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

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.