Numerical Study on a Focus-Control Method Using Breast Model With Intentionally Assigned High-Absorbing Layer Near Skin for High-Intensity Focused Ultrasound Treatment

To prevent undesirable skin burns that occur in high-intensity focused ultrasound (HIFU) treatment, we numerically study focus-control methods, such as phase compensation (PC) and amplitude adaptation (AA). We intentionally assign a high-absorbing layer (HAL) near the part of the skin, where heat generation and tissue ablation are observed, because of high energy loss in the interface between water and breast skin. Results show that PC improves the effectiveness of focusing by enhancing the focal peak and reducing the focal deviation; however, PC does not suppress skin burn. AA and PC eliminate skin burns only if appropriate amplitude weights are applied. A preliminary discussion on three algorithms for obtaining amplitude weights is conducted as follows; First, we switched off transducer channels using distance-to-HAL. This algorithm eliminates skin burns while causing other undesirable burns by preserving 100% input energy. Second, we use cross-correlated amplitude weights. It eliminates skin burn after properly limiting large-amplitude weights while producing focal necrosis in a smaller and slower manner. Third, we introduced root-mean-square (rms) level of back-propagated wave (BPW) into cross-correlated amplitude weights. This new algorithm produces focal ablation in 20 s without causing any skin burn. Although longer irradiation time brings back skin burn, the result is satisfying since short irradiation time is needed in HIFU treatment to avoid exceeding the physical endurance of human patients. Moreover, this work indicates that focus-control associated with an acoustic peak is insufficient. The effects of the high attenuation area are significant and should be captured.