Review of temperature dependence of thermal properties, dielectric properties, and perfusion of biological tissues at hyperthermic and ablation temperatures

Abstract

The application of supraphysiological temperatures (>40°C) to biological tissues causes changes at the molecular, cellular, and structural level, with corresponding changes in tissue function and in thermal, mechanical and dielectric tissue properties. This is particularly relevant for image-guided thermal treatments (e.g. hyperthermia and thermal ablation) delivering heat via focused ultrasound (FUS), radiofrequency (RF), microwave (MW), or laser energy; temperature induced changes in tissue properties are of relevance in relation to predicting tissue temperature profile, monitoring during treatment, and evaluation of treatment results. This paper presents a literature survey of temperature dependence of electrical (electrical conductivity, resistivity, permittivity) and thermal tissue properties (thermal conductivity, specific heat, diffusivity). Data of soft tissues (liver, prostate, muscle, kidney, uterus, collagen, myocardium and spleen) for temperatures between 5 to 90°C, and dielectric properties in the frequency range between 460 kHz and 3 GHz are reported. Furthermore, perfusion changes in tumors including carcinomas, sarcomas, rhabdomyosarcoma, adenocarcinoma and ependymoblastoma in response to hyperthmic temperatures up to 46°C are presented. Where appropriate, mathematical models to describe temperature dependence of properties are presented. The presented data is valuable for mathematical models that predict tissue temperature during thermal therapies (e.g. hyperthermia or thermal ablation), as well as for applications related to prediction and monitoring of temperature induced tissue changes.

Figure 1

Electrical conductivity of human and various animal tissues as a function of temperature. Colors are indicating frequencies [yellow: 450-500 kHz (RF), blue: ~1 GHz (low MW), green: >2 GHz (high MW)].

Figure 2

Electrical permittivity of various animal tissues as a function of temperature. Colors are indicating frequencies (blue: ~1 GHz, green: >2 GHz). Permittivity for frequencies in the RF heating range (450-500 kHz) is not presented; however the electrical permittivity has limited relevance at RF frequencies where conduction current is dominating.

Figure 3

Relative permittivity and conductivity measurements made at 915 MHz and 2.45 GHz during thermal ablation. Accumulated ablation times noted on each figure to identify temporal variations. Values tended to drop quickly in all cases when temperatures reached 100°C and continued to drop as temperature was maintained and the tissue became more dehydrated (64, 65).

Figure 4

Thermal conductivity of various tissues as a function of temperature.

Figure 5

Specific heat of various tissues as a function of temperature.

Figure 6

Effect of hyperthermic temperatures (41 – 46°C) on blood flow