Molecular Imaging in the College of Optical Sciences - An Overview of Two Decades of Instrumentation Development.

During the past two decades, researchers at the University of Arizona's Center for Gamma-Ray Imaging (CGRI) have explored a variety of approaches to gamma-ray detection, including scintillation cameras, solid-state detectors, and hybrids such as the intensified Quantum Imaging Device (iQID) configuration where a scintillator is followed by optical gain and a fast CCD or CMOS camera. We have combined these detectors with a variety of collimation schemes, including single and multiple pinholes, parallel-hole collimators, synthetic apertures, and anamorphic crossed slits, to build a large number of preclinical molecular-imaging systems that perform Single-Photon Emission Computed Tomography (SPECT), Positron Emission Tomography (PET), and X-Ray Computed Tomography (CT). In this paper, we discuss the themes and methods we have developed over the years to record and fully use the information content carried by every detected gamma-ray photon.

Imaging of rat cerebral ischemia-reperfusion injury using(99m)Tc-labeled duramycin.

OBJECTIVES: Prompt identification of necrosis and apoptosis in the infarct core and penumbra region is critical in acute stroke for delineating the underlying ischemic/reperfusion molecular pathologic events and defining therapeutic alternatives. The objective of this study was to investigate the capability of (99m)Tc-labeled duramycin in detecting ischemia-reperfusion injury in rat brain after middle cerebral artery (MCA) occlusion. METHODS: Ischemic cerebral injury was induced in ten rats by vascular insertion of a nylon suture in the left MCA for 3 hr followed by 21-24hr reperfusion. After i.v. injection of (99m)Tc-duramycin (1.0-3.5 mCi), dynamic cerebral images were acquired for 1 hr in six rats using a small-animal SPECT imager. Four other rats were imaged at 2 hr post-injection. Ex vivo images were obtained by autoradiography after sacrifice. Histologic analyses were performed to assess cerebral infarction and apoptosis. RESULTS: SPECT images showed that (99m)Tc-duramycin uptake in the left cerebral hemisphere was significantly higher than that in the right at 1 and 2 hr post-injection. The level of radioactive uptake in the ischemic brain varied based on ischemic severity. The average ratio of left cerebral hot-spot uptake to right hemisphere radioactivity, as determined by computerized ROI analysis, was 4.92±0.79. Fractional washout at 1 hr was 38.2±4.5% of peak activity for left cerebral hot-spot areas and 80.9±2.0% for remote control areas (P<0.001). Based on triphenyltetrazolium chloride staining and autoradiograph image data, the hotspot uptake may be associated primarily with the ischemic penumbra, in which high apoptotic activity was observed by cleaved caspase-3 immunocytochemical staining. CONCLUSIONS: (99m)Tc-duramycin SPECT imaging may be useful for detecting and quantifying ongoing apoptotic neuronal cell loss induced by ischemia-reperfusion injury.

Kinetic characterization of a novel cationic (99m)Tc(I)-tricarbonyl complex, (99m)Tc-15C5-PNP, for myocardial perfusion imaging.

BACKGROUND: Intense liver uptake of (99m)Tc-sestamibi (MIBI) often interferes with visualization of myocardial perfusion in the inferior wall of the left ventricle. To develop improved myocardial perfusion agents, crown ether-containing dithiocarbamates and bisphosphines have been introduced in recent years. This study was designed to investigate the myocardial imaging properties and in vivo kinetics of a cationic (99m)Tc(I)-tricarbonyl complex, (99m)Tc-15C5-PNP, in comparison with MIBI. METHODS: Dynamic cardiac images were acquired for 60 minutes after intravenous tracer injection using a small-animal SPECT system in healthy control rats and rats with myocardial infarcts. Myocardial and liver time-activity curves were generated for radiopharmaceutical kinetic analysis. RESULTS: Good visualization of the left ventricular wall and perfusion defects could be achieved 20 minutes after (99m)Tc-15C5-PNP administration. (99m)Tc-15C5-PNP images in all hearts with infarcts showed perfusion defects, which were comparable to MIBI images. The kinetic curves plotted from 1 to 60 minutes demonstrated that (99m)Tc-15C5-PNP has a shorter washout half-life (6.4 ± 3.2 vs 124 ± 30.5 minutes, P < .01) in the liver, lower residual liver activity (14.5 ± 10.2% vs 36.5 ± 28.9%, P < .01), and higher heart/liver ratio than MIBI. CONCLUSIONS: (99m)Tc-15C5-PNP has potential for rapid myocardial perfusion imaging with low liver uptake.

Evaluating the protective role of ischaemic preconditioning in rat hearts using a stationary small-animal SPECT imager and 99mTc-glucarate.

OBJECTIVE: To examine the protective role of ischaemic preconditioning (IPC) in rat hearts using Tc-glucarate (GLA) and a stationary SPECT imager, FastSPECT. METHODS: Twenty-four rats with 30 min myocardial ischaemia and 150 min reperfusion (IR) were studied as follows. The IPC group (n=6) underwent IPC (five cycles of 4 min ligation of the left coronary artery and reflow) before IR. The control group (n=7) was treated by IR without IPC. The SPT group (n=6) was subjected to IPC and an adenosine antagonist, 8-(p-sulfophenyl)-theophylline (SPT). The vehicle group (n=5) received IPC and SPT carrier vehicle. GLA was delivered intravenously 30 min post-reperfusion, and 2-h dynamic cardiac images were acquired by FastSPECT. RESULTS: GLA showed 'hot-spot' accumulation in the ischaemic area-at-risk (IAR) and exhibited lower retention (% 5 min peak) in the IPC and vehicle groups (33.8+/-2.6 vs. 35.7+/-9.2, P>0.05) than in the control and SPT groups (63.1+/-5.3 vs. 54.8+/-4.8, P>0.05). The infarct size (% IAR) was larger in the control and SPT groups (48.2+/-6.3 vs. 41.7+/-6.3, P>0.05) than that in the IPC and vehicle groups (21.0+/-1.9 vs. 19.1+/-4.6, P>0.05). In terms of the ex-vivo IAR-to-normal radioactivity ratio, there was a statistical difference between the control and IPC groups (7.4+/-0.9 vs. 3.0+/-0.4), as well as the SPT and vehicle groups (7.4+/-1.0 vs. 3.4+/-0.5). CONCLUSION: IPC offers cardioprotection and relates to the activation of adenosine receptors in rat hearts. FastSPECT GLA imaging is not only useful in detecting early ischaemia-reperfusion injury, but also valuable in evaluating cardioprotection.

An implantable synthetic SPECT lesion: a bridge from phantom to reality.

UNLABELLED: Small-animal imaging systems are often characterized using phantoms, which may not predict performance in clinical applications. An implantable synthetic SPECT lesion would facilitate characterization of lesion detectability in a living animal. METHODS: Anion-exchange columns with bed volumes of 100-300 nL were constructed from medical-grade polyvinyl chloride tubing and resin. The columns were tested in an excised mouse femur and implanted in the femur of a living mouse. Imaging was performed using a prototype dual-modality SPECT/CT system. RESULTS: Activity of 7.4-22.2 MBq (0.2-0.6 mCi) localized within the synthetic lesion. The synthetic lesions were reused multiple times. Mice tolerated the implanted columns without complications for up to 8 wk. CONCLUSION: A reusable, synthetic SPECT lesion was constructed and implanted in the femur of a living mouse. The synthetic lesion is useful for the development of imaging schemes and for more realistically evaluating imaging-system performance in the context of a living animal.

Imaging recognition of inhibition of multidrug resistance in human breast cancer xenografts using 99mTc-labeled sestamibi and tetrofosmin.

BACKGROUND: (99m)Tc-sestamibi (MIBI) and (99m)Tc-tetrofosmin (TF) are avid transport substrates recognized by the multidrug resistance (MDR) P-glycoprotein (Pgp). This study was designed to compare the properties of MIBI and TF in assessing the inhibition of Pgp by PSC833 in severe combined immunodeficient mice bearing MCF7 human breast tumors using SPECT imaging. METHODS: Animals with drug-sensitive (MCF/WT) and drug-resistant (MCF7/AdrR) tumors were treated by PSC833 and by carrier vehicle 1 h before imaging, respectively. Dynamic images were acquired for 30 min after intravenous injection of MIBI/TF using a SPECT system, FastSPECT. The biodistribution of MIBI and TF was determined at the end of the imaging session. RESULTS: MCF7/WT in the absence and presence of PSC833 could be visualized by MIBI and TF imaging within 5 min and remained detectable for 30 min postinjection. MCF7/AdrR could be visualized only 2-5 min without PSC833 treatment but could be detected for 30 min with PSC833, very similar to MCF7/WT. MCF7/AdrR without PSC833 showed significantly greater radioactive washout than MCF7/WT and MCF7/AdrR with PSC833 treatment. PSC833 increased the accumulation (%ID/g) in MCF7/AdrR 3.0-fold (1.62+/-0.15 vs. 0.55+/-0.05, P<.05) for TF and 1.9-fold (1.21+/-0.04 vs. 0.64+/-0.05, P<.05) for MIBI but did not affect MCF7/WT. CONCLUSIONS: The feasibility of MIBI and TF for assessment of MDR expression and inhibition was demonstrated in mice through FastSPECT imaging. The results indicate that TF may be at least comparable with MIBI in recognizing Pgp expression and modulation.

High-resolution imaging with (99m)Tc-glucarate for assessing myocardial injury in rat heart models exposed to different durations of ischemia with reperfusion.

UNLABELLED: (99m)Tc-Glucarate ((99m)Tc-GLA) is a novel infarct-avid imaging agent. The aim of this study was to evaluate the role of (99m)Tc-GLA for assessing the severity of myocardial ischemia-reperfusion injury in rat heart models exposed to varied durations of left coronary artery (LCA) occlusion with reperfusion using a high-resolution SPECT system, FASTSPECT. We also wanted to clarify whether a rapid sequence of 3-dimensional imaging with FASTSPECT can quantify uptake and washout kinetics of cardiovascular imaging agents in small-animal heart models. METHODS: The ischemic-reperfused rat heart models were created by ligating the LCA for 30 min (IR30, n = 12) or 90 min (IR90, n = 6) (IR = ischemia-reperfusion) and releasing the ligature for 30 min. Dynamic images were acquired over a 2-h period after (99m)Tc-GLA was intravenously injected. The ischemic area at risk (IAR) was determined by Evans blue staining. Necrosis was assessed with triphenyltetrazolium chloride (TTC) staining and a transmission electron microscope (TEM). RESULTS: The infarct size of the left ventricle (% IAR) on TTC staining was smaller in IR30 (49.2 +/- 4.3) than in IR90 (73.4 +/- 4.7, P < 0.05), which exhibited evidence of more severe irreversible injury than the IR30 heart on TEM. FASTSPECT images demonstrated hot spot accumulations of (99m)Tc-GLA in all hearts. The washout of (99m)Tc-GLA from the ischemic-reperfused area in IR90 was significantly slower than that in IR30. The ratio of the hot spot to normal myocardial activity was 4.1 +/- 0.3 in IR30 and 7.1 +/- 1.1 in IR90 (P < 0.05). The hot spot size (% IAR) (58.4 +/- 2.7 in IR30 vs. 75.9 +/- 2.7 in IR90, P < 0.05) related significantly to the infarct size. CONCLUSION: The severity of myocardial injury induced by ischemia-reperfusion can be assessed by FASTSPECT imaging with (99m)Tc-GLA. The results suggest that (99m)Tc-GLA will be clinically useful in detecting and quantifying acute necrotic myocardium. The FASTSPECT imaging with the rat heart models provides a solution-specific approach with high-resolution and fast dynamic acquisition for kinetic studies of new myocardial imaging agents.

99mTc glucarate high-resolution imaging of drug sensitive and drug resistant human breast cancer xenografts in SCID mice.

BACKGROUND AND AIM: Previous studies have showed that 99mTc labelled glucarate (GLA) might be an agent for non-invasive detection of breast tumours. In xenografted BT20 breast tumours, GLA was found to have higher uptake than 99mTc sestamibi (MIBI). It is unclear whether GLA can localize in all cell line breast cancer xenografts, as well as breast tumours with multidrug resistance (MDR). The present study aimed to investigate the properties of GLA in detecting drug sensitive and drug resistant MCF7 breast cancer xenografts in mice by using dynamic single photon emission computed tomography (SPECT) imaging. METHODS: MCF7/S cells are drug sensitive breast carcinoma cells. MCF7/D40 cells are 40-fold more resistant to doxorubicin compared to MCF7/S. Subcutaneous tumours were grown in SCID mice for 10-14 days after injection of 1 x 10(6) cells into the right thigh. Anaesthetized mice with MCF7/S (MIBI, n=9; GLA, n=8) and MCF7/D40 (MIBI, n=6; GLA, n=5) tumours were imaged using a high-resolution SPECT system called FASTSPECT. Dynamic images were acquired for 2 h after intravenous injection of GLA or MIBI. Expression of MDR P-glycoprotein (Pgp) in the tumours was demonstrated in the MCF7/D40 tumours by western blotting, not in the MCF7/S tumours. RESULTS: The xenografted tumours were visualized unequivocally within 10-30 min in GLA images and remained detectable for at least 2 h after injection. Drug resistant tumours, from which MIBI was rapidly expelled, retained GLA as readily as did drug sensitive tumours. The biodistribution data of GLA demonstrated significantly higher accumulation (%ID/g) compared to MIBI. CONCLUSION: MCF7 tumour xenografts can be detected by 99mTc glucarate imaging. More importantly, 99mTc glucarate can potentially localize drug resistant breast tumours.

Imaging recognition of multidrug resistance in human breast tumors using 99mTc-labeled monocationic agents and a high-resolution stationary SPECT system.

Imaging recognition of multidrug-resistance by 99mTc-labeled sestamibi, tetrofosmin and furifosmin in mice bearing human breast tumors was evaluated using a high-resolution SPECT, FASTSPECT. Imaging results showed that the washout rates in drug-resistant MCF7/D40 tumors were significantly greater than that in drug-sensitive MCF7/S tumors. Furifosmin exhibited greater washout from both MCF7/S and MCF7/D40 than sestamibi, while tetrofosmin washout was greater than sestamibi in MCF7/D40 only. Feasibility of the monocationic agents for characterizing MDR expression was well clarified with FASTSPECT imaging.

Quantitative analysis of acute myocardial infarct in rat hearts with ischemia-reperfusion using a high-resolution stationary SPECT system.

UNLABELLED: The purpose of this study was to develop an in vivo imaging protocol for a high-resolution stationary SPECT system, called FASTSPECT, in a rat heart model of ischemia-reperfusion (IR) and to compare 99mTc-sestamibi imaging and triphenyltetrazolium chloride (TTC) staining for reliability and accuracy in the measurement of myocardial infarcts. METHODS: FASTSPECT consists of 24 modular cameras and a 24-pinhole aperture with 1.5-mm spatial resolution and 13.3 cps/microCi (0.359 cps/kBq) sensitivity. The IR heart model was created by ligating the left coronary artery for 90 min and then releasing the ligature for 30 min. Two hours after 99mTc-sestamibi injection (5-10 mCi [185-370 MBq]), images were acquired for 5-10 min for 5 control rats and 11 IR rats. The hearts were excised, and the left ventricle was sectioned into 4 slices for TTC staining. RESULTS: Left and right ventricular myocardium in control rats was shown clearly, with uniform 99mTc-sestamibi distribution and 100% TTC staining for viable myocardium. Nine of 11 rats with IR survived throughout imaging and exhibited 50.8% +/- 2.7% ischemic area and 37.9% +/- 3.9% infarct in the left ventricle on TTC staining. The infarct size measured by FASTSPECT imaging was 37.6% +/- 3.6%, which correlated significantly with that measured by TTC staining (r = 0.974; P < 0.01). CONCLUSION: The results confirmed the accuracy of FASTSPECT imaging for measurement of acute myocardial infarcts in rat hearts. Application of FASTSPECT imaging in small animals may be feasible for investigating myocardial IR injury and the effects of revascularization.