Ultrasound shear wave elastography for detection of myocardial fibrosis

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.