Background: Myocardial fibrosis
has important clinical and prognostic implications. Cost-effective imaging
tools for fibrosis screening to enable personalized therapies are thus of major
interest. Echocardiographic shear wave (SW) elastography is an emerging
approach for measuring myocardial stiffness (MS). SWs occur after mechanical
excitation of the myocardium, e.g. after mitral valve closure (MVC, i.e natural
SW), and their propagation velocity is directly related to MS.
Purpose: To investigate
if natural SW velocities can detect myocardial fibrosis.
Methods: We
included 89 subjects: (30 heart transplant patients [52±17years, 80% male], 22 patients
with hypertrophic cardiomyopathy [55±16 years, 58% male] and 37 healthy
volunteers [HV, 48±17 years, 76% male]). SW elastography was performed in
parasternal long axis views of the left ventricle (LV) using an
experimental scanner (HD-PULSE) at 1134±255 frames per second. Tissue
acceleration maps were extracted from an anatomical M-mode line along the
midline of the LV septum. SW propagation velocity at MVC was measured as slope
in the M-mode image. Besides conventional echocardiography, patients underwent
also 1.5T cardiac magnetic resonance with T1 mapping as well as late gadolinium
enhancement (LGE) to assess the presence of myocardial fibrosis (Fig 1).
Subjects were divided into 4 groups: HV, patients with no fibrosis (NF), interstitial
fibrosis (MIF) and replacement fibrosis (MRF).
Results: SWVs differed significantly
among groups (p<0.001), with a significant post-test between MIF and MRF and subjects without fibrosis (6.5±1.1 m/s and
8.7±1.2 m/s), respectively. The NF group showed a tendency towards higher
values of SW velocities than HV, but the difference didn’t reach the
pre-defined significance level (3.4±1.0 m/s vs. 4.3±1.3 m/s, p=0.06. (Fig 2A)).
SW velocity showed significant
correlations with ECV values (r=0.70) and T1 values (r=0.47), and markers of LV diastolic function (E/e’: r=0.54; isovolumetric relaxation
time: r=0.39; deceleration time: r=0.28 (all p<0.01) (Fig 2B-D).
A cut-off SW velocity of 3.86m/s allowed to differentiate
between HV and the NF group with a sensitivity of 65% and a specificity of 76%
(area under the curve (AUC)= 0.73). A cut-off of 4.58m/s distinguished between
HV and all patient groups (sensitivity 95%, specificity 73%, AUC 0.88). SW velocities below 6.0 m/s
showed highest accuracy to identify patients without any type of fibrosis (sensitivity 97%, specificity 90%, AUC=0.97). A cut-off of 8.1 m/s could distinguish replacement fibrosis from
diffuse fibrosis with a sensitivity and specificity of 100% and 69%,
respectively (AUC=0.92).
Conclusions: SW velocity measurements can
differentiate between fibrotic myocardium and healthy tissue with very good accuracy.
Different types of fibrosis (interstitial vs. replacement) could also be
distinguished. Nevertheless, we hypothesize that stiffness changes relate
mainly to the fibrosis burden. Shear wave elastography appears as promising new
tool for the non-invasive assessment of myocardial fibrosis.
