Myocardial infarction in rats: high-resolution single-photon emission tomographic imaging with a pinhole collimator

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.

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.

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

INTRODUCTION

The aim of this study is to evaluate a non-invasive method for measuring myocardial perfusion defect size in mice using a clinical single-photon emission computed tomography system equipped with pinhole collimators (pinhole SPECT).

MATERIALS AND METHODS

Thirty days after ligation of the left anterior descending coronary artery, 13 mice (C57BL/6J) were imaged following intravenous injection of 370 MBq [99mTc]sestamibi. Eight control mice without myocardial infarction were likewise investigated. Image quality optimization had been achieved by repeated scanning of a multiple point phantom, with varying zoom factors, number of projection angles, and pinhole diameter. Volumetric sampling was used to generate polar maps, in which intensity was normalized to that of a standard septal region of interest (ROI), which was set at 100%. Receiver operating characteristic analyses were performed to define an optimal threshold as compared to histologically measured defect sizes, which were considered as gold standard.

RESULTS

A spatial resolution of 1.9 mm was achieved using a pinhole diameter of 0.5 mm, a zoom factor of 2, and 6 degrees projection angles. Histological results were best reproduced by a 60% threshold relative to the septal reference ROI. By applying this threshold, SPECT perfusion defect sizes revealed very high correlation to the histological results (R(2) = 0.867) with excellent intra- and interobserver reproducibility (intraclass correlation coefficients of 0.84 and 0.82).

CONCLUSIONS

We achieved a spatial resolution of 1.9 mm in myocardial perfusion imaging in mice using a clinical SPECT system mounted with pinhole collimators. Compared to a histological gold standard, the infarct sizes were accurately estimated, indicating that this method shows promise to monitor experimental cardiac interventions in mice.

Monitoring left ventricular function in small animals

Small animals such as mice and rats are extensively used to investigate the mechanisms and treatment of human cardiac diseases in vivo. The monitoring of left ventricular function is a key factor in this research. The measurement should be rapid, reproducible, and repeatable and allow the detection of subtle differences in function. Currently, echocardiography is most widely used in cardiac research laboratories for measuring left ventricular dimensions and function in small animals. Although the technique is rapid, the reproducibility of the calculations of left ventricular volumes is limited in some circumstances as a result of assumptions that do not necessarily hold true, such as in the setting of dilated, failing ventricles.

An analytical algorithm for skew-slit collimator SPECT with uniform attenuation correction

To replace the conventional pinhole (normal cone-beam) collimator, a novel skew-slit collimator was previously proposed and a Novikov-type algorithm developed to reconstruct images using the skew-slit geometry. The goal of this paper is to develop a reconstruction algorithm that has better noise control than the Novikov-type algorithm. The new algorithm is able to compensate for uniform attenuation, and computer simulation results show that reconstructed images are less noisy.

Resolution recovery in pinhole SPECT based on multi-ray projections: a phantom study

PURPOSE

Low sensitivity can become a major problem when very small pinholes are used in SPECT imaging. Although a larger pinhole aperture will improve the sensitivity, this will be at the cost of the spatial resolution. With a view to improving the resolution-sensitivity trade-off, this paper explores an iterative reconstruction algorithm that models the pinhole aperture based on multi-ray projections.

METHODS

This new implementation was validated using simulated data and phantom experiments. Two approaches were investigated. Firstly, the pinhole aperture was modelled in both the forward and the back projector. Secondly, the dual matrix implementation was investigated by modelling the pinhole aperture only in the forward projector. The systematic error, the full-width at half-maximum (FWHM) and the statistical error were quantified using the simulated data. Experimental phantom data were acquired for visual comparison with the reconstructions obtained from the simulated data.

RESULTS

For a predefined number of iterations, the systematic error, the FWHM and the statistical error could be decreased when the pinhole aperture was modelled during iterative reconstruction. For a fixed, predefined statistical error of +/-10%, smaller systematic errors and smaller FWHM were obtained when modelling the pinhole opening. When the dual matrix implementation was used, equivalent results could be obtained as when modelling the pinhole opening in both the forward and the back projector.

CONCLUSION

The multi-ray method to accomplish resolution recovery during the reconstruction of pinhole SPECT projection images offers a better trade-off between spatial resolution and noise compared with a reconstruction which does not model the pinhole aperture.

Resolution recovery in pinhole SPECT based on multi-ray projections: a phantom study

PURPOSE

Low sensitivity can become a major problem when very small pinholes are used in SPECT imaging. Although a larger pinhole aperture will improve the sensitivity, this will be at the cost of the spatial resolution. With a view to improving the resolution-sensitivity trade-off, this paper explores an iterative reconstruction algorithm that models the pinhole aperture based on multi-ray projections.

METHODS

This new implementation was validated using simulated data and phantom experiments. Two approaches were investigated. Firstly, the pinhole aperture was modelled in both the forward and the back projector. Secondly, the dual matrix implementation was investigated by modelling the pinhole aperture only in the forward projector. The systematic error, the full-width at half-maximum (FWHM) and the statistical error were quantified using the simulated data. Experimental phantom data were acquired for visual comparison with the reconstructions obtained from the simulated data.

RESULTS

For a predefined number of iterations, the systematic error, the FWHM and the statistical error could be decreased when the pinhole aperture was modelled during iterative reconstruction. For a fixed, predefined statistical error of +/-10%, smaller systematic errors and smaller FWHM were obtained when modelling the pinhole opening. When the dual matrix implementation was used, equivalent results could be obtained as when modelling the pinhole opening in both the forward and the back projector.

CONCLUSION

The multi-ray method to accomplish resolution recovery during the reconstruction of pinhole SPECT projection images offers a better trade-off between spatial resolution and noise compared with a reconstruction which does not model the pinhole aperture.

Small animal SPECT and its place in the matrix of molecular imaging technologies

Molecular imaging refers to the use of non-invasive imaging techniques to detect signals that originate from molecules, often in the form of an injected tracer, and observe their interaction with a specific cellular target in vivo. Differences in the underlying physical principles of these measurement techniques determine the sensitivity, specificity and length of possible observation of the signal, characteristics that have to be traded off according to the biological question under study. Here, we describe the specific characteristics of single photon emission computed tomography (SPECT) relative to other molecular imaging technologies. SPECT is based on the tracer principle and external radiation detection. It is capable of measuring the biodistribution of minute (<10(-10) molar) concentrations of radio-labelled biomolecules in vivo with sub-millimetre resolution and quantifying the molecular kinetic processes in which they participate. Like some other imaging techniques, SPECT was originally developed for human use and was subsequently adapted for imaging small laboratory animals at high spatial resolution for basic and translational research. Its unique capabilities include (i) the ability to image endogenous ligands such as peptides and antibodies due to the relative ease of labelling these molecules with technetium or iodine, (ii) the ability to measure relatively slow kinetic processes (compared with positron emission tomography, for example) due to the long half-life of the commonly used isotopes and (iii) the ability to probe two or more molecular pathways simultaneously by detecting isotopes with different emission energies. In this paper, we review the technology developments and design tradeoffs that led to the current state-of-the-art in SPECT small animal scanning and describe the position SPECT occupies within the matrix of molecular imaging technologies.

A REVIEW OF SMALL ANIMAL IMAGING PLANAR AND PINHOLE SPECT gamma CAMERA IMAGING

Scintigraphy (positron emission tomography (PET) or single photon emission computed tomography (SPECT) techniques) allows qualitative and quantitative measurement of physiologic processes as well as alterations secondary to various disease states. With the use of specific radioligands, molecular pathways and pharmaco-kinetic processes can be investigated. Radioligand delivery can be (semi)quantified in the region of interest in cross-sectional and longitudinal examinations, which can be performed under the same conditions or after physiologic or pharmacologic interventions. Most preclinical pharmacokinetic studies on physiological and experimentally altered physiological processes are performed in laboratory animals using high-resolution imaging systems. Single photon emission imaging has the disadvantage of decreased spatial and temporal resolution compared with PET. The advantage of SPECT is that equipment is generally more accessible and commonly used radionuclides have a longer physical half-life allowing for investigations over a longer time interval. This review will focus on single photon emission scintigraphy. An overview of contemporary techniques to measure biodistribution and kinetics of radiopharmaceuticals in small animal in vivo is presented. Theoretical as well as practical aspects of planar gamma camera and SPECT pinhole (PH) imaging are discussed. Current research is focusing on refining PH SPECT methodology, so specific regarding technical aspects and applications of PH SPECT will be reviewed.

Small animal imaging with high resolution single photon emission tomography

Molecular imaging of small animals in vivo is vital in the study of mouse and rat models of human diseases, and will provide important clues to the pathogenesis, progression and treatment of many disorders. Functional imaging of small animals using ultra-high resolution single photon emission tomography (SPECT) should be a valuable tool in the molecular imaging armamentarium. SPECT has been used to study cerebral binding sites, to image the expression of reporter genes, and in applications in cardiology and oncology. In this review, we summarize the most recent developments in SPECT imaging of small animals, with particular reference to the types of systems available, their application, and some of the potential limitations.

Pinhole single-photon emission computed tomography for myocardial perfusion imaging of mice

OBJECTIVES

Although transgenic mice have emerged as powerful experimental models of cardiovascular disease, methods for in vivo phenotypic assessment and characterization remain limited, motivating the development of new instruments for biologic measurement.

BACKGROUND

We have developed a single-photon emission computed tomography system with a pinhole collimator (pinhole SPECT) for high-resolution cardiovascular imaging of mice. In this study, we describe a protocol for myocardial perfusion imaging of mice using technetium-99m ((99m)Tc)-sestamibi and demonstrate the feasibility for measurement of perfusion defect size from pinhole SPECT images.

METHODS

Mice were anesthetized and injected with 370 MBq (10 mCi) of (99m)Tc-sestamibi. Tomographic projection images were acquired by rotating each mouse in a vertical axis in front of a stationary clinical scintillation camera equipped with a pinhole collimator. BALB/c mice (n = 15) were imaged after the permanent ligation of the left anterior descending coronary artery. The resulting defect size was measured from circumferential profiles of short-axis images. After imaging, the hearts were excised and sectioned to obtain ultra-high resolution digital autoradiographs of (99m)Tc-sestamibi, from which the actual infarct size was determined.

RESULTS

Reconstructed image quality was equivalent to that obtained for clinical myocardial perfusion imaging. Linear regression analysis produced a correlation coefficient of 0.83 (p < 0.001) between the measured and actual values of the defect size.

CONCLUSIONS

These results demonstrate that myocardial perfusion can be characterized qualitatively and quantitatively in mice using pinhole SPECT.

Performance evaluation of a pinhole SPECT system for myocardial perfusion imaging of mice

The increasing use of transgenic mice as models of human physiology and disease has motivated the development of dedicated in vivo imaging systems for anatomic and functional characterization of mice as an adjunct to or a replacement for established ex vivo techniques. We have developed a pinhole single photon emission computed tomography (SPECT) system for high resolution imaging of mice with cardiovascular imaging as the primary application. In this work, we characterize the system performance through phantom studies. The spatial resolution and sensitivity were measured from images of a line source and point source, respectively, and were reported for a range of object-to-pinhole distances and pinhole diameters. Tomographic images of a uniform cylindrical phantom, Defrise phantom, and grid phantom were used to characterize the image uniformity and spatial linearity. The uniform phantom image did not contain any ring or reconstruction artifacts, but blurring in the axial direction was evident in the Defrise phantom images. The grid phantom images demonstrated excellent spatial linearity. A novel phantom modeling perfusion of the left ventricle of a mouse was designed and built with perfusion defects of varying sizes to evaluate the system performance for myocardial perfusion imaging of mice. The defect volumes were measured from the pinhole SPECT images and correlated to the actual defect volumes calculated according to geometric formulas. Linear regression analysis produced a correlation coefficient of r = 0.995 (p < 0.001), demonstrating the feasibility for measurement of perfusion defect size in mice using pinhole SPECT. We have performed phantom studies to characterize the spatial resolution, sensitivity, image uniformity, and spatial linearity of the pinhole SPECT system. Measurement of the perfusion defect size is a valuable phenotypic assessment and will be useful for hypothesis testing in murine models of cardiovascular disease.

Noninvasive measurement of myocardial activity concentrations and perfusion defect sizes in rats with a new small-animal positron emission tomograph

BACKGROUND

We explored the feasibility of measuring regional tracer activity concentrations and flow defects in myocardium of rats with a high spatial resolution small-animal PET system (microPET).

METHODS AND RESULTS

Myocardial images were obtained after intravenous (18)F-fluorodeoxyglucose (18FDG) in 11 normal rats (group 1) and assembled into polar maps. Regional 18F activity concentrations were measured in 9 regions of interest and compared with tissue activity concentrations measured by well counting. In another 9 rats (group 2), myocardial perfusion images were acquired with 13N-ammonia at baseline and during coronary occlusion. On the polar maps recorded during coronary occlusion, the size of perfusion defects was measured as the myocardium with <50% of maximum activity and expressed as percent total myocardium and was correlated with the area at risk defined by postmortem staining. The diagnostic quality of 18FDG and 13N-ammonia microPET images was good to excellent; the images were easily assembled into polar maps. In group 1, regional (18)F concentrations by microPET and postmortem were correlated linearly (r=0.99; P<0.01 for average and r=0.97; P<0.01 for regional concentrations). In group 2, perfusion defect sizes by microPET and postmortem were correlated linearly (P<0.01; r=0.93).

CONCLUSIONS

The findings indicate the feasibility of noninvasive studies of the myocardium in rats with a dedicated small-animal PET-imaging device.

Imaging transgenic animals

Transgenic and eugenic animals as small as 30 g can be studied non-invasively by radionuclides with resolutions of 1-2 mm, by MRI with resolution of 100 microns and by light fluorescence and bioluminescence with high sensitivities. The technologies of radionuclide emission, magnetic resonance imaging, magnetic resonance spectroscopy, optical tomography, optical fluorescence and optical bioluminescence are currently being applied to small-animal studies. These technologies and examples of their applications are reviewed in this chapter.

High resolution SPECT in small animal research

Single Photon Emission Computed Tomography (SPECT) is a technique used to assess physiological and biochemical processes under in vivo conditions. SPECT generates tomographic images from blood flow, glucose metabolism and receptor characteristics using radioactively labelled substances. This paper reviews the state of the art of in vivo imaging of laboratory animals in modified human and dedicated animal SPECT scanners. SPECT cameras with special collimators currently reach spatial resolutions up to 1 mm and sensitivities of about 1000 cps/MBq, allowing observation of receptor activity concentration changes in the pico-mole range. The time resolution of such cameras strongly depends on the pharmacological behaviour of the tracer and can range from several minutes to hours. Within these limits the functional characterization of many processes is possible. SPECT also offers the possibility to set up dynamic study protocols and repeated measurements of the same animal. This technique reduces the need for sacrificing animals, as was commonly practiced before the development of animal cameras. Animal SPECT gives the opportunity to monitor physiological and biochemical processes in animals in vivo, without interfering with the system under observation, and may become a valuable adjunct to the instrumentation (autoradiography, in vitro methods) of animal research.

Evaluation of myocardial infarct size in rat heart by pinhole SPECT

BACKGROUND

High-resolution single photon emission computed tomography (SPECT) with a pinhole collimator is a new method for evaluating the regional properties of radiopharmaceuticals in small laboratory animals in vivo. Although several reports of normal images of rat taken by this new technique are available, there are as yet few reports on its use in disease models, such as myocardial infarction. In this study, we clearly visualized myocardial flow in the rat heart with myocardial infarction using this system, and evaluated the relationship between SPECT images and histologic analysis.

METHODS AND RESULTS

For visualization of myocardial flow in rat heart, 201Tl images were taken just before and 24 days after left coronary artery ligation. The images were taken using a 4-head SPECT scanner with pinhole collimators. The percent infarct size on 201Tl-SPECT imaging (%SI) and the defect score were then assessed and compared with the percent infarct size on histologic analysis (%HI). Both the %SI and defect score correlated well with %HI (r = 0.97 and 0.74, respectively).

CONCLUSION

Serial SPECT imaging using pinhole collimators permits estimates of myocardial flow even in small laboratory animals noninvasively in vivo.

The role of 99mTc-tetrofosmin Pinhole-SPECT in breast cancer axillary lymph node staging

The number of metastatic axillary nodes represents one of the most important prognostic factors in preoperative breast cancer patients. 99mTc-Tetrofosmin high resolution Pinhole (P)-SPECT was employed in 112 patients, 100 with breast cancer and 12 with benign mammary lesions, to ascertain axillary lymph node involvement. Axillary P-SPECT images were acquired utilizing specific software connected to a circular high resolution, single-head gamma camera equipped with a pinhole collimator with aperture size of 4.45 mm, rotating 180 degrees around the involved axilla. At the same time, patients also underwent conventional SPECT and planar acquisitions. Per-patient sensitivity and specificity were 100% and 93.6% for P-SPECT, 96.2% and 93.6% for SPECT and 56.6% and 100% for planar imaging, respectively. Moreover, P-SPECT detected more than 51% of lesions ascertained by histology, whereas SPECT and planar detected 32.6% and 20.3%, respectively. Only P-SPECT succeeded in identifying the exact number of metastatic axillary lesions in patients with multiple nodes; this procedure was able to correctly differentiate 88.67% of patients with 3 or less nodes from those with more than 3, thus giving important prognostic information. These data suggest 99mTc-Tetrofosmin P-SPECT is a reliable imaging method both for staging and prognostic purposes in breast cancer, and its routine use is recommended.

Performance stability of SHR-2000 high resolution PET for animal research

The performance of a high resolution positron emission tomography (PET) system SHR-2000 for animal studies was re-evaluated six years after its installation. The system employs a detector array consisting of BGO crystals that are 1.7 mm (transaxially) by 10 mm (axially) by 30 mm (deep). A block detector, which is a position-sensitive photomultiplier tube (PMT) coupled to 4 arrays of BGO crystals has been adopted to the system. There are 15 block detectors positioned to form a 35 cm diameter ring with a field of view (FOV) of 17 cm by 4.6 cm axially, giving the system a 7 slice imaging capability. For six year workload in spatial resolution (FWHM), there were approximately a 2.6% increase at tangential FOV and a 7.5% increase at radial FOV. In axial resolution (FWHM) there was almost no change. The count rate loss for the true count rate increased 1.3% at 200 kBq/ ml. The average slice sensitivity showed a decrease of approximately 4.1%, and in scatters it showed an increase of approximately 1.4%. In animal experiments, the bones of guinea pigs were clearly identified with 18F fluoride ion. These experiments show that after a six year workload, the system also maintains good performance and has good stability.