Phase-contrast MRI-based elastography technique detects early hypertensive changes in ex vivo porcine aortic wall.

Hypertension is a major risk factor associated with atherosclerosis contributing to a greater increase in cardiac events. It has been shown to contribute to fundamental pathologic processes such as systemic remodeling of blood vessel walls, changes in mural extracellular matrix, and abnormal smooth muscle tone contributing to increased wall stiffness (1). This is important because the elastic and muscular properties of the vascular wall determine the propagation of the pulse wave generated with every beat of the heart. The central arteries, i.e. the aorta and its branches, are rich in the elastic components, which smooth out blood flow and pressure (so-called “Windkessel effect”) (2). On the contrary, the muscular elements predominate in the peripheral arteries and determine vascular resistance. Advancing age and systemic arterial hypertension change the structural relaxation properties of the central arteries much more than the muscular arteries by increasing the collagen-elastin ratio (3). This leads to a reduction in the elasticity of the central arteries as well as a reduction of the Windkessel effects with an acceleration of the pulse wave and its reflection in the periphery (2,4). The hemodynamic consequences are augmentation of systolic rather than diastolic blood pressure with an increase in the systolic work-load on the heart and a decrease in the diastolic perfusion of the heart (3). Hence, loss of elasticity of the central conduits over time leads to left ventricular hypertrophy, myocardial ischemia, and increased degenerative changes of the arterial system, thereby adding an important dimension to cardiovascular morbidity and mortality (3,5). Indeed, increased blood pressure is an important determinant of pulse wave velocity, which has been utilized as an index of arterial stiffness (6). Different methodologies have tried to assess vascular stiffness such as analysis of pulse wave velocity (PWV), arterial pressure waveform analysis, and relating change in arterial diameter (or area) to distending pressure. Even though a number of tonometry- and ultrasound-based devices within these three categories are easy to use, their indirect estimate of elasticity, the data bias, and the reduced validity relating to operator experience have been outlined as potential barriers to widespread reliable use (7,8). Hence, while recognized to be an important element for some time, direct measurement of aortic elasticity has not become readily available and no diagnostic gold standard has been established so far. Two MR-based methods, phase contrast MRI and MR elastography, with the potential for non-invasive study of the arterial wall have emerged. Phase contrast MR imaging has been studied in a normotensive porcine model and given good results (9). The other method being studied utilizes a phase contrast MRI-based elastography technique (7). The developed phase contrast MRI-based elastography technique can image micron-amplitude propagating waves induced in the walls of blood vessels by harmonic mechanical stimulation at acoustic frequencies and can be used to quantify vascular wall stiffness (10-12). Additionally, it is also able to determine very small changes in wall thickness and elasticity as determined in model systems (10). Arteries are naturally conducive to this methodology due to the natural wave guide conduction properties (10,13,14). In this study, we therefore hypothesized that early changes in arterial wall elasticity, secondary to hypertension, can be visualized and measured using this novel technique in a previously characterized animal model (15).