Myocardial perfusion imaging is feasible for infarct size quantification in mice using a clinical single-photon emission computed tomography system equipped with pinhole collimators

Multi-Scale Imaging of Vascular Pathologies in Cardiovascular Disease

Cardiovascular disease entails systemic changes in the vasculature. The endothelial cells lining the blood vessels are crucial in the pathogenesis of cardiovascular disease. Healthy endothelial cells direct the blood flow to tissues as vasodilators and act as the systemic interface between the blood and tissues, supplying nutrients for vital organs, and regulating the smooth traffic of leukocytes into tissues. In cardiovascular diseases, when inflammation is sensed, endothelial cells adjust to the local or systemic inflammatory state. As the inflamed vasculature adjusts, changes in the endothelial cells lead to endothelial dysfunction, altered blood flow and permeability, expression of adhesion molecules, vessel wall inflammation, thrombosis, angiogenic processes, and extracellular matrix production at the endothelial cell level. Preclinical multi-scale imaging of these endothelial changes using optical, acoustic, nuclear, MRI, and multimodal techniques has progressed, due to technical advances and enhanced biological understanding on the interaction between immune and endothelial cells. While this review highlights biological processes that are related to changes in the cardiac vasculature during cardiovascular diseases, it also summarizes state-of-the-art vascular imaging techniques. The advantages and disadvantages of the different imaging techniques are highlighted, as well as their principles, methodologies, and preclinical and clinical applications with potential future directions. These multi-scale approaches of vascular imaging carry great potential to further expand our understanding of basic vascular biology, to enable early diagnosis of vascular changes and to provide sensitive diagnostic imaging techniques in the management of cardiovascular disease.

Accuracy of cardiac functional parameters measured from gated radionuclide myocardial perfusion imaging in mice

BACKGROUND

Quantitative cardiac contractile function assessment is the primary indicator of disease progression and therapeutic efficacy in small animals. Operator dependency is a major challenge with commonly used echocardiography. Simultaneous assessment of cardiac perfusion and function in nuclear scans would reduce burden on the animal and facilitate longitudinal studies. We evaluated the accuracy of contractile function measurements obtained from electrocardiogram-gated nuclear perfusion imaging compared with anatomic imaging.

METHODS AND RESULTS

In healthy C57Bl/6N mice (n = 11), 99mTc-sestamibi SPECT and 13N-ammonia PET underestimated left ventricular volumes (23 to 28%, P = 0.02) compared to matched anatomic images, though ejection fraction (LVEF) was comparable (%, SPECT: 73 ± 8 vs CMR: 72 ± 6, P = 0.1). At 1 week after myocardial infarction (n = 13), LV volumes were significantly lower in perfusion images compared to CMR and contrast CT (P = 0.003), and LVEF was modestly overestimated (%, SPECT: 37 ± 8, vs CMR: 27 ± 7, P = 0.003). Nuclear images exhibited good intra- and inter-reader agreement. Perfusion SPECT accurately calculated infarct size compared to histology (r = 0.95, P < 0.001).

CONCLUSIONS

Cardiac function can be calculated by gated nuclear perfusion imaging in healthy mice. After infarction, perfusion imaging overestimates LVEF, which should be considered for comparison to other modalities. Combined functional and infarct size analysis may optimize imaging protocols and reduce anaesthesia duration for longitudinal studies.

Rubidium-82 PET imaging is feasible in a rat myocardial infarction model

BACKGROUND

Small-animal myocardial infarct models are frequently used in the assessment of new cardioprotective strategies. A validated quantification of perfusion using a non-cyclotron-dependent PET tracer would be of importance in monitoring response to therapy. We tested whether myocardial PET perfusion imaging is feasible with Rubidium-82 (82Rb) in a small-animal scanner using a rat myocardial infarct model.

METHODS

18 Sprague-Dawley rats underwent permanent coronary artery ligation (infarct group), and 11 rats underwent ischemia-reperfusion (reperfusion group) procedure. 82Rb-PET and magnetic resonance imaging (MRI) were conducted before and after the intervention. Perfusion was compared to both left ventricle ejection fraction (LVEF) and infarct size assessed by MRI.

RESULTS

Follow-up global 82Rb-uptake correlated significantly with infarct size (infarct group: r = -0.81, P < 0.001 and reperfusion group: r = -0.61, P = 0.04). Only 82Rb-uptake in the infarct group correlated with LVEF. At follow-up, a higher segmental 82Rb-uptake in the infarct group was associated with better wall motion (β = 0.034, CI [0.028;0.039], P < 0.001, R2 = 0.30), and inversely associated with scar transmurality (β = -2.4 [-2.6; -2.2], P < 0.001, R2 = 0.59). The associations were similar for the reperfusion group.

CONCLUSION

82Rb-PET is feasible in small animal scanners despite the long positron range and enables fast and time-efficient myocardial perfusion imaging in rat models.

Cardiac Radionuclide Imaging in Rodents: A Review of Methods, Results, and Factors at Play

The interest around small-animal cardiac radionuclide imaging is growing as rodent models can be manipulated to allow the simulation of human diseases. In addition to new radiopharmaceuticals testing, often researchers apply well-established probes to animal models, to follow the evolution of the target disease. This reverse translation of standard radiopharmaceuticals to rodent models is complicated by technical shortcomings and by obvious differences between human and rodent cardiac physiology. In addition, radionuclide studies involving small animals are affected by several extrinsic variables, such as the choice of anesthetic. In this paper, we review the major cardiac features that can be studied with classical single-photon and positron-emitting radiopharmaceuticals, namely, cardiac function, perfusion and metabolism, as well as the results and pitfalls of small-animal radionuclide imaging techniques. In addition, we provide a concise guide to the understanding of the most frequently used anesthetics such as ketamine/xylazine, isoflurane, and pentobarbital. We address in particular their mechanisms of action and the potential effects on radionuclide imaging. Indeed, cardiac function, perfusion, and metabolism can all be significantly affected by varying anesthetics and animal handling conditions.

Longitudinal imaging of the ageing mouse

Several non-invasive imaging techniques are used to investigate the effect of pathologies and treatments over time in mouse models. Each preclinical in vivo technique provides longitudinal and quantitative measurements of changes in tissues and organs, which are fundamental for the evaluation of alterations in phenotype due to pathologies, interventions and treatments. However, it is still unclear how these imaging modalities can be used to study ageing with mice models. Almost all age related pathologies in mice such as osteoporosis, arthritis, diabetes, cancer, thrombi, dementia, to name a few, can be imaged in vivo by at least one longitudinal imaging modality. These measurements are the basis for quantification of treatment effects in the development phase of a novel treatment prior to its clinical testing. Furthermore, the non-invasive nature of such investigations allows the assessment of different tissue and organ phenotypes in the same animal and over time, providing the opportunity to study the dysfunction of multiple tissues associated with the ageing process. This review paper aims to provide an overview of the applications of the most commonly used in vivo imaging modalities used in mouse studies: micro-computed-tomography, preclinical magnetic-resonance-imaging, preclinical positron-emission-tomography, preclinical single photon emission computed tomography, ultrasound, intravital microscopy, and whole body optical imaging.

Heme Oxygenase-1 Gene Therapy Provides Cardioprotection Via Control of Post-Ischemic Inflammation: An Experimental Study in a Pre-Clinical Pig Model

BACKGROUND

Heme oxygenase-1 (HO-1) is an inducible stress-responsive enzyme converting heme to bilirubin, carbon monoxide, and free iron, which exerts anti-inflammatory and antiapoptotic effects. Although efficient cardioprotection after HO-1 overexpression has been reported in rodents, its role in attenuating post-ischemic inflammation is unclear.

OBJECTIVES

This study assessed the efficacy of recombinant adenoassociated virus (rAAV)-encoding human heme oxygenase-1 (hHO-1) in attenuating post-ischemic inflammation in a murine and a porcine ischemia/reperfusion model.

METHODS

Murine ischemia was induced by 45 min of left anterior descending occlusion, followed by 24 h of reperfusion and functional as well as fluorescent-activated cell sorting analysis. Porcine hearts were subjected to 60 min of ischemia and 24h of reperfusion before hemodynamic and histologic analyses were performed.

RESULTS

Human microvascular endothelial cells transfected with hHO-1 displayed an attenuated interleukin-6 and intercellular adhesion molecule 1 expression, resulting in reduced monocytic THP-1 cell recruitment in vitro. In murine left anterior descending occlusion and reperfusion, the post-ischemic influx of CD45(+) leukocytes, Ly-6G(+) neutrophils, and Ly-6C(high) monocytes was further exacerbated in HO-1-deficient hearts and reversed by rAAV.hHO-1 treatment. Conversely, in our porcine model of ischemia, the post-ischemic influx of myeloperoxidase-positive neutrophils and CD14(+) monocytes was reduced by 49% and 87% after rAAV.hHO-1 transduction, similar to hHO-1 transgenic pigs. Functionally, rAAV.hHO-1 and hHO-1 transgenic left ventricles displayed a smaller loss of ejection fraction than control animals.

CONCLUSIONS

Whereas HO-1 deficiency exacerbates post-ischemic cardiac inflammation in mice, hHO-1 gene therapy attenuates inflammation after ischemia and reperfusion in murine and porcine hearts. Regional hHO-1 gene therapy provides cardioprotection in a pre-clinical porcine ischemia/reperfusion model.

Impact of injection dose, post-reconstruction filtering, and collimator choice on image quality of myocardial perfusion SPECT using cadmium-zinc telluride detectors in the rat

Background

The aims of this study were (1) to evaluate the impact of injection dose, post-reconstruction filtering, and collimator choice on image quality of myocardial perfusion single-photon emission computed tomography (SPECT) using cadmium-zinc telluride (CZT) detectors and (2) to determine how these factors affect measured infarct size in the in vivo rat.

Methods

Twenty-four healthy and eight myocardial infarct (MI) rats underwent myocardial perfusion SPECT imaging after injection of various doses (25 to 200 MBq) of ^99mTc-tetrofosmin using a standard (STD) five-pinhole collimator and high-sensitivity (HS) five-pinhole collimator. Image quality score, contrast-to-noise ratio, sharpness index, coefficient of variation (CV), and measured defect size were assessed as measures of image quality.

Results

The image quality score increased and CV decreased as a function of injection dose. The contrast-to-noise ratio increased and sharpness index decreased as a function of Gaussian kernel size. When STD and HS were compared, HS tended to show higher image quality score and lower CV than STD. The use of post-reconstruction filter significantly improved image quality score and lessened CV. The reproducibility of defect size measurements, as assessed by intraclass correlation coefficients (ICC), between the collimators was poor-to-moderate (ICC = −0.31~0.57) with low (25 MBq) injection dose and with no or light (1.5-mm kernel size) filtering, whereas it was good-to-excellent (ICC = 0.75~0.97) with high (200 MBq) dose or low dose with heavy (2.5-mm kernel size) filtering. The filtering-related reproducibility was poor (ICC = −0.18~0.17) for STD with low injection dose, whereas it was good-to-excellent (ICC = 0.79~0.89) for HS. Furthermore, there was a filtering-related underestimation of defect size particularly with the use of heavy smoothing.

Conclusions

Appropriate imaging setting is important to obtain high quality images and thereby reliable measurements using a preclinical myocardial SPECT in the rat. When only a low injection dose (25 MBq) is allowed, we would recommend to use HS with light (1.5-mm kernel size) filtering.

Advances in pinhole and multi-pinhole collimators for single photon emission computed tomography imaging

The collimator in single photon emission computed tomography (SPECT), is an important part of the imaging chain. One of the most important collimators that used in research, preclinical study, small animal, and organ imaging is the pinhole collimator. Pinhole collimator can improve the tradeoff between sensitivity and resolution in comparison with conventional parallel-hole collimator and facilities diagnosis. However, a major problem with pinhole collimator is a small field of view (FOV). Multi-pinhole collimator has been investigated in order to increase the sensitivity and FOV with a preserved spatial resolution. The geometry of pinhole and multi-pinhole collimators is a critical factor in the image quality and plays a key role in SPECT imaging. The issue of the material and geometry for pinhole and multi-pinhole collimators have been a controversial and much disputed subject within the field of SPECT imaging. On the other hand, recent developments in collimator optimization have heightened the need for appropriate reconstruction algorithms for pinhole SPECT imaging. Therefore, iterative reconstruction algorithms were introduced to minimize the undesirable effect on image quality. Current researches have focused on geometry and configuration of pinhole and multi-pinhole collimation rather than reconstruction algorithm. The lofthole and multi-lofthole collimator are samples of novel designs. The purpose of this paper is to provide a review on recent researches in the pinhole and multi-pinhole collimators for SPECT imaging.

In-vivo monitoring of erythropoietin treatment after myocardial infarction in mice with [⁶⁸Ga]Annexin A5 and [¹⁸F]FDG PET

BACKGROUND

Several studies substantiate the cardioprotective effects of erythropoietin (EPO). Our goal was to quantify the effects of EPO treatment on the early expression of the apoptosis marker phosphatidylserine as well as on the left ventricular volumes and function by means of small animal PET.

METHODS AND RESULTS

Myocardial infarction (MI) was induced in C57BL/6 mice. Animals were assigned to saline or EPO groups and underwent Annexin PET (day 2) and gated FDG PET (days 6 and 30). Annexin uptake was significantly higher in the infarction than in remote myocardium, with no differences between treatment groups. Infarct size showed a slight decrease in the EPO group and a slight increase in the controls, which did not reach statistical significance. Follow-up analyses revealed a significant increase of end-diastolic and end-systolic volumes in the EPO group, in which a stable left ventricular ejection fraction (LVEF) was maintained.

CONCLUSION

We find that deleterious effects of EPO can outweigh cardioprotective effects. The present EPO treatment did not significantly reduce apoptosis after MI, but seemingly provoked significant myocardial dilation while maintaining a stable LVEF. Molecular mechanisms of EPO treatment may need further elucidation to optimize therapy regimens.

Impact of parathyroid hormone on bone marrow-derived stem cell mobilization and migration

Parathyroid hormone (PTH) is well-known as the principal regulator of calcium homeostasis in the human body and controls bone metabolism via actions on the survival and activation of osteoblasts. The intermittent administration of PTH has been shown to stimulate bone production in mice and men and therefore PTH administration has been recently approved for the treatment of osteoporosis. Besides to its physiological role in bone remodelling PTH has been demonstrated to influence and expand the bone marrow stem cell niche where hematopoietic stem cells, capable of both self-renewal and differentiation, reside. Moreover, intermittent PTH treatment is capable to induce mobilization of progenitor cells from the bone marrow into the bloodstream. This novel function of PTH on modulating the activity of the stem cell niche in the bone marrow as well as on mobilization and regeneration of bone marrow-derived stem cells offers new therapeutic options in bone marrow and stem cell transplantation as well as in the field of ischemic disorders.

[68Ga]-albumin-PET in the monitoring of left ventricular function in murine models of ischemic and dilated cardiomyopathy: comparison with cardiac MRI

PURPOSE

The purpose of this study is to evaluate left ventricular functional parameters in healthy mice and in different murine models of cardiomyopathy with the novel blood pool (BP) positron emission tomography (PET) tracer [68Ga]-albumin.

PROCEDURES

ECG-gated microPET examinations were obtained in healthy mice, and mice with dilative (DCM) and ischemic cardiomyopathy (ICM) using the novel BP tracer [68Ga]-albumin (AlbBP), as well as [18F]-FDG microPET. Cine-magnetic resonance imaging (MRI) examination performed on a clinical 1.5-T MRI provided the reference standard measurements.

RESULTS

When considering the combined group of healthy controls, DCM and ICM AlbBP-PET significantly overestimated the magnitudes of EDV (AlbBP, 181±86 μl; cine-MRI, 125±80 μl; P<0.001) and ESV (AlbBP, 136±92 μl; cine-MRI, 96±77 μl; P<0.001), whereas the EF (AlbBP, 31±16%; cine-MRI, 33±21%; P=0.910) matched closely to cine-MRI results, as did findings with [18F]-FDG. High correlations were found between the measured cardiac parameters (EDV: R=0.978, ESV: R=0.989, and LVEF: R=0.992).

CONCLUSIONS

Measuring left ventricular function in mice with [68Ga]-albumin BP PET is feasible and showed a high correlation compared to cine-MRI, which was used as a reference standard.

Myocardial infarction area quantification using high-resolution SPECT images in rats

Background

Imaging techniques enable in vivo sequential assessment of the morphology and function of animal organs in experimental models. We developed a device for high-resolution single photon emission computed tomography (SPECT) imaging based on an adapted pinhole collimator.

Objective

To determine the accuracy of this system for quantification of myocardial infarct area in rats.

Methods

Thirteen male Wistar rats (250 g) underwent experimental myocardial infarction by occlusion of the left coronary artery. After 4 weeks, SPECT images were acquired 1.5 hours after intravenous injection of 555 MBq of 99mTc-Sestamibi. The tomographic reconstruction was performed by using specially developed software based on the Maximum Likelihood algorithm. The analysis of the data included the correlation between the area of perfusion defects detected by scintigraphy and extent of myocardial fibrosis assessed by histology.

Results

The images showed a high target organ/background ratio with adequate visualization of the left ventricular walls and cavity. All animals presenting infarction areas were correctly identified by the perfusion images. There was no difference of the infarct area as measured by SPECT (21.1 ± 21.2%) and by histology (21.7 ± 22.0%; p=0.45). There was a strong correlation between individual values of the area of infarction measured by these two methods.

Conclusion

The developed system presented adequate spatial resolution and high accuracy for the detection and quantification of myocardial infarction areas, consisting in a low cost and versatile option for high-resolution SPECT imaging of small rodents.

Myocardial infarct size quantification in mice by SPECT using a novel algorithm independent of a normal perfusion database

Background

There is a growing interest in developing non-invasive imaging techniques permitting infarct size (IS) measurements in mice. The aim of this study was to validate the high-resolution rodent Linoview single photon emission computed tomography (SPECT) system for non-invasive measurements of IS in mice by using a novel algorithm independent of a normal database, in comparison with histology.

Methods

Eleven mice underwent a left coronary artery ligature. Seven days later, animals were imaged on the SPECT 2h30 after injection of 173 ± 27 MBq of Tc-99m-sestamibi. Mice were subsequently killed, and their hearts were excised for IS determination with triphenyltetrazolium chloride (TTC) staining. SPECT images were reconstructed using the expectation maximization maximum likelihood algorithm, and the IS was calculated using a novel algorithm applied on the 20-segment polar map provided by the commercially available QPS software (Cedars-Sinai Medical Center, CA, USA). This original method is attractive by the fact that it does not require the implementation of a normal perfusion database.

Results

Reconstructed images allowed a clear delineation of the left ventricles borders in all mice. No significant difference was found between mean IS determined by SPECT and by TTC staining [37.9 ± 17.5% vs 35.6 ± 17.2%, respectively (P = 0.10)]. Linear regression analysis showed an excellent correlation between IS measured on the SPECT images and IS obtained with TTC staining (y = 0.95x + 0.03 (r = 0.97; P < 0.0001)), without bias, as demonstrated by the Bland-Altman plot.

Conclusion

Our results demonstrate the accuracy of the method for the measurement of myocardial IS in mice with the Linoview SPECT system.

High-resolution positron emission tomography/computed tomography imaging of the mouse heart

Different animal models have been used to reproduce coronary heart disease, but in recent years mice have become the animals of choice, because of their short life cycle and the possibility of genetic manipulation. Various techniques are currently used for cardiovascular imaging in mice, including high-resolution ultrasound, X-ray computed tomography (CT), magnetic resonance imaging and nuclear medicine procedures. In particular, molecular imaging with cardiac positron emission tomography (PET) allows non-invasive evaluation of changes in myocardial perfusion, metabolism, apoptosis, inflammation and gene expression or measurement of changes in left ventricular functional parameters. With technological advances, dedicated small laboratory PET/CT imaging has emerged in cardiovascular research, providing in vivo a non-invasive, serial and quantitative assessment of left ventricular function, myocardial perfusion and metabolism at a molecular level. This non-invasive methodology might be useful in longitudinal studies to monitor cardiac biochemical parameters and might facilitate studies to assess the effect of different interventions after acute myocardial ischaemia.

Left ventricular functional assessment in murine models of ischemic and dilated cardiomyopathy using [18 F]FDG-PET: comparison with cardiac MRI and monitoring erythropoietin therapy

BACKGROUND

We performed an initial evaluation of non-invasive ECG-gated [18 F]FDG-positron emission tomography (FDG-PET) for serial measurements of left ventricular volumes and function in murine models of dilated (DCM) and ischemic cardiomyopathy (ICM), and then tested the effect of erythropoietin (EPO) treatment on DCM mice in a preliminary FDG-PET therapy monitoring study.

METHODS

Mice developed DCM 8 weeks after injection with Coxsackievirus B3 (CVB3), whereas ICM was induced by ligation of the left anterior descending artery. LV volumes (EDV and ESV) and the ejection fraction (LVEF) of DCM, ICM and healthy control mice were measured by FDG-PET and compared with reference standard results obtained with 1.5 T magnetic resonance imaging (MRI). In the subsequent monitoring study, LVEF of DCM mice was evaluated by FDG-PET at baseline, and after 4 weeks of treatment, with EPO or saline.

RESULTS

LV volumes and the LVEF as measured by FDG-PET correlated significantly with the MRI results. These correlations were higher in healthy and DCM mice than in ICM mice, in which LVEF measurements were somewhat compromised by absence of FDG uptake in the area of infarction. LV volumes (EDV and ESV) were systematically underestimated by FDG-PET, with net bias such that LVEF measurements in both models of heart disease exceeded by 15% to 20% results obtained by MRI. In our subsequent monitoring study of DCM mice, we found a significant decrease of LVEF in the EPO group, but not in the saline-treated mice. Moreover, LVEF in the EPO and saline mice significantly correlated with histological scores of fibrosis.

CONCLUSIONS

LVEF estimated by ECG-gated FDG-PET significantly correlated with the reference standard MRI, most notably in healthy mice and mice with DCM. FDG-PET served for longitudinal monitoring of effects of EPO treatment in DCM mice.

Migration of bone marrow-derived cells and improved perfusion after treatment with erythropoietin in a murine model of myocardial infarction

Erythropoietin (EPO) was shown to have protective effects after myocardial infarction (MI) by neovascularization and antiapoptotic mechanisms. Beside direct receptor-dependent mechanisms, mobilization and homing of bone marrow-derived cells (BMCs) may play a pivotal role in this regard. In this study, we intended to track different subpopulations of BMCs and to assess serially myocardial perfusion changes in EPO-treated mice after MI. To allow tracking of BMCs, we used a chimeric mouse model. Therefore, mice (C57BL/6J) were sublethally irradiated, and bone marrow (BM) from green fluorescent protein transgenic mice was transplanted. Ten weeks later coronary artery ligation was performed to induce MI. EPO was injected for 3 days with a total dose of 5000 IU/kg. Subpopulations (CD31, c-kit, CXCR-4 and Sca-1) of EGFP⁺ cells were studied in peripheral blood, bone marrow and hearts by flow cytometry. Myocardial perfusion was serially investigated in vivo by pinhole single-photon emission computed tomography (SPECT) at days 6 and 30 after MI. EPO-treated animals revealed an enhanced mobilization of BMCs into peripheral blood. The numbers of these cells in BM remained unchanged. Homing of all BMCs subpopulations to the ischaemic myocardium was significantly increased in EPO-treated mice. Among the investigated subpopulations, EPO predominantly affected migration of CXCR-4⁺ (4.3-fold increase). Repetitively SPECT analyses revealed a reduction of perfusion defects after EPO treatment over time. Our study shows that EPO treatment after MI enhances the migration capacity of BMCs into ischaemic tissue, which may attribute to an improved perfusion and reduced size of infarction, respectively.

Reproducibility and accuracy of non-invasive measurement of infarct size in mice with high-resolution PET/CT

BACKGROUND

This study assessed the reproducibility and accuracy of 2-deoxy-2[(18)F]fluoro-D-glucose ((18)F-FDG) for non-invasive quantification of myocardial infarct size in mice by a high-resolution positron emission tomography (PET)/computed tomography (CT) system.

METHODS AND RESULTS

Mice were studied by (18)F-FDG PET/CT 1 week after induction of myocardial infarction by permanent coronary occlusion or sham procedure. In a subset of mice, PET/CT was repeated 2 days apart to assess the reproducibility of infarct size measurements. Histological analysis was used as reference method to validate imaging data. The average difference in infarct size measurements between the first and the second study was -0.42% ± 2.07% (95% confidence interval -2.6 to 1.75) with a repeatability coefficient of 4.05%. At Bland-Altman analysis, the lower and upper limits of agreement between the two repeated studies were -4.46% and 3.63%, respectively, and no correlation between difference and mean was found (P = .89). The concordance correlation coefficient was 0.99 (P < .001) and the intraclass coefficient of correlation was 0.99. A high correlation between PET/CT and histology was found for measurement of infarct size (P < .001). Using Bland-Altman analysis, the mean difference in infarct size measurement (PET/CT minus histology) was 1.9% (95% confidence interval 0.94% to 2.86%).

CONCLUSIONS

In a mice model of permanent coronary occlusion non-invasive measurement of infarct size with high-resolution (18)F-FDG, PET/CT has excellent reproducibility and accuracy. These findings support the use of this methodology in serial studies.

Systematic evaluation of 99mTc-tetrofosmin versus 99mTc-sestamibi to study murine myocardial perfusion in small animal SPECT/CT

BACKGROUND

The "back-translation" of clinically available protocols to measure myocardial perfusion to preclinical imaging in mouse models of human disease is attractive for basic biomedical research. With respect to single-photon emission computed tomography (SPECT) approaches, clinical myocardial perfusion imaging protocols are established with different 99mTc-labeled perfusion tracers; however, studies evaluating and optimizing protocols for these tracers in high-resolution pinhole SPECT in mice are lacking. This study aims at evaluating two clinically available 99mTc-labeled myocardial perfusion tracers (99mTc-sestamibi vs. 99mTc-Tetrofosmin) in mice using four different imaging protocols.

METHODS

Adult C57BL/6 male mice were injected with 99mTc-sestamibi (MIBI) or 99mTc-Tetrofosmin (TETRO) (4 MBq/g body weight) either intravenously through the tail vein (n = 5) or retroorbitally (n = 5) or intraperitoneally (i.p.) under anesthesia (n = 3) or i.p. in an awake state (n = 3) at rest. Immediately after injection, a multi-frame single-photon emission computed tomography/computed tomography (SPECT/CT) acquisition was initiated with six subsequent time frames of 10 min each. Reconstructed images of the different protocols were assessed and compared by visual analysis by experts and by time-activity-curves generated from regions-of-interest for various organs (normalized uptake values).

RESULTS

Visually assessing overall image quality, the best image quality was found for MIBI for both intravenous injection protocols, whereas TETRO only had comparable image quality after retroorbital injections. These results were confirmed by quantitative analysis where left ventricular (LV) uptake of MIBI after tail vein injections was found significantly higher for all time points accompanied with a significantly slower washout of 16% for MIBI vs. 33% for TETRO (p = 0.009) from 10 to 60 min post injection (PI). Interestingly, LV washout from 10 to 60 min PI was significantly higher for TETRO when applied by tail vein injections when compared to retroorbital injections (22%, p = 0.008). However, liver uptake was significant and comparable for both tracers at all time points. Radioactivity concentration in the lungs was negligible for all time points and both tracers.

CONCLUSION

Intravenous MIBI injection (both tail vein and retroorbital) results in the best image quality for assessing myocardial perfusion of the murine heart by SPECT/CT. TETRO has a comparable image quality only for the retroorbital injection route.

PET/CT imaging in mouse models of myocardial ischemia

Different species have been used to reproduce myocardial infarction models but in the last years mice became the animals of choice for the analysis of several diseases, due to their short life cycle and the possibility of genetic manipulation. Many techniques are currently used for cardiovascular imaging in mice, including X-ray computed tomography (CT), high-resolution ultrasound, magnetic resonance imaging, and nuclear medicine procedures. Cardiac positron emission tomography (PET) allows to examine noninvasively, on a molecular level and with high sensitivity, regional changes in myocardial perfusion, metabolism, apoptosis, inflammation, and gene expression or to measure changes in anatomical and functional parameters in heart diseases. Currently hybrid PET/CT scanners for small laboratory animals are available, where CT adds high-resolution anatomical information. This paper reviews mouse models of myocardial infarction and discusses the applications of dedicated PET/CT systems technology, including animal preparation, anesthesia, radiotracers, and images postprocessing.

Dual stem cell therapy after myocardial infarction acts specifically by enhanced homing via the SDF-1/CXCR4 axis

BACKGROUND

G-CSF based stem cell mobilization and stabilization of cardiac SDF-1 by DPP-IV-inhibition (dual stem cell therapy) improve heart function and survival after myocardial infarction. However, it is barely understood whether this new approach acts specifically through the SDF-1/CXCR4 axis, stimulation of resident cardiac stem cells and improved myocardial perfusion. Therefore, we aimed to clarify the role of the SDF1/CXCR4 axis with respect to the benefits of a dual stem cell based therapy.

METHODOLOGY/PRINCIPAL FINDINGS

After surgically induced ligation of the LAD, SDF-1/CXCR4 interactions were specifically blocked by the CXCR4 receptor antagonist AMD3100 in G-CSF and Diprotin A treated C57BL/6 mice. G-CSF+DipA treated and non-treated animals served as controls. Because AMD3100 is known to mobilize bone marrow derived stem cells (BMCs) in high concentrations, the optimal dosage (1.25mg per kg body weight) sufficient to block CXCR4 without stimulating mobilization was established. AMD3100 treatment of G-CSF and Diprotin A stimulated mice significantly decreased myocardial homing of circulating stem cells (FACS analysis) and inverted the beneficial effects of (i) cardiac remodeling (histological analyses), (ii) heart function (Millar tip catheterization) and (iii) survival (Kaplan-Meier curves). G-CSF treatment in combination with DPP-IV inhibition enhanced neovascularization at the infarct border zone which was related to an improved myocardial blood flow as measured by SPECT. Moreover, dual stem cell treatment effectively stimulated the pool of resident cardiac stem cells (FACS) which was reversed by AMD3100 treatment.

CONCLUSIONS/SIGNIFICANCE

Our data give final proof that homing through the SDF-1/CXCR-4 axis is essential for the success of dual stem cell therapy.

Dual stem cell therapy after myocardial infarction acts specifically by enhanced homing via the SDF-1/CXCR4 axis

BACKGROUND

G-CSF based stem cell mobilization and stabilization of cardiac SDF-1 by DPP-IV-inhibition (dual stem cell therapy) improve heart function and survival after myocardial infarction. However, it is barely understood whether this new approach acts specifically through the SDF-1/CXCR4 axis, stimulation of resident cardiac stem cells and improved myocardial perfusion. Therefore, we aimed to clarify the role of the SDF1/CXCR4 axis with respect to the benefits of a dual stem cell based therapy.

METHODOLOGY/PRINCIPAL FINDINGS

After surgically induced ligation of the LAD, SDF-1/CXCR4 interactions were specifically blocked by the CXCR4 receptor antagonist AMD3100 in G-CSF and Diprotin A treated C57BL/6 mice. G-CSF+DipA treated and non-treated animals served as controls. Because AMD3100 is known to mobilize bone marrow derived stem cells (BMCs) in high concentrations, the optimal dosage (1.25mg per kg body weight) sufficient to block CXCR4 without stimulating mobilization was established. AMD3100 treatment of G-CSF and Diprotin A stimulated mice significantly decreased myocardial homing of circulating stem cells (FACS analysis) and inverted the beneficial effects of (i) cardiac remodeling (histological analyses), (ii) heart function (Millar tip catheterization) and (iii) survival (Kaplan-Meier curves). G-CSF treatment in combination with DPP-IV inhibition enhanced neovascularization at the infarct border zone which was related to an improved myocardial blood flow as measured by SPECT. Moreover, dual stem cell treatment effectively stimulated the pool of resident cardiac stem cells (FACS) which was reversed by AMD3100 treatment.

CONCLUSIONS/SIGNIFICANCE

Our data give final proof that homing through the SDF-1/CXCR-4 axis is essential for the success of dual stem cell therapy.