator. Both experimental scans with 99mTc, 111In, and 123I, and simulated scans with 67Ga and 90Y were performed for evaluation. FMC was compared with two currently used approaches, one based on a set of point source measurements with 99mTc (dubbed traditional method), and the other based on an energy-dependent ray-tracing simulation (ray-tracing method). The reconstruction results show better image quality when using FMC-based matrices than when applying the traditional or ray-tracing matrices in various cases. FMC-based matrices generalise better than the traditional matrices when imaging radionuclides with energies deviating too much from the energy used in the calibration and are computationally more efficient for very-high-resolution imaging than the ray-tracing matrices. In addition, FMC has the advantage of easily combining energies in a single matrix which is relevant when imaging radionuclides with multiple photopeak energies (e.g. 67Ga and 111In) or with a continuous energy spectrum (e.g. 90Y). To conclude, FMC is an efficient, accurate, and versatile tool for creating system matrices for ultra-high-resolution pinhole SPECT.

Accurate Monte Carlo Modeling of Small-Animal Multi-Pinhole SPECT for Non-Standard Multi-Isotope Applications.

Recent advances in preclinical SPECT instrumentation enable non-standard multi-isotope acquisitions at the edge of physical feasibility to improve efficiency of pharmaceutical research. Due to the variety of applications, optimization of imaging hardware, acquisition protocols and reconstruction algorithms is a central and recurring task. For this purpose, we developed a Monte Carlo simulation model of a preclinical state-of-the-art multi-pinhole SPECT system, the NanoSPECT/CTPLUS, with emphasis on high accuracy for multi-isotope experiments operating near the system range limits. The GATE/ GEANT4 model included an accurate description of multi-pinhole collimators and all substructures of the detector back compartment. The readout electronics was modeled with a variety of signal processors partially extended to incorporate non-simplified measured response functions. The final model was able to predict energy spectra, planar images and tomographic reconstructions with high accuracy for both standard and non-standard multi-isotope experiments. Complex activity distributions could be reproduced for a wide range of noise levels and different modes of angular undersampling. Using the example of a dual-isotope triple-tracer experiment, the model has proven to be a powerful tool for protocol optimization and quantitative image correction at the performance range limits of multi-isotope multi-pinhole SPECT.

Design, Performance Evaluation, and Modeling of an Ultrahigh Resolution Detector Dedicated for Simultaneous SPECT/MRI

In this paper, we report design and preliminary performance evaluation of a CdTe detector module for use in an MR-compatible preclinical SPECT system. The detector module consists of four CdTe detectors. Each of them has a CdTe crystal with a dimension of 1.125 cm × 2.25 cm × 2 mm (thickness). The crystal anode side is divided into 32 × 64 square pixels with a width of 350 μm and attached to a custom-designed energy-resolved photon-counting (ERPC) ASIC. The ERPC CdTe detector module has a median energy resolution of 7.0 keV across 8192 (4 × 32 × 64 ) pixels and 95% of the pixels have energy resolution better than 9 keV inside a Siemens clinical 3-T MR scanner; it has a median detection efficiency of 0.47 and 95% of the pixels have detection efficiency between 0.44 and 0.49; the detector spatial resolution enables one to observe the point response function (PRF) distortion caused by a 3T magnetic field. An analytical method has been developed to model this distortion and the corresponding modeled PRF has a root of mean square error within 5%, compared to the experimental PRF.

Performance assessment of the ALBIRA II pre-clinical SPECT S102 system for 99mTc imaging.

The performance characteristics of the SPECT sub-system S102 of the ALBIRA II PET/SPECT/CT are analyzed for the 80mm field of view (FOV) to evaluate the potential in-vivo imaging in rats, based on measurements of the system response for the commonly used Technetium-99m (99mTc) in small animal imaging. The ALBIRA II tri-modal µPET/SPECT/CT pre-clinical system (Bruker BioSpin, Ettlingen, Germany) was used. The SPECT modality is made up of two opposite gamma cameras (Version S102) with Sodium doped Cesium Iodide (CsI(Na)) single continuous crystal detectors coupled to position-sensitive photomultipliers (PSPMTs). Imaging was performed with the NEMA NU-4 image quality phantom (Data Spectrum Corporation, Durham, USA). Measurements were performed with a starting activity concentration of 4.76MBq/mL 99mTc. An energy window of 20% at 140keV was selected in this study. The system offers a 20mm, 40mm, 60mm and an 80mm field of view (FOV) and in this study the 80mm FOV was used for all the acquisitions. The data were reconstructed with an ordered subset expectation maximization (OSEM) algorithm. Sensitivity, spatial resolution, count rate linearity, convergence of the algorithm and the recovery coefficients (RC) were analyzed. All analyses were performed with PMOD and MATLAB software. The sensitivities measured at the center of the 80mm FOV with the point source were 23.1 ± 0.3 cps/MBq (single pinhole SPH) and 105.6 ± 5.5 cps/MBq (multi pinhole MPH). The values for the axial, tangential and radial full width at half maximum (FWHM) were 2.51, 2.54, and 2.55mm with SPH and 2.35, 2.44 and 2.32mm with MPH, respectively. The corresponding RC values for the 5mm, 4mm, 3mm and 2mm rods were 0.60 ± 0.28, 0.61 ± 0.24, 0.29 ± 0.11 and 0.20 ± 0.06 with SPH and 0.56 ± 0.20, 0.50 ± 0.18, 0.38 ± 0.09 and 0.23 ± 0.06 with MPH. To obtain quantitative imaging data, the image reconstructions should be performed with 12 iterations. The ALBIRA II preclinical SPECT sub-system S102 has a favorable sensitivity and spatial resolution for the 80mm FOV setting for both the SPH and MPH configurations and is a valuable tool for small animal imaging.

Super Spatial Resolution (SSR) method for small animal SPECT imaging: A Monte Carlo study

This work used Monte Carlo (MC) simulations in order to investigate the feasibility of the application of a super-resolution method, namely Super Spatial Resolution (SSR), to a preclinical SPECT system. The SSR method is used to obtain a resolution enhancement by combining several images containing slightly different perspectives of the same scene.MC simulations are a useful tool in the development of new imaging devices, as well as in the assessment of new reconstruction methods. In this respect, GATE (Geant4 Application for Emission Tomography) is an open source platform dedicated to numerical MC simulations that has become a well validated toolkit for the simulation of SPECT and PET systems.We modelled a four-headed preclinical SPECT scanner capable of the proper movements in order to obtain the SSR acquisition sequences. Each detector is based on a CsI(Tl) pixelated scintillator coupled to a low-energy tungsten collimator with parallel square holes.Several phantoms and radioactive sources were simulated, in order to assess the performance of the proposed SPECT scanner. In addition, a complex voxelized phantom of a mouse was used to obtain realistic images with the aim of evaluating the system as a suitable tool for small animal imaging.The whole system sensitivity was 164.1 cps/MBq. In order to assess the impact of the SSR this value must be divided by the number of the images acquired to perform this method. The average value of the trans-axial resolution improves from 2.4 mm to 1.54 mm, 1.21 mm and 1.03 mm by respectively applying the SSR based on two, three or four images. While the average axial resolution changes from 1.69 mm to 1.49 mm, 1.15 mm and 0.98 mm by respectively using the SSR based on two, three or four images. The mouse images obtained by using the voxel phantom have demonstrated the good capability of the system as a suitable tool for small animal imaging.Finally, a comparison with commercial preclinical scanners has proved that the presented SSR scanner provides an alternative to pinhole SPECT systems for many preclinical research studies.

Preclinical SPECT and SPECT-CT in Oncology.

Molecular imaging enables both spatial and temporal understanding of the complex biologic systems underlying carcinogenesis and malignant spread. Single-photon emission tomography (SPECT) is a versatile nuclear imaging-based technique with ideal properties to study these processes in vivo in small animal models, as well as to identify potential drug candidates and characterize their antitumor action and potential adverse effects. Small animal SPECT and SPECT-CT (single-photon emission tomography combined with computer tomography) systems continue to evolve, as do the numerous SPECT radiopharmaceutical agents, allowing unprecedented sensitivity and quantitative molecular imaging capabilities. Several of these advances, their specific applications in oncology as well as new areas of exploration are highlighted in this chapter.

A new High-Resolution Imaging System (HiRIS2) detector for preclinical SPECT imaging

The aim of the work is to describe a new high-performance scintigraphic detector, derived from our previous work based on a Position Sensitive Photomultiplier Tube (PSPMT) with 256 output anodes. This detector is suitable for direct integration into multi-modality imaging systems. Indeed, the detector has been developed in order to be integrated into a pre-clinical system.The detector is based on an H9500 Hamamatsu PSPMT coupled to an LYSO pixelated scintillator and a low-energy tungsten collimator with parallel square holes. In order to limit the overall size of the device, a dedicated compact electronics has been developed. The device design was performed using Monte Carlo transport calculations to aid the development.Our results show a detector performance suitable for application on small animals. The intrinsic spatial resolution was experimentally determined to be about 1.6 mm. The measured energy resolution was ∼16% at 140 keV and the maximum recorded sensitivity was ∼76 cps/MBq. In addition, the results agree quite well with those of Monte Carlo simulations.We have successfully improved the performance of our previous detector design in order to integrate the new device into a complex system focused on pre-clinical imaging.

Characterization of the Detection Module of the INSERT SPECT/MRI Clinical System

Within the vast panorama of multimodal systems for clinical diagnosis, PET/MRI has been receiving significant attention. In parallel, new technological solutions have been paving the way to simultaneous SPECT/MRI systems, whose development has been so far delayed due to the challenging compatibility between MRI, gamma-ray detectors, and collimators. This paper presents an silicon photomultiplier (SiPM)-based Anger camera designed for preclinical and clinical SPECT static inserts for standard MR scanners. The gamma-ray detector is based on a continuous CsI(Tl) scintillator readout by arrays of SiPMs and custom ASICs. The design has been adapted to fit with the limited space conditions (23 mm thickness), but also considering mutual compatibility issues. Despite these constraints and thanks to the adoption of an iterative statistical event estimation method simply requiring a flood image, the detector maintains state-of-the-art performance in terms of intrinsic spatial resolution (1.0 mm FWHM within the usable field of view) and an energy resolution below 14% FWHM at 140 keV. The experimental studies here performed show that the camera, designed to operate at 0 °C to reduce the dark current of SiPMs, can operate up to 10 °C without significant worsening of imaging performance.

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.

Performance Evaluation and Quantitative Accuracy of Multipinhole NanoSPECT/CT Scanner for Theranostic Lu-177 Imaging

SPECT plays important role in peptide receptor targeted radionuclide therapy using theranostic radionuclides such as Lu-177 for the treatment of various cancers. However, SPECT studies must be quantitatively accurate because the reliable assessment of tumor uptake and tumor-to-normal tissue ratios can only be performed using quantitatively accurate images. Hence, it is important to evaluate performance parameters and quantitative accuracy of preclinical SPECT systems for therapeutic radioisotopes before conducting pre- and post-therapy SPECT imaging or dosimetry studies. In this study, we evaluated system performance and quantitative accuracy of NanoSPECT/CT scanner for Lu-177 imaging using point source and uniform phantom studies. We measured recovery coefficient, uniformity, spatial resolution, system sensitivity and calibration factor for mouse whole body standard aperture. We also performed the experiments using Tc-99m to compare the results with that of Lu-177. We found that the recovery coefficient of more than 70% for Lu-177 at the optimum noise level when nine iterations were used. The spatial resolutions of Lu-177 with and without adding uniform background was comparable to that of Tc-99m in axial, radial and tangential directions. System sensitivity measured for Lu-177 was almost three times less than that of Tc-99m.

Simulation study of the second-generation MR-compatible SPECT system based on the inverted compound-eye gamma camera design

In this paper, we present simulation studies for the second-generation MRI compatible SPECT system, MRC-SPECT-II, based on an inverted compound eye (ICE) gamma camera concept. The MRC-SPECT-II system consists of a total of 1536 independent micro-pinhole-camera-elements (MCEs) distributed in a ring with an inner diameter of 6 cm. This system provides a FOV of 1 cm diameter and a peak geometrical efficiency of approximately 1.3% (the typical levels of 0.1%–0.01% found in modern pre-clinical SPECT instrumentations), while maintaining a sub-500 μm spatial resolution. Compared to the first-generation MRC-SPECT system (MRC-SPECT-I) (Cai 2014 Nucl. Instrum. Methods Phys. Res. A 734 147–51) developed in our lab, the MRC-SPECT-II system offers a similar resolution with dramatically improved sensitivity and greatly reduced physical dimension. The latter should allow the system to be placed inside most clinical and pre-clinical MRI scanners for high-performance simultaneous MRI and SPECT imaging.

Spinning slithole collimation for high-sensitivity small animal SPECT: Design and assessment using GATE simulation

PurposeWhile traditional collimations are widely used in preclinical SPECT imaging, they usually suffer from possessing a low system sensitivity leading to noisy images. In this study, we are aiming at introducing a novel collimator, the slithole, offering a superior resolution-sensitivity tradeoff for small animal SPECT. MethodsThe collimator was designed for a molecular SPECT scanner, the HiReSPECT. The slithole is a knife-edge narrow long aperture extended across long-axis of the camera’s head. To meet the data completeness requirement, the collimator-detector assembly spins at each regular SPECT angle. The collimator was modeled within GATE Monte Carlo simulator and the data acquisition was performed for NEMA Image Quality (IQ) phantom. In addition, a dedicated 3D iterative reconstruction algorithm based upon plane-integral projections was also developed. ResultsThe mean sensitivity of the slithole is 285cps/MBq while the current parallel-hole collimator holds a sensitivity of 36cps/MBq at a 30mm distance. The slithole collimation gives rise to a tomographic resolution of 1.8mm compared to a spatial resolution of∼1.7mm for the parallel-hole one (even after resolution modeling). A 1.75 reduction factor in the noise level was observed when the current parallel-hole collimator is replaced by the slithole. Furthermore, quantitative analysis proves that 3 full-iterations of our dedicated image reconstruction lead to optimal image quality. For the largest rod in the NEMA IQ phantom, a recovery coefficient of∼0.83 was obtained. ConclusionThe slithole collimator outperforms the current parallel-hole collimation by exhibiting a better resolution-sensitivity compromise for preclinical SPECT studies.

Preclinical imaging characteristics and quantification of Platinum-195m SPECT.

In vivo biodistribution imaging of platinum-based compounds may allow better patient selection for treatment with chemo(radio)therapy. Radiolabeling with Platinum-195m (195mPt) allows SPECT imaging, without altering the chemical structure or biological activity of the compound. We have assessed the feasibility of 195mPt SPECT imaging in mice, with the aim to determine the image quality and accuracy of quantification for current preclinical imaging equipment. Enriched (>96%) 194Pt was irradiated in the High Flux Reactor (HFR) in Petten, The Netherlands (NRG). A 0.05M HCl 195mPt-solution with a specific activity of 33MBq/mg was obtained. Image quality was assessed for the NanoSPECT/CT (Bioscan Inc., Washington DC, USA) and U-SPECT+/CT (MILabs BV, Utrecht, the Netherlands) scanners. A radioactivity-filled rod phantom (rod diameter 0.85-1.7mm) filled with 1MBq 195mPt was scanned with different acquisition durations (10-120min). Four healthy mice were injected intravenously with 3-4MBq 195mPt. Mouse images were acquired with the NanoSPECT for 120min at 0, 2, 4, or 24h after injection. Organs were delineated to quantify 195mPt concentrations. Immediately after scanning, the mice were sacrificed, and the platinum concentration was determined in organs using a gamma counter and graphite furnace - atomic absorption spectroscopy (GF-AAS) as reference standards. A 30-min acquisition of the phantom provided visually adequate image quality for both scanners. The smallest visible rods were 0.95mm in diameter on the NanoSPECT and 0.85mm in diameter on the U-SPECT+. The image quality in mice was visually adequate. Uptake was seen in the kidneys with excretion to the bladder, and in the liver, blood, and intestine. No uptake was seen in the brain. The Spearman correlation between SPECT and gamma counter was 0.92, between SPECT and GF-AAS it was 0.84, and between GF-AAS and gamma counter it was0.97 (all p < 0.0001). Preclinical 195mPt SPECT is feasible with acceptable tracer doses and acquisition times, and provides good image quality and accurate signal quantification.

Monte Carlo Optimization of Crystal Configuration for Pixelated Molecular SPECT Scanners

Resolution-sensitivity-PDA tradeoff is the most challenging problem in design and optimization of pixelated preclinical SPECT scanners. In this work, we addressed such a challenge from a crystal point-of-view by looking for an optimal pixelated scintillator using GATE Monte Carlo simulation. Various crystal configurations have been investigated and the influence of different pixel sizes, pixel gaps, and three scintillators on tomographic resolution, sensitivity, and PDA of the camera were evaluated. The crystal configuration was then optimized using two objective functions: the weighted-sum and the figure-of-merit methods. The CsI(Na) reveals the highest sensitivity of the order of 43.47cps/MBq in comparison to the NaI(Tl) and the YAP(Ce), for a 1.5×1.5mm2 pixel size and 0.1mm gap. The results show that the spatial resolution, in terms of FWHM, improves from 3.38 to 2.21mm while the sensitivity simultaneously deteriorates from 42.39cps/MBq to 27.81cps/MBq when pixel size varies from 2×2mm2 to 0.5×0.5mm2 for a 0.2mm gap, respectively. The PDA worsens from 0.91 to 0.42 when pixel size decreases from 0.5×0.5mm2 to 1×1mm2 for a 0.2mm gap at 15° incident-angle. The two objective functions agree that the 1.5×1.5mm2 pixel size and 0.1mm Epoxy gap CsI(Na) configuration provides the best compromise for small-animal imaging, using the HiReSPECT scanner. Our study highlights that crystal configuration can significantly affect the performance of the camera, and thereby Monte Carlo optimization of pixelated detectors is mandatory in order to achieve an optimal quality tomogram.

High-resolution clustered pinhole 131Iodine SPECT imaging in mice

IntroductionHigh-resolution pre-clinical 131I SPECT can facilitate development of new radioiodine therapies for cancer. To this end, it is important to limit resolution-degrading effects of pinhole edge penetration by the high-energy γ-photons of iodine. Here we introduce, optimize and validate 131I SPECT performed with a dedicated high-energy clustered multi-pinhole collimator. MethodsA SPECT–CT system (VECTor/CT) with stationary gamma-detectors was equipped with a tungsten collimator with clustered pinholes. Images were reconstructed with pixel-based OSEM, using a dedicated 131I system matrix that models the distance- and energy-dependent resolution and sensitivity of each pinhole, as well as the intrinsic detector blurring and variable depth of interaction in the detector. The system performance was characterized with phantoms and in vivo static and dynamic 131I-NaI scans of mice. ResultsReconstructed image resolution reached 0.6mm, while quantitative accuracy measured with a 131I filled syringe reaches an accuracy of +3.6±3.5% of the gold standard value. In vivo mice scans illustrated a clear shape of the thyroid and biodistribution of 131I within the animal. Pharmacokinetics of 131I was assessed with 15-s time frames from the sequence of dynamic images and time–activity curves of 131I-NaI. ConclusionsHigh-resolution quantitative and fast dynamic 131I SPECT in mice is possible by means of a high-energy collimator and optimized system modeling. This enables analysis of 131I uptake even within small organs in mice, which can be highly valuable for development and optimization of targeted cancer therapies.

Effect of pinhole shape on projection resolution

We are designing a dual-resolution pre-clinical SPECT system based on square-pinhole apertures for use in applications with a small field-of-view (FOV), such as cardiac imaging of mice. Square pinholes allow for increased sensitivity due to more efficient projection tiling on the detector compared to circular pinholes. Aperture fabrication techniques cannot produce a perfect square, giving the square pinholes some amount of roundedness at the corners. This work investigates how this roundedness affects the physical properties of projection images in terms of spatial resolution. Different pinhole full-acceptance angles and roundedness values were simulated. To facilitate a fair comparison, properties of the non-square pinholes were manipulated to yield pinholes with approximately the same sensitivity (to within 0.1%) and FOV (to within 0.5%) as those of the square pinholes, subsequently referred to as matched apertures. The aperture size (flat-to-flat edge length) of each non-square aperture was increased until its sensitivity was approximately equal to that of the square pinhole. Next, the full acceptance angle was increased until the FOV of each non-square aperture was approximately equivalent to that of the square pinhole. Sensitivity was calculated to include both the geometric and penetrative sensitivity of a point source, as well as the packing faction of the multi-pinhole collimator. Using the sensitivity-matched and FOV-matched apertures, spatial resolution was estimated. For the 0.3 mm, 0.5 mm, and 1 mm edge-length square apertures studied, the full-width at half-maximum widened as pinhole shape changed from square to circle, while full-width tenth-maximum showed little change. These results indicate that a perfect square pinhole shape is more desirable than a rounded-square pinhole with regard to spatial resolution when sensitivity and FOV-matched pinholes are compared.

Simultaneous acquisition of (99m)Tc- and (123)I-labeled radiotracers using a preclinical SPECT scanner with CZT detectors.

Simultaneous acquisition of (99m)Tc and (123)I was evaluated using a preclinical SPECT scanner with cadmium zinc telluride (CZT)-based detectors. 10-ml cylindrical syringes contained about 37MBq (99m)Tc-tetrofosmin ((99m)Tc-TF) or 37MBq (123)I-15-(p-iodophenyl)-3R,S-methyl pentadecanoic acid ((123)I-BMIPP) were used to assess the relationship between these SPECT radioactive counts and radioactivity. Two 10-ml syringes contained 100 or 300MBq (99m)Tc-TF and 100MBq (123)I-BMIPP to assess the influence of (99m)Tc upscatter and (123)I downscatter, respectively. A rat-sized cylindrical phantom also contained both 100 or 300MBq (99m)Tc-TF and 100MBq (123)I-BMIPP. The two 10-ml syringes and phantom were scanned using a pinhole collimator for rats. Myocardial infarction model rats were examined using 300MBq (99m)Tc-TF and 100MBq (123)I-BMIPP. Two 1-ml syringes contained 105MBq (99m)Tc-labeled hexamethylpropyleneamine oxime ((99m)Tc-HMPAO) and 35MBq (123)I-labeled N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl) nortropane ((123)I-FP-CIT). The two 1-ml syringes were scanned using a pinhole collimator for mice. Normal mice were examined using 105MBq (99m)Tc-HMPAO and 35MBq (123)I-FP-CIT. The relationship between SPECT radioactive counts and radioactivity was excellent. Downscatter contamination of (123)I-BMIPP exhibited fewer radioactive counts for 300MBq (99m)Tc-TF without scatter correction (SC) in 125-150keV. There was no upscatter contamination of (99m)Tc-TF in 150-175keV. In the rat-sized phantom, the radioactive count ratio decreased to 4.0% for 300MBq (99m)Tc-TF without SC in 125-150keV. In the rats, myocardial images and radioactive counts of (99m)Tc-TF with the dual tracer were identical to those of the (99m)Tc-TF single injection. Downscatter contamination of (123)I-FP-CIT was 4.2% without SC in 125-150keV. In the first injection of (99m)Tc-HMPAO and second injection of (123)I-FP-CIT, brain images and radioactive counts of (99m)Tc-HMPAO with the dual tracer in normal mice also were the similar to those of the (99m)Tc-HMPAO single injection. In the first injection of (123)I-FP-CIT and second injection of (99m)Tc-HMPAO, the brain images and radioactive counts with the dual tracer were not much different from those of the (123)I-FP-CIT single injection. Dual-tracer imaging of (99m)Tc- and (123)I-labeled radiotracers is feasible in a preclinical SPECT scanner with CZT detector. When higher radioactivity of (99m)Tc-labeled radiotracers relative to (123)I-labeled radiotracers is applied, correction methods are not necessarily required for the quantification of (99m)Tc- and (123)I-labeled radiotracers when using a preclinical SPECT scanner with CZT detector.