Application of Clinical Scanners in Rats: Experimental Carotid Imaging Using Magnetic Resonance Imaging, Spiral Computed Tomography, and Color Duplex Ultrasound

Cardiovascular magnetic resonance imaging in small animals

Noninvasive imaging studies involving small animals are becoming increasingly important in preclinical pharmacological, genetic, and biomedical cardiovascular research. Especially small animal magnetic resonance imaging (MRI) using high field and clinical MRI systems has gained significant importance in recent years. Compared to other imaging modalities, like computer tomography, MRI can provide an excellent soft tissue contrast, which enables the characterization of different kinds of tissues without the use of contrast agents. In addition, imaging can be performed with high spatial and temporal resolution. Small animal MRI cannot only provide anatomical information about the beating murine heart; it can also provide functional and molecular information, which makes it a unique imaging modality. Compared to clinical MRI examinations in humans, small animal MRI is associated with additional challenges. These included a smaller size of all cardiovascular structures and a up to ten times higher heart rate. Dedicated small animal monitoring devices make a reliable cardiac triggering and respiratory gating feasible. MRI in combination with molecular probes enables the noninvasive imaging of biological processes at a molecular level. Different kinds of iron oxide or gadolinium-based contrast agents can be used for this purpose. Compared to other molecular imaging modalities, like single photon emission computed tomography (SPECT) and positron emission tomography (PET), MRI can also provide imaging with high spatial resolution, which is of high importance for the assessment of the cardiovascular system. The sensitivity for detection of MRI contrast agents is however lower compared to sensitivity of radiation associated techniques like PET and SPECT. This chapter is divided into the following sections: (1) "Introduction," (2) "Principals of Magnetic Resonance Imaging," (3) "MRI Systems for Preclinical Imaging and Experimental Setup," and (4) "Cardiovascular Magnetic Resonance Imaging."

Cardiovascular MRI in small animals

Imaging studies of cardiovascular disease in small rodents have become a prerequisite in preclinical cardiovascular research. Transgenic and gene-knockout models of cardiovascular diseases enables the investigation of the influence of single genes or groups of genes on disease pathogenesis. In addition, experimental and genetically altered models provide valuable in vivo platforms to investigate the efficacy of novel drugs and contrast agents. Owing to the excellent soft tissue contrast, high spatial and temporal resolution, as well as the tomographic nature of MRI, anatomy and function can be assessed with unique accuracy and reproducibility. Furthermore, using novel targeted MRI contrast agents, molecular changes associated with cardiovascular disease can be investigated in the same imaging session. This review focuses on recent advances in hardware, imaging sequences and probe design.

VCAM-1 siRNA reduces neointimal formation after surgical mechanical injury of the rat carotid artery

OBJECTIVE

Restenosis is one of several complications following carotid endarterectomy (CEA). The pathogenesis of restenosis may be related to postsurgery inflammation and leukocyte recruitment mediated by cellular adhesion molecules. In this study, we examine the role of vascular cell adhesion molecule-1 (VCAM-1) in carotid neointimal hyperplasia following carotid surgical mechanical de-endothelialization (CSMDE) in a rat model of CEA.

METHODS

The inhibition of siRNA on VCAM-1 protein expression was determined by using the methods of immunostaining and Western blot. Ultrasound imaging and morphometric analysis were applied to measure the degree of CSMDE-induced carotid artery neointimal hyperplasia of rats.

RESULTS

We found that a lentivirus-based construct expressing a small interfering RNA (siRNA) against VCAM-1 could effectively (P < .05, n = 10 per group) reduce VCAM-1 protein expression in the carotid arteries of rats undergoing CSMDE (CSMDE+RNAi: 135.0 +/- 27.6%) when compared that of CSMDE with scrambled siRNA (CSMDE+CON: 182.7 +/- 36.4%). Doppler ultrasonography revealed that CSMDE+RNAi was accompanied by a significant reduction in the extent of stenosis demonstrated by increased blood velocity (665.85 +/- 48.37 mm/s) and linear diameter (0.59 +/- 0.77 mm) compared to CSMDE+CON (46.72 +/- 28.67 mm/s with undetectable linear diameter, P < .05, n = 10 per group). In addition, morphometric analysis of hematoxylin and eosin (HE)-stained sections indicated that the intima (innermost layer of media at lesion site)/media area ratio (I/M) was significantly increased (P < .05, n = 10 per group) both in the CSMDE (3.99 +/- 0.65) and CSMDE+CON (4.33 +/- 0.59) groups compared with the SHAM group (0.35 +/- 0.13). However, CSMDE+RNAi resulted in a significant (P < .05, n = 10 per group) decrease in the I/M ratio (1.79 +/- 0.43) compared to CSMDE+CON, whereas there were no significant differences in the total arterial area and medial areas among the groups.

CONCLUSION

These results suggest that perivascular events mediated by VCAM-1 are likely to play an important role in the pathogenesis of carotid artery neointimal hyperplasia in rats after CSMDE.

Magnetic resonance angiography of carotid and cerebral arterial occlusion in rats using a clinical scanner

In rat models to induce both focal cerebral ischemia and chronic cerebral hypoperfusion, it is highly desirable to verify the success of vessel occlusion and reopening with non-invasive method. The contrast-agent free 3D time-of-flight magnetic resonance angiography (TOF-MRA), diffusion-weighted imaging (DWI) and T2-weighted imaging by 3.0-T MR clinical scanner were applied when unilateral middle cerebral artery (MCA) was occluded and reopened, and after bilateral common carotid arteries were in ligation. The arterial angiograms of the rat brain and neck were achieved successfully in all chosen directions by the 3D TOF-MRA. It was shown that MCA in occlusion presented no signal in MRA, and the parenchyma of the ipsilateral MCA territory hypointensity signal in maps of apparent diffusion coefficient (ADC). After reperfusion, the signal intensity of ipsilateral MCA was resumed in MRA, and the decreased ADC was restored simultaneously. However, after 5h of reperfusion, it was found that the value of ADC deteriorated second time with high T2 value. In bilateral common carotid artery occlusion (BCCAO) rats, it can be confirmed by MRA that the effectively occluded BCCA presented the absent signal and the basilar artery became tortuous. As a result, MRA by clinical scanner was proved of a valuable method to validate transient middle cerebral artery occlusion (MCAO) and permanent BCCAO rat model.

Fischer-344 rats are unsuitable for the MCAO filament model due to their cerebrovascular anatomy

Middle cerebral artery occlusion (MCAO) in Fischer-344 rats results in a small variance of infarct size. However, complications are frequent especially in aged Fisher-344 rats undergoing endovascular suture occlusion of the middle cerebral artery. Analyzing our experiences with 165 Wistar, 13 Sprague-Dawley and 10 F-344 rats, we compared the incidence of impossible thread advancement and subarachnoid hemorrhage, respectively. Magnetic resonance angiography (MRA) was applied to study the course of the internal carotid artery (ICA) in Fischer and Wistar rats. Finally, we performed a structured review of the literature from 1991 to 2005 evaluating reports on Fischer rats subjected to intraluminal filament MCAO. Complications like fruitless filament advancement or subarachnoid hemorrhage were found to be significantly more frequent in Fischer rats than in other strains. MRA revealed significantly more pronounced kinking of the ICA in F-344 than in Wistar rats. In seven publications available on filament MCAO in F-344 rats, complication rates of 50-100% were reported, corroborating our data. Surgical difficulties accompanied by high complication rates due to their cerebrovascular anatomy make Fischer rats unsuitable for filament MCAO. If the use of Fischer rats for studies on focal cerebral ischemia is indicated, other ischemia models than intraluminal suture occlusion should be chosen.