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

Abstract

Publisher Summary High-intensity focused ultrasound (HIFU) is a minimally invasive medical procedure used for thermal ablation of tumors and uterine fibroids, vessel cauterization, thrombolysis, drug delivery, and gene activation. 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. 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. This chapter focuses on the new preclinical testing methods developed for acoustic intensity and temperature measurements at HIFU energy levels. A preliminary step in the analysis of new medical devices involving high-intensity ultrasound is the determination of the acoustic intensity field in a liquid medium. Medical ultrasound fields generated by focused transducers are usually characterized in water using calorimetry methods, hydrophones, and radiation force balance techniques. The chapter also describes alternate characterization methods and acoustic streaming-based method. Thin wire thermocouples embedded in tissue-mimicking materials or excised tissues have been popularly used as a cost-effective method for measuring the HIFU-induced transient temperature rise. An improvement in HIFU characterization studies is the development of nonperturbing methods, to assess the thermal field in a tissue medium. In this method direct sonication of thermocouple junctions is avoided to prevent artifacts in the temperature data. The beam is focused at locations off the thermocouple junctions and the temperature rise at the remote junctions is measured.