Introduction

Preclinical SPECT scanners are an important tool for molecular imaging in biomedical research and the development of new medicine [1]. Several recent scanners incorporate semiconductor materials such as cadmium zinc telluride (CZT) [2–6]. The CZT-based scanners improve energy and spatial resolution, as compared with sodium iodide (NaI) scintillation detectors, which are used in conventional SPECT scanners, in not only preclinical animal imaging but also clinical imaging.

With conventional SPECT scanners, it is difficult to separate 99mTc (140 keV) and 123I (159 keV) images because the emission energies of these radiotracers are close. Accurate quantification of dual-isotope imaging is also difficult because the images are affected by scatter, cross-talk, attenuation and distance-dependent collimator response. Especially, there is substantial influence on not only downscattered 123I photons in the energy window of 99mTc, but also cross-talk from primary photons of each radionuclide. Attempts have already been made to discriminate downscatter contamination and cross-talk using an energy window correction method in clinical SPECT scanners with NaI scintillation detectors [6–12]. Constrained spectral factor analysis and artificial neural networks are two promising approaches to compensate for scatter, cross-talk, and high-energy septal penetration in simultaneous 99mTc and 123I imaging [7, 8]. Another method, a Monte Carlo-based joint ordered-subset expectation maximization iterative reconstruction algorithm, was applied to 99mTc/123I brain or cardiac SPECT imaging [6, 9–11]. However, these methods are generally complicated and require many energy windows.

In this study, simultaneous acquisition of 99mTc- and 123I-labeled radiotracers was evaluated on a preclinical SPECT scanner with a CZT-based semiconductor detector. To assess each image and the quantification, we used cylindrical syringes, cylindrical phantoms and myocardial infarction model rats injected with 99mTc-labeled tetrofosmin (99mTc-TF) [13] and 123I-labeled 15-(p-iodophenyl)-3R,S-methyl pentadecanoic acid (123I-BMIPP) [14–16], and syringes and normal mice injected with 99mTc-labeled hexamethylpropyleneamine oxime (99mTc-HMPAO) and 123I-labeled N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane (123I-FP-CIT).

Materials and methods

Phantoms

Two kinds of phantoms for the rat study were created: 10-ml cylindrical syringes (diameter 15.8 mm) and 250-ml rat-sized cylindrical phantoms (diameter 80 mm). For evaluation of relationship between SPECT radioactive counts (1 min acquisition) and radioactivity of 99mTc and 123I, 10-ml cylindrical syringes contained about 37 MBq 99mTc-TF or 123I-BMIPP were prepared and then attenuated in accordance with the radioactive decay. The radioactivity was measured using dose calibrator (Capintec, Inc.)

In two experiments with the two 10-ml cylindrical syringes, one syringe contained 100 MBq 99mTc-TF (Nihon Medi-physics Co., Ltd.) and the other contained 100 MBq 123I-BMIPP (Nihon Medi-physics Co., Ltd.) to evaluate equal doses of 99mTc-TF and 123I-BMIPP. In another experiment, one syringe contained 300 MBq 99mTc-TF and the other contained 100 MBq 123I-BMIPP to evaluate high doses of 99mTc-TF and 123I-BMIPP. For the rat-sized cylindrical phantoms, we evaluated two different versions. The first contained both 100 MBq 99mTc-TF and 100 MBq 123I-BMIPP and second had both 300 MBq 99mTc-TF and 100 MBq 123I-BMIPP.

For the mice study, one 1-ml cylindrical syringe (diameter 4.7 mm) contained 105 MBq 99mTc-HMPAO (Nihon Medi-physics Co., Ltd.) and the other contained 35 MBq 123I-FP-CIT (Nihon Medi-physics Co., Ltd.).

Animals

All animal procedures were approved by the institutional committee at the Medical and Pharmacological Research Center Foundation and were conducted in compliance with the American Heart Association requirements regarding the use of research animals. Eight male Wister rats (8–11 weeks old, 250–320 g) and 10 male normal mice (6–8 weeks old, 31–35 g) were purchased from Japan SLC Inc. and were housed for 1 week under a 12-h light/12-h dark cycle with free access to food and water. The rats were anesthetized with an intraperitoneal injection of pentobarbital (30 mg/kg) and maintained with 2 % isoflurane (Abbott Laboratories), and then intubated and ventilated using a small animal ventilator (SN-480-7 × 2T, Shinano). After left thoracotomy and exposure of the heart, a 7–0 polypropylene suture on a small curved needle was passed through the myocardium beneath the proximal portion of the left coronary artery (LCA), and both ends of the suture were passed through a small vinyl tube to make a snare. The suture material was pulled tightly against the vinyl tube to occlude the LCA. Myocardial ischemia was confirmed by ST-segment elevation on the electrocardiogram and regional cyanosis of the myocardial surface. The LCA was occluded for 30 min, and reperfusion was obtained by release of the snare and confirmed by a myocardial blush over the area at risk.

Description of preclinical SPECT/CT camera

The performance and features of the eXplore speCZT (GE Healthcare) have been reported previously [4]. Briefly, this system has a stationary detector with interchangeable rotating collimators. The detector consists of 10 CZT-based detector panels surrounding the field of view (FOV). Each CZT detector panel consists of 32 × 32 arrays with a pixel size of 2.46 mm × 2.46 mm.

We used a 5-pinhole collimator for rat imaging because the pinhole collimators are designed for high-resolution images with small regions of interest [17, 18]. The main characteristics of the pinhole collimators are: diameter of pinhole 1 mm, FOV diameter of single-bed position 76 mm, axial FOV of single-bed position 38 mm, bore diameter 89 mm and radius of rotation/focal length 50/70 mm. The system saves each event of the acquired data in a list-mode format including its energy, and gated electrocardiogram (ECG)/respiratory motion trigger for later, specialized analysis. Performance tests with the pinhole collimator showed that the transaxial and axial system resolution with resolution correction, and sensitivity of the scanner were, respectively, 1.20/2.18 mm [full-width half maximum (FWHM)/full-width tenth maximum (FWTM)] and 1.11/2.02 mm (FWHM/FWTM), and 138.5 cpm/MBq using a 99mTc radiotracer.

SPECT/CT scan with a pinhole collimator for rats

Experiments with cylindrical syringes, cylindrical phantoms and rats were performed using the eXplore speCZT SPECT/CT scanner with a 5-pinhole collimator for rat imaging. The voxel size of the image matrix was 0.5 mm × 0.5 mm × 0.5 mm because the voxel size of the single-orbit SPECT scan was fixed to 0.5 mm, which was determined by the step size of the table motion. The energy window for SPECT acquisition was set to 50–180 keV by default.

For the syringe and phantom experiments, two 10-ml cylindrical syringes or 250-ml rat-sized cylindrical phantoms were placed on the scanner table in the center of the FOV and scanned for approximately 30 min using the pinhole collimator.

In rat experiments, the helical scan mode was used to image the myocardium of the rats. Eight rats with myocardial infarction were fasted, with water supplied ad libitum and no food, at least 4 h before the SPECT experiments. The rats were anesthetized with 1.5–2.0 % isoflurane (Abbott Laboratories) and placed in a supine position on the scanner table. Limbs were fixed using surgical tape. Stainless steel electrodes were subsequently inserted into the skin of the right and left forelimbs to monitor and record the ECG data. Before SPECT scanning, the orientation of the myocardium was determined using a laser beam and CT imaging on the scanner. For CT images, the tube voltage and current of the X-ray tube were 60 kVp and 40 mA, respectively. The rats were injected with approximately 300 MBq of 99mTc-TF via the tail vein. Rats were scanned from 5 min after the injection using imaging parameters of 25 s/view and 72 views/pinhole (total 360 view) in 1° increments. Immediately after the first scan of 99mTc-TF, the same rats were injected with 100 MBq of 123I-BMIPP and scanned again using the same scanning protocols and parameters.

SPECT/CT scan with a pinhole collimator for mice

For the 1-ml cylindrical syringe and mice study, experiments were performed using a 7-pinhole collimator for mice imaging. The voxel size of the image matrix was 0.5 mm × 0.5 mm × 0.5 mm, which was determined by the step size of the table motion. The energy window for SPECT acquisition was set to 50–180 keV by default in the list mode.

The setup for the syringes and 10 normal mice was the same as that for the rat study. Before SPECT scanning, the orientation of the brain was determined using a laser beam and CT imaging on the scanner. For CT images, the tube voltage and current of the X-ray tube were 60 kVp and 40 mA, respectively. Five of the 10 mice were injected with approximately 105 MBq of 99mTc-HMPAO via the tail vein. Mice were scanned from 5 min after the injection using imaging parameters of 36 s/view and 50 views/pinhole (total 350 view) in 1.06° increments. Immediately after the first scan of 99mTc-HMPAO, the same rats were injected with approximately 35 MBq of 123I-FP-CIT and scanned again using the same scanning protocols and parameters from approximately 3 h after the injection.

The other five mice were injected with approximately 35 MBq of 123I-FP-CIT via the tail vein and then scanned from approximately 3 h after the injection. Immediately after the first scan of 123I-FP-CIT, the same rats were injected with approximately 105 MBq of 99mTc-HMPAO and scanned again from 5 min after the injection.

Reconstruction settings

All SPECT data were reconstructed using the three-dimensional maximum-likelihood expectation maximization algorithm with 50 iterations, with distance-dependent collimator response correction (DCC) [4]. For the parameters, the energy window was set to 125–150 keV for 99mTc imaging and 150–175 keV for 123I imaging using acquisition data of 50–180 keV. Attenuation correction (AC) was not applied because AC provided low uniformity in the phantom study, and radioactive counts with and without AC were not that much different in the rat myocardial [19] mice brain studies (data not shown) in this study. Post-reconstruction filtering was also not applied because the filter affects the radioactive counts in the SPECT images. The scatter correction (SC) uses the triple-energy window (TEW) method for experiments with a single tracer of 99mTc [20]. Specifically, the TEW method was applied to the experiments with 10-ml cylindrical syringes and rat-sized phantom, and rats in the experiments with a single tracer of 99mTc-TF, and we set up to 3 keV in width for the left and right scatter as sub-windows by default. In the experiments with the dual tracers, the TEW method was not available because we could not set up the sub-window. AC, SC and DCC can be selected for the reconstruction method in our scanner, but DCC is always applied to improve image resolution.

Image analysis

After reconstruction of SPECT data, radioactive counts were obtained using the AMIDE medical imaging data examiner and OsiriX (http://homepage.mac.com/rossetantoine/osirix). The voxels of interest (VOIs) were located in the center of the syringes and phantoms. The spherical VOIs were approximately 5 mm in diameter for the experiments with 1-ml syringes, and approximately 10 mm in diameter for the 10-ml syringes and 250-ml rat-sized cylindrical phantoms. We placed spherical VOIs over normal and myocardial infarction areas in the rat study and striatum and whole brain in the mice study. In experiments with the rat-sized cylindrical phantoms and 1-ml syringes for the mice study, radioactive counts of 99mTc at 125–150 keV in the dual-tracer images were divided by those of the 99mTc single-tracer images in 125–150 keV. In addition, radioactive counts of 123I at 150–175 keV in the dual-tracer images were divided by those of the 123I single-tracer images in 150–175 keV, respectively.

Statistical analyses

A statistical software package (JMP® version 9, SAS Institute Inc.) was used for statistical analysis. We applied a paired t test, and statistically significant differences were defined as P < 0.01.

Results

In 10-ml cylindrical syringes including 100 MBq 99mTc or 100 MBq 123I, the relationship between SPECT radioactive counts and radioactivity was excellent (Fig. 1).

Fig. 1
figure 1

The relationship between SPECT radioactive counts and 100 MBq 99mTc (a) and 100 MBq 123I (b) images

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In two syringes containing 100 MBq of 99mTc-TF or 123I-BMIPP, the peaks of the energy spectrum were at the same levels (Fig. 2a). Accumulation of 99mTc-TF images in the 125–150 keV energy window were slightly higher than that of 123I-BMIPP images in the 150–175 keV energy window (Fig. 2b). Mean radioactive counts per voxel of 99mTc-TF in 125–150 keV were 7 % higher than those of 123I-BMIPP in 150–175 keV (Table 1). When SC was applied, downscatter contamination of 123I-BMIPP was approximately 3.5 % of the counts per voxel of 99mTc-TF in 125–150 keV, whereas in the case without SC, the downscatter contamination was approximately 13.2 % of the counts per voxel of 99mTc-TF in 125–150 keV. With and without SC, no upscatter contamination of 99mTc-TF was observed in 150–175 keV.

Fig. 2
figure 2

Energy spectrum (a) and SPECT images without SC (b) of two cylindrical syringes with 100 MBq 99mTc-TF or 100 MBq 123I-BMIPP. Energy peak of 99mTc-TF was almost the same as that of 123I-BMIPP. Accumulation of 99mTc-TF images in the 125–150 keV energy window were slightly higher than that of 123I-BMIPP images in the 150–175 keV energy window. Downscatter contaminations of 123I-BMIPP in 125–150 keV were approximately 3.5 and 13.2 % of the radioactive counts for 99mTc-TF with and without SC, respectively. Upscatter contamination of 99mTc-TF was not observed in 150–175 keV

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Table 1 Radioactive counts per voxel in the 10-ml cylindrical syringes including 100 MBq 99mTc-TF and 100 MBq 123I-BMIPP
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In two syringes including 300 MBq 99mTc-TF and 100 MBq 123I-BMIPP, the peak of the energy spectrum of 99mTc-TF was almost 3-fold higher than that of 123I-BMIPP (Fig. 3a). Mean radioactive counts per voxel of 99mTc-TF in 125–150 keV were also 3.3-fold higher than that of 123I-BMIPP in 150–175 keV without SC (Table 2). Downscatter contaminations of 123I-BMIPP were approximately 0.9 and 4.0 % of the radioactive counts for 99mTc-TF in 125–150 keV with and without SC, while there was no upscatter contamination of 99mTc-TF in 150–175 keV.

Fig. 3
figure 3

Energy spectrum (a) and SPECT images without SC (b) of two cylindrical syringes with 300 MBq 99mTc-TF or 100 MBq 123I-BMIPP. Energy peak of 99mTc-TF was approximately 3-fold higher than that of 123I-BMIPP. 99mTc-TF image in 125–150 keV was significantly different from that of 123I-BMIPP in 150–175 keV. Downscatter contaminations of 123I-BMIPP in 125–150 keV were, respectively approximately 0.9 and 4.1 % of the radioactive counts for 99mTc-TF with and without SC

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Table 2 Radioactive counts per voxel in the 10-ml cylindrical syringes including 300 MBq 99mTc-TF and 100 MBq 123I-BMIPP
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In the rat-sized cylindrical phantom including 100 MBq of 99mTc-TF and 123I-BMIPP as the dual tracer (Table 3), the SC of the TEW method was not applied because we could not increase the number of energy windows in the TEW method for dual-tracer imaging. The ratios of radioactive counts of the dual tracer without SC to single tracer of 99mTc-TF in 125–150 keV became 1.21. When 300 MBq 99mTc-TF and 100 MBq 123I-BMIPP were used, the ratios were 1.08. There was almost no upscatter contamination of 99mTc-TF in 150–175 keV.

Table 3 Ratios of radioactive counts in dual tracer without SC to single tracer with SC on 99mTc-TF images in 125–150 keV and 123I-BMIPP images in 150–175 keV in the rat-sized cylindrical phantom
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In the rat model of myocardial infarction (Fig. 4), the 99mTc-TF images in 125–150 keV for the dual tracer without SC were not different from those of 99mTc-TF for the single tracer with SC. 123I-BMIPP images showed no accumulation in the infarction area compared to 99mTc-TF. Mean radioactive counts of 99mTc-TF in 125–150 keV for the dual tracer were not much different from those of 99mTc-TF for the single tracer in the normal and infarction areas when SC was applied for the single tracer (Table 4). In the two syringes including high 99mTc-HMPAO dose and 123I-FP-CIT, the peak of the energy spectrum of 99mTc-HMPAO was almost three times that of 123I-FP-CIT (Fig. 5). Mean radioactive counts per voxel of 99mTc-HMPAO in 125–150 keV were 2.9-fold higher than that of 123I-FP-CIT in 150–175 keV without SC (Table 5). Downscatter contaminations of 123I-FP-CIT were approximately 1.4 % and 4.2 % of the radioactive counts for 99mTc-HMPAO in 125–150 keV with and without SC, while there was no upscatter contamination of 99mTc-HMPAO in 150–175 keV.

Fig. 4
figure 4

SPECT images in a rat model of myocardial infarction using 300 MBq 99mTc-TF in the single tracer and 300 MBq 99mTc-TF and 100 MBq 123I-BMIPP in the dual tracer. The myocardial SPECT images of 99mTc-TF in the dual tracer without SC were similar to those of 99mTc-TF in the single tracer with SC. 123I-BMIPP images showed a slight mismatch with 99mTc-TF images in the posterior wall at risk of acute myocardial infarction as memory imaging

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Table 4 Radioactive counts per voxel of 99mTc-TF images in single tracer with SC and in dual tracer without SC for the myocardial infarction model rats
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Fig. 5
figure 5

Energy spectrum (a) and SPECT images without SC (b) of two 1-ml cylindrical syringes with 105 MBq 99mTc-HMPAO or 35 MBq 123I-FP-CIT. Energy peak of 99mTc-HMPAO was approximately 3-fold higher than that of 123I-FP-CIT. 99mTc-HMPAO image in 125–150 keV was significantly different from that of 123I-FP-CIT in 150–175 keV. Downscatter contaminations of 123I-FP-CIT in 125–150 keV were, respectively, approximately 1.4 and 4.2 % of the radioactive counts for 99mTc-HMPAO with and without SC

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Table 5 Radioactive counts per voxel in the 1-ml cylindrical syringes including high 99mTc-HMPAO dose and 123I-FP-CIT
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In the mice study, the 99mTc-HMPAO images for the dual tracer in 125–150 keV without SC were not different from those of 99mTc-HMPAO for the single tracer with SC in the first injection of 99mTc-HMPAO and second injection of 123I-FP-CIT (Fig. 6). In the first injection of 123I-FP-CIT and second injection of 99mTc-HMPAO, the brain images and mean radioactive counts were also not much different from those of the 123I-FP-CIT single injection (Fig. 7; Table 6).

Fig. 6
figure 6

Fusion images of SPECT and CT in normal mice using a single tracer of 105 MBq 99mTc-HMPAO, and dual tracers of the first injection with 105 MBq 99mTc-HMPAO and second injection with 35 MBq 123I-FP-CIT. 99mTc-HMPAO images in the dual tracer without SC were similar to those of 99mTc-HMPAO in the single tracer with SC

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Fig. 7
figure 7

Fusion images of SPECT and CT in normal mice using 35 MBq 123I-FP-CIT in the single tracer, and the first injection with 105 MBq 99mTc-HMPAO and second injection with 35 MBq 123I-FP-CIT in the dual tracer. 123I-FP-CIT images in the dual tracer without SC were not different from those of 123I-FP-CIT in the single tracer with SC

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Table 6 Radioactive counts per voxel of 99mTc-HMPAO images in 125–150 keV and 123I-FP-CIT images in 150–175 keV on normal mice injected single tracer with SC and dual tracer without SC
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Discussion

The simultaneous acquisition of 99mTc-TF and 123I-BMIPP yields myocardial blood flow and metabolism of rats under identical conditions. To detect cardiac disease more clearly, it is important for cardiac studies of rats to achieve more accurate quantification and anatomical registration between the two images of 99mTc-TF and 123I-BMIPP. The quantification and anatomical registration are also crucial for brain studies of mice using 99mTc-HMPAO and 123I-FP-CIT. This dual-tracer imaging accordingly reduces the experiment time and burden for research staff [6–12]. The radioactive counts and each image of simultaneously acquired 99mTc and 123I were evaluated using a preclinical SPECT scanner with a CZT-based semiconductor detector. Although we can select if AC, SC and DCC are applied in the reconstruction method on this SPECT scanner, AC is not applied because AC provided low uniformity and there is no hard substance such as bone in the chest region including the heart and scull of the mice (0.2 mm) which is much smaller thickness than that of rats (0.5–1.0 mm) and human (about 10 mm) [21], but, DCC is always applied to improve significant SPECT images. Scatter and cross-talk effects of 99mTc and 123I mainly hamper the simultaneous acquisition of dual tracers whose photon energies are close.

The relationship between SPECT radioactive counts and radioactivity of 99mTc or 123I was excellent (Fig. 1). However, each radioactivity of 99mTc and 123I in dual tracers could not be accurately separated and measured by dose calibrator and ex vivo autoradiography. Therefore, we used SPECT radioactive counts for evaluation of dual tracers in our study.

In the experiment with the cylindrical syringe, upscatter contamination of 99mTc-TF was not incorporated in the 150–175 keV energy window of 123I-BMIPP without AC and SC (Tables 1, 2, 5). The ratio of downscatter contamination from 123I-BMIPP in 125–150 keV divided by 300 MBq 99mTc-TF without SC (4.0 %) produces results similar to when divided by 100 MBq 99mTc-TF with SC in 125–150 keV (3.5 %) because downscatter contamination of 123I-BMIPP exhibited fewer radioactive counts of 300 MBq 99mTc-TF.

In experiments with a rat-sized cylinder phantom (Table 3), when 300 MBq 99mTc-TF and 100 MBq 123I-BMIPP were used, there was little influence from the downscatter contamination of 123I-BMIPP in 125–150 keV despite SC as the radioactivity of 99mTc-TF was higher than 123I-BMIPP. CZT detectors and pinhole collimators yield a narrow energy spectrum [2–6], and preclinical SPECT images are typically much less degraded by photon scattering than clinical SPECT images because of smaller body dimensions. Kao et al. has also reported that downscatter contamination of 123I-ADAM was less affected by radioactive counts of 99mTc-TRODAT in dual-tracer imaging of 99mTc-TRODAT and 123I-ADAM when the radioactivity of 99mTc-TRODAT was approximately 3.5-fold higher than that of 123I-ADAM [12]. In addition, there was no upscatter contamination of 99mTc-TF in 150–175 keV. Therefore, each accurate radioactive count of 99mTc and 123I will be provided by the narrow energy spectrum of radionuclides using a CZT detector with pinhole collimator and/or SC and DCC if the radioactivity of the injected 99mTc-TF tracer is increased. Although the simultaneous acquisition of 99mTc and 201Tl has already been reported using a clinical SPECT scanner with a CZT detector, AC and DCC were not applied to measure accurate radioactive counts [6]. Especially, DCC should be applied to the simultaneous acquisition of 99mTc and 123I in preclinical SPECT imaging with CZT detector. If the correction methods are unavailable, we should increase the radioactivity of 99mTc to decrease the downscatter contamination of 123I.

In the rat model of myocardial infarction using 300 MBq 99mTc-TF and 100 MBq 123I-BMIPP, the 99mTc-TF images in 125–150 keV without SC of the TEW method in the dual tracer were not significantly different from those of the single tracer with SC of the TEW method when DCC was applied (Fig. 3). The uptake of 99mTc-TF in the myocardial infarction significantly decreased in comparison with that in the normal area (Table 5). In 123I-BMIPP images, the infarct areas showed no accumulation compared to those in 99mTc-TF images because 123I-BMIPP images yielded memory imaging of the infarction area [14, 15]. In the 125–150 keV energy window, radioactive counts in dual tracers were not significantly different from those of 99mTc-TF in the single tracer in normal and infarction areas when DCC were applied (Table 5).

Since 123I-FP-CIT specifically accumulates in striatum [23], we evaluated the accumulation in both 99mTc-HMPAO and 123I-FP-CIT images. In striatum and whole brain of normal mice, the 99mTc-HMPAO images and radioactive counts in 125–150 keV for the dual tracer without SC were not different from those for the single tracer with SC in the first injection of 99mTc-HMPAO (Fig. 6; Table 6) because, in human study, accumulation of 99mTc-HMPAO maintains steady-state from early phase of 5 min to late phase of 8 h after the injection [22]. Brain images and radioactive counts of 123I-FP-CIT in striatum were also similar between the single tracer of 123I-FP-CIT (Fig. 7; Table 6). To decrease radioactivity of 99mTc-labeled radiotracers for dual-tracer imaging, we will develop a program for the TEW method for dual-tracer imaging in the future. In addition, we should consider the cardiac motion of rats in this study. More accurate quantification of the dual tracers may be achieved using gated SPECT.

Conclusion

Dual-tracer imaging of 99mTc- and 123I-labeled radiotracers is feasible in small animal studies using a preclinical SPECT scanner with CZT detectors. Radioactivity of the injected 99mTc-labeled radiotracers should be increased as much as possible to improve quantification of 99mTc-labeled radiotracers in dual-tracer imaging if the correction methods cannot be appropriately applied. In this case, correction methods are not necessarily required for the quantification of 99mTc- and 123I-labeled radiotracers when using a preclinical SPECT scanner with CZT detector.