Magnetic resonance imaging of myocardial kinematics. technique to detect, localize, and quantify the strain rates of the active human myocardium

Abstract

A magnetic resonance imaging (MRI) method is presented to detect, localize, and quantify myocardial kinematics by measuring the material rate-of-strain tensor at each pixel in gated NMR images of the heart. The immediate, local effect of muscular activity is self-deformation, and the strain tensor is the basic mathematical device by which such deformation may be quantified. The present method, called “strain-phase” MRI (SP-MRI), entails four steps: (1) the velocity of the myocardium is encoded by means of a set of motion-sensitive NMR image acquisitions, one image per velocity component; (2) the spatial derivatives of the velocity are computed at each pixel; (3) the velocity-derivative data are combined to compute an approximation of the strain-rate tensor of the myocardium at each pixel; and (4) the strain-rate tensor data are simplified to produce a color-coded functional image which represents strain-rate components which are of particular biomedical interest in the myocardium. We present a quantitative SP-MRI methodology suited to conventional MRI, and in addition present an “echo-planar” methodology, able to produce qualitative functional images of myocardial kinematics at almost real-time speeds. Two-dimensional strain-phase MRI data acquired in normal human subjects are presented. These data demonstrate the practicability of SP-MRI in vivo, that SP-MRI resolves myocardial kinematics at the single-pixel scale, having resolution comparable to that of conventional MRI, and that SP-MRI data may have a signal-to-noise ratio up to 50% as great as that of the conventional MRI data from which they are produced. SP-MRI measurements of the local instantaneous strain rates in the human left ventricular myocardium are quantitatively consistent with known transmural average values of myocardial strain.