Cardiovascular magnetic resonance imaging of the vessel wall

Atherosclerosis (AS) is a chronic immune-mediated inflammatory state and multifactorial disease of large and medium sized arteries leading to cardiovascular diseases such as stroke and myocardial infarction.
Atherogenesis is studied by the use of animal models (mouse, rabbit, swine) and the most popular strategy to induce lesion development in a rabbit model of atherosclerosis (New Zealand White rabbit) is the combination of a dietary trigger and balloon catheter denudation (mechanical intervention). Because the aorta is narrowing from proximal to distal a constant balloon volume introduces significant pressure variations when retracted. Consequently, a not well-defined, heterogeneous plaque development can be expected, which represents a major drawback of this technique. Moreover, catheter induced injury may also vary between operator's as the handling of syringe and balloon filling requires training and adjusting a prescribed pressure to vessel diameter variations seems to be challenging. To compensate for that operator driven errors a new technique (Patent Cooperation Treaty (PCT) application number PCT/EP2017/071300) is proposed for the generation of more well-defined and reproducible aortic atherosclerotic plaques by a pressure controlled mechanical balloon injury device. Balloon inflation pressure recordings support the superiority of the proposed method compared to the conventional technique.
Multicontrast magnetic resonance imaging (MRI) has become a well-recognized tool to study the complexity of an atherosclerotic plaque in order to predict lesion vulnerability. Diffusion tensor imaging (DTI) could be an additional measure that provides information on the microstructural changes of the vessel wall due to tissue remodeling and plaque burden, which may help to understand the formation of a complex disease. In this work, the feasibility to investigate the orientation of diffusion anisotropy in healthy human carotids in vivo is demonstrated. Tangential and radial diffusion were measured by the combination of a read-out segmented EPI (rs-EPI) with a 2D special gradient direction scheme. A significant linear relationship between age and fractional anisotropy (FA) was found which may indicate a possible alteration of tissue's microstructural integrity. Moreover, this finding point FA being a sensitive metric able to uncover changes in the wall architecture with age.
Beside the use of multispectral protocols or all-in-one solutions to identify plaque morphology and composition a complementary method to study lesion progression is molecular imaging. A technique presented in this work allows the simultaneous measurement of endothelial barrier function (interendothelial junction integrity) and nitric oxide induced endothelium-independent (smooth-muscle cell dependent) vasomotor response of the abdominal aorta in rabbits. The infusion of the nanoparticle caused a prominent relaxation of the vessel wall and a decrease in vascular endothelial permeability of the control animal. The AS induced rabbit with comparable wall appearance (similar normalized wall index) showed an impaired vasodilation as well as loss of the interendothelial junction integrity (endothelial damage). The rabbit with a marked wall thickening showed a paradoxical vasoconstriction and a reconstituted delayed endothelial function. These data assume specific patterns depending on the degree of plaque burden. The use of dynamic nanoparticle enhanced plaque MR imaging for assessing vasomotion and permeability seems to be a potential strategy for risk stratification of the vascular fitness.