Characterization Methods of High-Intensity Focused Ultrasound-Induced Thermal Field

High-intensity focused ultrasound (HIFU) is a minimally invasive medical procedure which has shown considerable potential for a variety of therapeu­ tic applications. Some of the medical applications of HIFU include thermal ablation of tumors and uterine fibroids, vessel cauterization, thrombolysis, drug delivery, and gene activation (Curra and Crum [1]). HIFU ablation, unlike other minimally invasive hyperthermia techniques, is completely noninvasive with minimum patient recovery time (ter Haar [2]). Tissue damage via ultrasound is achieved by the conversion of the mechanical energy of acoustic waves to thermal energy as the ultrasound propagates through the tissue (Fig. 1(a)). Temperature rise of the order of 40–60 C is achieved within a few seconds, causing immediate cell death or necrosis in the targeted region (ter Haar [2]). Since the size of the single lesion is small (few centimeters in length few millimeters in width) in comparison to the tumor (Fig. 1), multiple sonications are typically required to comple­ tely destroy the tumor (Fig. 1 (b)). Unlike other hyperthermia techniques such as radio frequency (RF) and laser ablation procedures, during a typical HIFU procedure, a large amount of energy is deposited in a short duration causing sudden, drastic, and localized rise in tissue temperature. Cell necrosis time for a typical HIFU procedure is a few seconds, while it is of the order of several minutes for other hyperthermia techniques (Hariharan et al. [3]). Consequently, to prevent excessive or collateral tissue damage, the HIFU beam should