In Vivo Monitoring of HIFU Induced Temperature Rise in Porcine Liver Using Magnetic Resonance Thermometry1

Abstract

High intensity focused ultrasound (HIFU) is a noninvasive medical procedure during which a large amount of energy is deposited in a short duration, which causes sudden localized rise in tissue temperature, and ultimately, cell necrosis. In the preclinical characterization of thermal fields generated by HIFU systems, the temperature rise in an ex vivo or an in vivo tissue must be accurately measured. The temperature rise can be measured using thin wire thermocouples or magnetic resonance (MR) thermometry. Among the two methods, thermocouples can be embedded invasively in the animal tissue, and HIFU induced temperature rise can be measured by focusing the beam on the thermocouple junction for the desired sonication time. However, the temperature rise measured using thermocouples is subject to several significant sources of error. One source of error associated with direct HIFU sonication of thermocouples is viscous-heating artifact [1]. Positioning errors represent another challenge in measuring temperature by locating beam atop a thermocouple [2]. Consequently, it has been difficult to accept that the temperature recorded by the thermocouple is the actual temperature at the focus of the HIFU beam. Therefore, there is a need for a method that can address these limitations. Unlike the measurement of temperature using thermocouples, the MR thermometry is a noninvasive method that does not suffer from the problems of positioning error and thermal artifacts. The magnetic resonance imaging (MRI) scanner is capable of assessing the transient temperature rise across the treatment volume, as well as measuring the volume of the thermal lesion [3]. Although the MR temperature monitoring was used to evaluate the feasibility of MR-guided HIFU ablation in the liver and kidney [4], the effect of higher acoustic powers on initiation of cavitation and possible maximum temperature rise in these organs were not assessed using MR thermometry. In this study, MR thermometry was used to monitor HIFU ablations performed on in vivo porcine livers, at elevated acoustic powers of 10 W, 30 W, and 40 W to assess the temperature field where initiation of cavitation is known to occur. Temperature rise during the heating phase as well as the temperature decay during the cooling phase was acquired. The transient temperature profiles measured during the heating and cooling phases for the three powers were compared, in order to check the HIFU induced maximum temperature rise.