Focused ultrasound waves in heterogeneous tissue

When focused ultrasound waves propagate through the human body, the combined effects of nonlinearity, absorption, refraction, and scattering can significantly alter the position and shape of the focal volume compared to the response in water. This can affect the desired clinical outcome, and lead to adverse events such as near-field heating and skin burns. In this chapter, these wave phenomena are discussed and their respective importance is demonstrated through numerical simulations based on a generalized Westervelt equation. Simple analytical formulae for the response in reference media are also provided. Both homogeneous and heterogeneous media are considered, including sonications of the kidney and liver through the muscles surrounding the abdominal cavity. The effect of frequency selection on the focal intensity, volume, and heating rate is also discussed