Investigation of the effects of myocardial anisotropy for shear wave elastography using acoustic radiation force and harmonic vibration

Shear wave elastography with acoustic radiation force (ARF) or harmonic vibration (HV) has been applied in animals and humans to evaluate myocardial material properties. The anisotropic myocardial structure presents a unique challenge to wave propagation methods because the fiber direction changes through the wall thickness. To investigate the effects of the frequency of excitation in the myocardium we constructed systolic and diastolic finite element models (FEMs) and performed an experiment on an ex vivo porcine heart. Both models were constructed with multiple elastic, transverse isotropic layers with a shear wave velocity (SWV) along and across the fibers where each layer has 1 mm thickness with the top and bottom in contact with water. The orientation of the muscle fibers was changed for each layer ranging from -50° to 80° from top to bottom. Harmonic excitations at 30, 50, 100, and 200 Hz and an impulsive force were used. An ex vivo porcine heart was tested using ARF excitations with a transesophogeal probe driven with a Verasonics ultrasound system applied directly to the left ventricular wall. We evaluated the measured orientation of the fibers in each layer by evaluating the angle with the highest SWV. The 30 and 50 Hz results showed little or no variation in the measured orientation angle in the layers. The 100 and 200 Hz results showed some variation of the orientation with respect to the layer. The impulse simulation results showed good agreement with the true orientations except near the top and bottom boundaries. The values of SWV were found to have different levels of bias depending on the excitation. The experimental results in the ex vivo heart showed similar trends as the FEM model results where the waves at lower frequencies had lower sensitivity to fiber direction. This multi-layered anisotropic model demonstrates how to resolve different anisotropic layers in the myocardium using ARF or HV while also revealing that using lower frequencies results in measurements that are less sensitive to anisotropy variation through the thickness of the myocardial wall.