Imaging myocardial strain

Measuring the local mechanical activity of the heart has lagged behind the measurement of electrical activity due to a lack of measurement tools. Myocardial wall motion abnormalities have been studied for years in the context of regional ischemia. Implanted beads and screws have been used to measure the mechanical activity of the heart in a few isolated regions. Over the past decade, precise and accurate methods for measuring local three-dimensional (3-D) myocardial motion with magnetic resonance imaging (MRI) have been developed using presaturation tagging patterns, velocity encoded phase maps, and displacement encoded phase maps. Concurrently, the quality of cardiac MRI images improved greatly with the use of customized receiver coils and the speed of acquisition has increased dramatically with the advent of undersampling techniques and new generations of MR machines with faster switching gradient coils. The use of these cardiac MRI techniques to produce an image of the local deformation of the heart in the form of a myocardial strain image is described. Using these images, the "mechanical activation" of the heart are defined, that is, the time of onset of contraction. A map of the mechanical activation over the heart is a direct analogy to an electrical activation map of the heart.