Temperature- and frequency-dependent dielectric properties of biological tissues within the temperature and frequency ranges typically used for magnetic resonance imaging-guided focused ultrasound surgery

Abstract

Purpose: This study aimed to obtain the temperature- and frequency-dependent dielectric properties of tissues subjected to magnetic resonance (MR) scanning for MR imaging-guided focused ultrasound surgery (MRgFUS). These variables are necessary to calculate radio frequency electromagnetic fields distribution and specific radio frequency energy absorption rate (SAR) in the healthy tissues surrounding the target tumours, and their variation may affect the efficacy of advanced RF pulses. Materials and methods: The dielectric properties of porcine uterus, liver, kidney, urinary bladder, skeletal muscle, and fat were determined using an open-ended coaxial probe method. The temperature range was set from 36 °C to 60 °C; and the frequencies were set at 42.58 (1 T), 64 (1.5 T), 128 (3 T), 170 (4 T), 298 (7 T), 400 (9 T), and 468 MHz (11 T). Results: Within the temperature and frequency ranges, the dielectric constants were listed as follows: uterus 49.6–121.64, liver 44.81–127.68, kidney 37.3–169.26, bladder 42.43–125.95, muscle 58.62–171.7, and fat 9.2327–20.2295. The following conductivities were obtained at the same temperature and frequency ranges: uterus 0.5506–1.4419, liver 0.5174–0.9709, kidney 0.8061–1.3625, bladder 0.6766–1.1817, muscle 0.8983–1.3083, and fat 0.1552–0.2316. Conclusions: The obtained data are consistent with the temperature and frequency ranges typically used in MRgFUS and thus can be used as reference to calculate radio frequency electromagnetic fields and SAR distribution inside the healthy tissues subjected to MR scanning for MRgFUS.