CMR is considered the “gold” standard for noninvasively characterizing cardiac function.
In order to track the motion of the ventricles during the cardiac cycle, imaging sequences are gated using
a 3-lead ECG. Typically, ventricular cines are imaged with 12-25 phases spanning the cardiac cycle.
Ventricular cines in either the cardiac short-axis or long-axis can provide an accurate estimation of the
following functional parameters:
Ejection fraction (EF)
Systolic wall thickening
Myocardial tagging is a CMR method to measure regional myocardial strain. This specialized imaging sequence tags
the myocardium by saturating the spins of the hydrogen atoms in specific slabs of the tissue in the shape of a grid.
Saturation of the spins attenuates the MR signal, thus creating black lines in the images. The saturated spins become
a local property of the tissue during the pulse sequence; therefore, the tags or lines move with the tissue during
the cardiac cycle. With tagging, the deformation of the myocardium (strain) can be measured at any point in the cardiac cycle.
Strain tagging is an important imaging technique in identifying regional myocardial disorders, such as a myocardial
infarction. Tagging can identify scars or regions in the myocardium which are not contracting; the systolic circumferential
strain in scarred or infarcted regions will be depressed.
Short-axis strain tagging of a canine heart after a four week ligation of the left anterior descending artery. Circumferential
shortening is indicated in blue during the systolic phase. The blue marker lies in the infracted region while the orange
marker lies in an unaffected region.