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

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.