Exploring the temperature dependent dielectric properties of adipose tissue in the THz range

Abstract

Terahertz (THz) frequency region of the electromagnetic spectrum is defined as radiation of 0.1 to 10.0 x 1012 Hz (corresponding to wavelengths of 3.0 mm to 0.03 mm). Water in the liquid state has a very high absorption coefficient in the lower THz region1 (80-350 cm-1 at 0.1-2.0 THz), with ~90% of the energy being absorbed in the first 0.10 mm at 0.6- 0.9 THz at 350 C. The THz absorption coefficient of ice, on the other hand is only in the order of 1.0 -7.0 cm-1 in the same range2. This, two orders of magnitude difference between the THz absorption of ice and liquid water is a unique feature of the 0.1-2.0 THz range. The water content of most normal tissue, including the dermis and the deeper layers of the epidermis is in the order of 70-73%. The water content of body adipose tissue (fat) is about 20% adults3, thus, freezing the water content in tissues will have a significant influence on THz absorption properties even in adipose tissue. The properties of other, non-water, non-fat components of adipose tissue will also have an influence. The potential for medical imaging or therapeutic intervention at body, room or freezing temperature becomes dependent, in part, on the behavior of the dielectric properties non-water elements of living tissues. These elements have a much lower absorption coefficient, generally in the order of 10-20 cm-1, and do not change on freezing to the same extent as water4,5. The preliminary exploration of the concept of the viability of the THz-skin freezing imaging technique in skin was undertaken using computational modelling6. The depth of the dermis in humans is in the range of 2 to 5 mm and thus freezing the skin for examination may involve subcutaneous adipose. It follows that before any advance can be made the temperature dependent properties of adipose tissue need to be understood. One poorly understood aspect is the presence of a phase change in the adipose tissue, analogous to the one observed with butter becoming soft at room temperature, after being firm at refrigerator temperatures (40 C). The attenuated total reflection (ATR) apparatus at the Australian synchrotron provides for a rapid acquisition of data in a temperature controlled environment, with individual sets of readings taking in the order of 1-3 minutes. This provides an appropriate environment for the study of the changes in absorption coefficients in the samples, and to ascertain the utility of ATR for diagnostic applications.