(C-88) Characterizing temperature-dependent tissue properties for focused ultrasound modeling

Introduction:: High-intensity focused ultrasound (HIFU) is a non-invasive, therapeutic technique used to ablate tumors. HIFU uses concentrated ultrasound waves that are absorbed by human tissue, increasing the local temperature, and destroying the tissue. A successful HIFU treatment requires a patient-specific treatment plan that is generated before the therapy by clinicians. Computer modeling can assist clinicians by simulating HIFU treatments and predicting treatment outcomes. However, accurate computer simulations are currently limited due to poorly characterized temperature-dependent tissue properties. An assessment of these properties will make treatments safer and more efficacious. Our research aims to experimentally determine temperature-dependent acoustic and thermal properties of porcine muscle tissue for more accurate HIFU simulations.


Materials and Methods:: Thermal properties under investigation include thermal diffusivity [mm²/s], thermal conductivity [W/m·K], and specific heat capacity [kJ/kg·K]. Acoustic properties include the attenuation coefficient [dB/cm], and tissue speed of sound [m/s]. To determine how properties vary with respect to temperature, tissue samples are immersed and allowed to equilibrate in a temperature-controlled water bath prior to measurements. The thermal properties of each sample are measured with commercially available thermal sensors (TEMPOS Thermal Properties Analyzer, Meter Group, Pullman, WA, USA and MCDSC, TA Instruments, New Castle, DE, USA). The insertion-loss through-transmission technique is used to calculate the speed of sound and attenuation. The attenuation coefficient of samples is also determined by using radiation-force insertion-loss measurements.


Results, Conclusions, and Discussions:: Results

Both acoustic and thermal properties were found to have temperature-dependent variation in porcine muscle. Specific heat capacity increased by 9% (approximately 3.4 kJ/kg·K to 3.75 kJ/kg·K) between 20°C and 70°C. Thermal diffusivity increased by 6% (0.141 mm²/s to 0.150 mm²/s) between 20°C to 60°C. Thermal conductivity decreased by 5% (0.494 W/m·K to 0.467 W/m·K) between 20°C to 40°C, and then increased from 45°C to 60°C by 10% (0.465 W/m·K to 0.510 W/m·K). Figure 1 shows the thermal property measurements versus temperature for (left to right) thermal diffusivity, thermal conductivity, and specific heat capacity.

For acoustic properties, the speed of sound at 1 MHz frequency increased by 3% (1575 m/s to 1621 m/s) for temperature increases from 21 to 50 °C. Through-transmission measurements of acoustic attenuation at 3 MHz showed a 16% increase (2.14 dB/cm to 2.54 dB/cm) from 21°C to 50°C. Figure 2 shows the temperature-dependent speed of sound (left) and acoustic attenuation (right) measurements for multiple ultrasound frequencies.
Using the radiation force balance technique at a frequency of 476 kHz, the attenuation coefficient in the porcine tissue increased by 36% (1.54 dB/cm to 2.10 dB/cm) from 21°C to 40°C.

Conclusions

We have developed a systematic approach to measuring temperature-dependent tissue properties. Our results in porcine muscle tissue show trends consistent with results in the scientific literature. Additional experiments are being performed to demonstrate the repeatability of the experiment and assess the reliability of the data.

Discussions

While previous research has focused on smooth muscle tissue, such as porcine kidney and liver, our measurements compliment those data by focusing on striated muscle tissue, specifically porcine loin tissue. Thermal properties follow trends from previous studies, with thermal diffusivity and specific heat capacity having values near porcine kidney, and thermal conductivity resembling porcine liver. When comparing acoustic measurements to porcine properties in different studies, we obtained similar results in the attenuation and speed of sound for a 3MHz signal at room temperature (21°C). Results at other frequency-temperature combinations we investigated were not found in the literature. Future work includes conducting more experiments to assess the reliability and repeatability of measurements.













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