Numerical model of Lamb wave propagation in the tapered septal wall of the heart

Abstract

Shear Wave Elastography (SWE) has been proposed to investigate cardiac health by non-invasively monitoring tissue stiffness. Previous work has shown that the plate-like geometry of the Interventricular Septum (IVS) may result in a dispersion similar to Lamb waves, complicating the link between shear wave speed and cardiac stiffness. However, the IVS is not a simple plate, e.g., its thickness tapers across its length. We have used 2-D Finite Element simulations to investigate the effects of tapering on Lamb waves. The model consists of an elastic slab immersed in water, with a thickness decreasing smoothly in space from 9 to 3 mm. Pulses with low (0–80 Hz) and high (0–700 Hz) frequency contents were used to excite natural and acoustic radiation force induced waves. The results show that natural waves can decelerate by up to 20% during propagation, leading to ambiguities in speed estimation. Moreover, neglecting tapering when fitting their dispersion curves can introduce errors in shear modulus estimation by up to 30%. In contrast, fits performed on waves with high frequency content yielded shear modulus estimations with < 5%. These results suggest that septal geometry can affect cardiac stiffness estimation performed by SWE, especially when natural waves are employed.