Atherosclerotic lesions rich in macrophages or smooth muscle cells discriminated in rabbit iliac arteries based on T1 relaxation time and lipid content

[Image Characteristics of T1 Weighted Magnetic Resonance Imaging Techniques for Plaque Tissue Characterization (Comparison with Conventional Spin Echo Method)]

In the tissue characterization of plaques using magnetic resonance imaging (MRI), T₁-weighted imaging is important. However, T₁-weighted imaging are obtained by various imaging methods, and show different contrasts depending on parameters such as repetition time, echo time, and inversion time. To evaluate the tissue characterization of plaques using MRI, the characteristics are estimated and evaluated using the strength of the plaque-to-muscle signal intensity ratio (PMR), which is the value obtained by dividing the signal intensity of the plaque by that of the sternocleidomastoid muscle or myocardium. In the present research, we aim to obtain the PMR by phantom experiment and grasp the image characteristics for T₁ and T₂ values of different T₁-weighted imaging methods. In addition, since the PMR of the conventional spin echo (SE) method of T₁-weighted imaging (two-dimensional (2D) T₁WI SE) is reported to have high discrimination ability in plaque tissue characterization, the experimental results were compared with those of 2D T₁WI SE. Among the protocols examined, 3D sampling perfection with application optimized contrasts using different flip angle evolutions, T₁-variable, motion-sensitized driven equilibrium (1-axis 300 ms^2*mT/m) + had the same tissue characterization ability as 2D T₁ WI SE, and was the most suitable imaging method. Moreover, in the gradient echo method, the effect of T₂ values was smaller than that of 2D T₁ WI SE, and it was suggested that the PMR of the plaque may be lowered when there is a change in the tissue properties that the T₂ value and T₁ value are prolonged due to liquefaction. The results of this phantom experiment are expected help in selecting the imaging method aimed at optimization and the image characteristics of different T₁-weighted imaging method can be grasped.

Assessment of global cardiac uptake of radiolabeled iron oxide nanoparticles in apolipoprotein-E-deficient mice: implications for imaging cardiovascular inflammation

PURPOSE

Atherosclerosis is a leading cause of death in industrialized countries and is characterized by the accumulation of lipids and inflammatory cells, including macrophages, in blood vessel walls. Therefore, the ability to image macrophages could help identify plaques that are precursors of acute thrombotic events. Previous research has shown that long-circulating nanoparticles could be used to detect macrophages within atherosclerotic plaques of the aorta. By conducting this study, we investigated whether global cardiac uptake of radiolabeled nanoparticles could allow assessment of total macrophage burden in the coronary arteries.

PROCEDURES

Dextran-coated iron oxide nanoparticles (IONPs) were labeled with iodine-125 via Bolton-Hunter (sulfosuccinimidyl-3-[4-hydroxyphenyl]propionate) method. IONPs were characterized by means of dynamic light scattering and transmission electronic microscopy. Biodistribution studies were performed in healthy and atherosclerotic mice. Additionally, digital autoradiography of hearts from both healthy and atherosclerotic mice was performed to assess regional and global atherosclerotic burden.

RESULTS

The [(125)I]IONPs exhibited high radiolabel stability and long blood circulation, which eventually led to high heart uptake in apoE -/- mice when compared with healthy controls. Furthermore, digital autoradiography showed substantially enhanced emission of signals from the hearts of atherosclerotic mice, while no or minimal cardiac signals were detected in healthy mice.

CONCLUSIONS

This preparation showed adequate physical-chemical properties for in vivo studies, such as small size (∼30 nm), good radiolabel stability, and long circulation time. There was also significant accumulation in the heart of apoE-/- mice compared with that of healthy control animals. These findings suggest that radiolabeled dextran-coated iron oxide nanoparticles may have potential to become a useful tool to detect macrophages in the atherosclerosis plaques of coronary arteries; however, these preliminary findings should be confirmed by further studies in a larger scale in various atherosclerosis models.

A rabbit model of thrombosis on atherosclerotic lesions

Thrombus formation on a disrupted atherosclerotic plaque is a key event that leads to atherothrombosis. Because thrombus is induced by chemical or physical injury of normal arteries in most animal models of thrombosis, the mechanisms of thrombogenesis and thrombus growth in atherosclerotic vessels should be investigated in diseased arteries of appropriate models. Pathological findings of human atherothrombosis suggest that tissue factor, an initiator of the coagulation cascade, significantly affects enhanced platelet aggregation and fibrin formation after plaque disruption. We established a rabbit model of atherothrombosis based on human pathology in which differences in thrombus formation between normal and atherosclerotic arteries, factors contributing to thrombus growth, and mechanisms of plaque erosion can be investigated. Emerging transgenic and stem cell technologies should also provide an invaluable rabbit experimental model in the near future.