Myocardial relaxation diagnosis possible with SWA for cardiac MR elastography

Low frequency shear-wave amplitude (SWA) vibration for cardiac MR elastography offers valuable diagnostic information on myocardial relaxation, according to a study published online Jan. 19 by Radiology.

Myocardial relaxation, which is related to increases and decreases in the shear modulus, is identified by active relaxation process and passive stiffness. Myocardial shear modulus is a direct diagnostic marker of diastolic dysfunction, which can be measured with elastography. “However,” the study’s lead author Thomas Elgeti, MD, of the Charité-Universitätsmedizin Berlin, and colleagues wrote, “elastography—although widely applied to all kinds of body tissue—remains particularly challenging in the heart for several reasons, including the heterogeneity and temporal variability of myocardial tissue, as well as the mechanically shielded position of the heart.”

Time-harmonic shear waves at low-vibration frequencies have been proven to safely and reproducibly inflict gentle mechanical deformations to the beating heart during elastography. Elgeti and colleagues designed their study to determine if SWAs in the myocardium measured with cardiac MR elastography can diagnose myocardial relaxation abnormalities in patients with diastolic dysfunction.

Fifty participants were divided into groups of asymptomatic young, asymptomatic old, and subjects with echocardiographically proved mild, moderate, or severe diastolic dysfunction. All subjects underwent electrocardiographically triggered SWA-based cardiac MR elastography with 24.13-Hz external vibration frequency. The authors examined the SWA images in the left ventricular (LV) region and then normalized the images against reference SWA of the thoracic wall.

The normalized mean of LV SWA for young control subjects was 0.67 and 0.56 for the older control group. Patients with mild, moderate, and severe diastolic dysfunction had a significantly reduced normalized mean of LV SWA. The means were 0.37, 0.34, and 0.29, respectively. Researchers found that the normalized mean of LV SWA was inversely correlated to the severity of diastolic dysfunction. The cutoff value of 0.43 best differentiated between asymptomatic volunteers and patients, with an area under the receiver operating characteristic curve of 0.92. Sensitivity was 90 percent and specificity was 89.7 percent.

“In the future, maps of SWAs may provide a mechanically based image contrast of the heart that allows assessment of myocardial relaxation,” wrote the study’s authors.

Due to several limitations, Elgeti and colleagues suggest that proceeding studies consider the sensitivity of unnormalized SWA through investigation of groups that have comparable body mass indexes and excitation amplitudes.