Assessment of Heart Valve Function by Magnetic Resonance Velocity Mapping

Valvular diseases cause an increased hemodynamic load on the heart. Chronic valvular incompetence may lead to impaired myocardial function and eventual progression toward heart failure. Accordingly, current diagnostic modalities aim at the detection of progressing valvular lesions before irreversible changes of the myocardium have developed. Symptomatic patients with severe valvular lesions are referred for valve replacement or valve repair surgery. In asymptomatic and mildly symptomatic patients, however, there is ongoing controversy about the optimal timing for intervention. Thus, a quantitative diagnostic modality providing indices for prediction of optimal post-operative results is a premise for improved patient care. In clinical practice, methods of echocardiography are routinely involved for evaluation of heart valve disease and heart valve function. While these modalities provide sufficient morphological information, the assessment of hemodynamic characteristics is often hampered by the inadequacy to resolve spatial distributions of velocities, limitation due to the restricted echo window, and the inability to register multiple velocity components. Magnetic resonance imaging has evolved to an accurate diagnostic complement for evaluating ventricular geometry and function. The ability to encode blood velocity in arbitrary spatial directions holds potential for accurate quantification of transvalvular blood flow and thus for quantification of valvular incompetence. However, several limitations apply to current methods used for magnetic resonance velocity mapping near or within the heart. Accurate positioning of an imaging slice is hampered by the excursion of the valvular plane during the cardiac cycle. Myocardial contraction and relaxation are referred to as the active motion of the heart. The active motion is a potential source for misregistration of low blood velocities. Acquired images represent velocity magnitudes resulting from blood velocities superimposed by the velocity of through-plane motion of the heart. Accordingly, leakage flow through heart valves may be considerably underestimated if the active motion of the heart is not taken into account. Aim of the present work was to develop improved acquisition and post-processing methods allowing for accurate quantification of blood velocities around heart valves and in the coronary vessels. Dedicated techniques for compensation of the active motion of the heart are presented in combination with improved respiratory gating strategies for image artifact reduction. The accuracy and feasibility of the methods proposed are shown in healthy volunteers and in studies including patients with valvular heart disease.