Multi-Isotope Capabilities of a Small-Animal Multi-Pinhole SPECT System

CdTe XG-Cam: A new high-resolution x-ray and gamma-ray camera for studies of the pharmacokinetics of radiopharmaceuticals in small animals

BACKGROUND

The recent emergence of targeted radionuclide therapy has increased the demand for imagers capable of visualizing pharmacokinetics in developing radiopharmaceuticals in the preclinical phase. Some radionuclides emit hard x-rays and gamma-rays below 100 keV, in which energy range the performance of conventional NaI scintillators is poor. Multipinhole collimators are also used for small animal imaging with a good spatial resolution but have a limited field of view (FOV).

PURPOSE

In this study, a new imager with high sensitivity over a wide FOV in the low-energy band ( < $<$ 100 keV) was developed for the pharmacokinetic study.

METHODS

We developed an x-ray and gamma-ray camera for high-resolution spectroscopy, named "CdTe XG-Cam," equipped with a cadmium telluride semiconductor detector and a parallel-hole collimator using a metal 3D printer. To evaluate the camera-system performance, phantom measurements with single and dual nuclides (99 m Tc $^{\rm 99m}{\rm Tc}$ ,111 In $^{111}{\rm In}$ , and125 I ) $^{125}{\rm I)}$ were performed. The performance for in vivo imaging was evaluated using tumor-bearing mice to which a nuclide (99 m Tc $^{\rm 99m}{\rm Tc}$ or125 I ) $^{125}{\rm I)}$  administered.

RESULTS

We simultaneously obtained information on111 In $^{111}{\rm In}$ and125 I $^{125}{\rm I}$ , which emit emission lines in the low-energy band with peak energies close to each other (23-26 keV for111 In $^{111}{\rm In}$ and 27-31 keV for125 I ) $^{125}{\rm I)}$ , and applied an analytical method based on spectral model fitting to determine the individual radioactivities accurately. In the small animal imaging, the distributions of the nuclide in tumors were accurately quantified and time-activity curves in tumors are obtained.

CONCLUSIONS

The demonstrated capability of our system to perform in vivo imaging suggests that the camera can be used for applications of pharmacokinetics research.

Effects of isoflurane anaesthesia depth and duration on renal function measured with [99mTc]Tc-mercaptoacetyltriglycine SPECT in mice

BACKGROUND

The influence of anaesthetic depth and the potential influence of different anaesthetic beds and thus different handling procedures were investigated in 86 severe combined immunodeficient (SCID) mice using semi-stationary dynamic single photon emission computed tomography (SPECT) for kidney scintigraphy. Therefore, isoflurane concentrations were adjusted using respiratory rate for low (80-90 breath/min) and deep anaesthesia (40-45 breath/min). At low anaesthesia, we additionally tested the influence of single bed versus 3-mouse bed hotel; the hotel mice were anaesthetized consecutively at ~ 30, 20, and 10 min before tracer injections for positions 1, 2, and 3, respectively. Intravenous [99mTc]Tc-MAG3 injection of ~ 28 MBq was performed after SPECT start. Time-activity curves were used to calculate time-to-peak (Tmax), T50 (50% clearance) and T25 (75% clearance).

RESULTS

Low and deep anaesthesia corresponded to median isoflurane concentrations of 1.3% and 1.5%, respectively, with no significant differences in heart rate (p = 0.74). Low anaesthesia resulted in shorter aortic blood clearance half-life (p = 0.091) and increased relative renal tracer influx rate (p = 0.018). A tendency toward earlier Tmax occurred under low anaesthesia (p = 0.063) with no differences in T50 (p = 0.40) and T25 (p = 0.24). Variance increased with deep anaesthesia. Compared to single mouse scans, hotel mice in position 1 showed a delayed Tmax, T50, and T25 (p < 0.05 each). Furthermore, hotel mice in position 1 showed delayed Tmax versus position 3, and delayed T50 and T25 versus position 2 and 3 (p < 0.05 each). No difference occurred between single bed and positions 2 (p = 1.0) and 3 (p = 1.0).

CONCLUSIONS

Deep anaesthesia and prolonged low anaesthesia should be avoided during renal scintigraphy because they result in prolonged blood clearance half-life, delayed renal influx and/or later Tmax. Vice versa, low anaesthesia with high respiratory rates of 80-90 rpm and short duration (≤ 20 min) should be preferred to obtain representative data with low variance.

Dual-radionuclide in vivo imaging of micro-metastasis and lymph tract with submillimetre resolution

Multi-radionuclide in vivo imaging with submillimetre resolution can be a potent tool for biomedical research. While high-resolution radionuclide imaging faces challenges in sensitivity, multi-radionuclide imaging encounters difficulty due to radiation contamination, stemming from crosstalk between radionuclides and Compton scattering. Addressing these challenges simultaneously is imperative for multi-radionuclide high-resolution imaging. To tackle this, we developed a high-spatial-resolution and high-energy-resolution small animal single-photon emission computed tomography (SPECT) scanner, named CdTe-DSD SPECT-I. We first assessed the feasibility of multi-tracer SPECT imaging of submillimetre targets. Using the CdTe-DSD SPECT-I, we performed SPECT imaging of submillimetre zeolite spheres absorbed with 125I- and subsequently imaged 125I-accumulated spheroids of 200-400 µm in size within an hour, achieving clear and quantitative images. Furthermore, dual-radionuclide phantom imaging revealed a distinct image of the submillimetre sphere absorbed with 125I- immersed in a 99mTc-pertechnetate solution, and provided a fair quantification of each radionuclide. Lastly, in vivo imaging was conducted on a cancer-bearing mouse with lymph node micro-metastasis using dual-tracers. The results displayed dual-tracer images of lymph tract by 99mTc-phytic acid and the submillimetre metastatic lesion by 125I-, shown to align with the immunofluorescence image.

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

BACKGROUND

The 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.

METHODS

NEMA 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.

RESULTS

The 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%.

CONCLUSIONS

NEMA 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.

Joint estimation of interaction position and energy deposition in semiconductor SPECT imaging sensors using fully connected neural network

Objective.Pixelated semiconductor detectors such as CdTe and CZT sensors suffer spatial resolution and spectral performance degradation induced by charge-sharing effects. It is critical to enhance the detector property through recovering the energy-deposition and position estimation.Approach.In this work, we proposed a fully-connected-neural-network-based charge-sharing reconstruction algorithm to correct the charge-loss and estimate the sub-pixel position for every multi-pixel charge-sharing event.Main results.Evident energy resolution improvement can be observed by comparing the spectrum produced by a simple charge-sharing addition method and the proposed energy correction methods. We also demonstrate that sub-pixel resolution can be achieved in projections obtained with a small pinhole collimator and an innovative micro-ring collimator.Significance.These achievements are crucial for multiple-tracer SPECT imaging applications, and for other semiconductor detector-based imaging modalities.

Performance evaluation of a preclinical SPECT/CT system for multi-animal and multi-isotope quantitative experiments

The aim was to study the performance of the U-SPECT⁶/CT E-class system for preclinical imaging, to later demonstrate the viability of simultaneous multi-animal and multi-isotope imaging with reliable quantitative accuracy. The performance of the SPECT was evaluated for two collimators dedicated for mouse (UHS-M) and rat imaging (UHR-RM) in terms of sensitivity, energy resolution, uniformity and spatial resolution. Point sources, hot‑rod and uniform phantoms were scanned, and additional tests were carried out to evaluate singular settings such as simultaneous multi-isotope acquisition and imaging with a multi-bed system. For in-vivo evaluation, simultaneous triple-isotope and multi-animal studies were performed on mice. Sensitivity for ^99mTc was 2370 cps/MBq for the UHS-M collimator and 493 cps/MBq for the UHR-RM. Rods of 0.6 mm and 0.9 mm were discernible with the UHS-M and UHR-RM collimators respectively, with optimized reconstruction. Uniformity in low counting conditions has proven to be poor (> 75%). Multi-isotope and multi-bed phantom acquisitions demonstrated accurate quantification. In mice, simultaneous multi-isotope imaging provided the separate distribution of 3 tracers and image quality of the multi-mouse bone scan was adequate. The U-SPECT⁶/CT E-class has shown good sensitivity and spatial resolution. This system provides quantitative images with suitable image quality for multi-mouse and multi-isotope acquisitions.

Simultaneous visualization of multiple radionuclides in vivo

The insufficient energy and spatial resolutions of radionuclide imaging with conventional scintillation detectors restrict the visualization of multiple radionuclides and of microstructures in tissue. Here we report the development and performance of an imaging system equipped with a cadmium telluride diode detector that achieves an energy resolution of 1.7% at 140 keV and a spatial resolution of 250 μm. The combination of high-resolution spectra fitted to an X-ray analysis model of the emission lines of the radionuclides in a chosen energy band allowed us to accurately determine individual radiation activities from three radionuclides to simultaneously visualize thyroid tissue (via intravenously administered iodine-125), mandibular lymph nodes (via the intramuscular injection of indium-111) and parotid lymph nodes (via a subcutaneous injection of technetium-99m) in mice. Multi-radionuclide imaging may find advantageous applications in biomedical imaging.

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/CT^PLUS, 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.

Performance evaluation of fifth-generation ultra-high-resolution SPECT system with two stationary detectors and multi-pinhole imaging

Background

Small-animal single-photon emission computed tomography (SPECT) systems with multi-pinhole collimation and large stationary detectors have advantages compared to systems with moving small detectors. These systems benefit from less labour-intensive maintenance and quality control as fewer prone parts are moving, higher accuracy for focused scans and maintaining high resolution with increased sensitivity due to focused pinholes on the field of view. This study aims to investigate the performance of a novel ultra-high-resolution scanner with two-detector configuration (U-SPECT5-E) and to compare its image quality to a conventional micro-SPECT system with three stationary detectors (U-SPECT⁺).

Methods

The new U-SPECT5-E with two stationary detectors was used for acquiring data with ^99mTc-filled point source, hot-rod and uniformity phantoms to analyse sensitivity, spatial resolution, uniformity and contrast-to-noise ratio (CNR). Three dedicated multi-pinhole mouse collimators with 75 pinholes each and 0.25-, 0.60- and 1.00-mm pinholes for extra ultra-high resolution (XUHR-M), general-purpose (GP-M) and ultra-high sensitivity (UHS-M) imaging were examined. For CNR analysis, four different activity ranges representing low- and high-count settings were investigated for all three collimators. The experiments for the performance assessment were repeated with the same GP-M collimator in the three-detector U-SPECT⁺ for comparison.

Results

Peak sensitivity was 237 cps/MBq (XUHR-M), 847 cps/MBq (GP-M), 2054 cps/MBq (UHS-M) for U-SPECT5-E and 1710 cps/MBq (GP-M) for U-SPECT⁺. In the visually analysed sections of the reconstructed mini Derenzo phantoms, rods as small as 0.35 mm (XUHR-M), 0.50 mm (GP-M) for the two-detector as well as the three-detector SPECT and 0.75 mm (UHS-M) were resolved. Uniformity for maximum resolution recorded 40.7% (XUHR-M), 29.1% (GP-M, U-SPECT5-E), 16.3% (GP-M, U-SPECT⁺) and 23.0% (UHS-M), respectively. UHS-M reached highest CNR values for low-count images; for rods smaller than 0.45 mm, acceptable CNR was only achieved by XUHR-M. GP-M was superior for imaging rods sized from 0.60 to 1.50 mm for intermediate activity concentrations. U-SPECT5-E and U-SPECT⁺ both provided comparable CNR.

Conclusions

While uniformity and sensitivity are negatively affected by the absence of a third detector, the investigated U-SPECT5-E system with two stationary detectors delivers excellent spatial resolution and CNR comparable to the performance of an established three-detector-setup.

Capabilities of multi-pinhole SPECT with two stationary detectors for in vivo rat imaging

We aimed to investigate the image quality of the U-SPECT5/CT E-Class a micro single-photon emission computed tomography (SPECT) system with two large stationary detectors for visualization of rat hearts and bones using clinically available ^99mTc-labelled tracers. Sensitivity, spatial resolution, uniformity and contrast-to-noise ratio (CNR) of the small-animal SPECT scanner were investigated in phantom studies using an ultra-high-resolution rat and mouse multi-pinhole collimator (UHR-RM). Point source, hot-rod, and uniform phantoms with ^99mTc-solution were scanned for high-count performance assessment and count levels equal to animal scans, respectively. Reconstruction was performed using the similarity-regulated ordered-subsets expectation maximization (SROSEM) algorithm with Gaussian smoothing. Rats were injected with ~ 100 MBq [^99mTc]Tc-MIBI or ~ 150 MBq [^99mTc]Tc-HMDP and received multi-frame micro-SPECT imaging after tracer distribution. Animal scans were reconstructed for three different acquisition times and post-processed with different sized Gaussian filters. Following reconstruction, CNR was calculated and image quality evaluated by three independent readers on a five-point scale from 1 = “very poor” to 5 = “very good”. Point source sensitivity was 567 cps/MBq and radioactive rods as small as 1.2 mm were resolved with the UHR-RM collimator. Collimator-dependent uniformity was 55.5%. Phantom CNR improved with increasing rod size, filter size and activity concentration. Left ventricle and bone structures were successfully visualized in rat experiments. Image quality was strongly affected by the extent of post-filtering, whereas scan time did not have substantial influence on visual assessment. Good image quality was achieved for resolution range greater than 1.8 mm in bone and 2.8 mm in heart. The recently introduced small animal SPECT system with two stationary detectors and UHR-RM collimator is capable to provide excellent image quality in heart and bone scans in a rat using standardized reconstruction parameters and appropriate post-filtering. However, there are still challenges in achieving maximum system resolution in the sub-millimeter range with in vivo settings under limited injection dose and acquisition time.