## 1 Introduction  
_Overview_  
Terahertz radiation (THz) is highly absorbed by liquid water and this presents the possibility to image biological materials using the difference in water content between normal and pathological lesions [1]. Efforts have been made in the past to evaluate THz properties of biological tissues with a view to devising diagnostic imaging techniques [2, 3, 4]. The small-scale variability of biological tissue produces large \"normal\" ranges for THz dielectric parameters, which have substantial overlaps with the dielectric parameters of diseased tissue [5]. THz examination can also be time consuming and involves highly specialised expertise. These problems have resulted in difficulties in devising a diagnostic test which is both clinically relevant and practical. This paper examines the viability of investigating biological tissues by employing the temperature dependent variation of THz dielectric parameters using an attenuated total reflection (ATR) apparatus, in both the \"true\" attenuated total reflection mode and in a novel \"reflection/transmission\" mode. The Australian Synchrotron's THz-far infrared beamline provides a high brightness source, which results in rapid evaluation, enhanced spectral quality and superior spatial resolution when compared to a conventional source. Useful data can be obtained in spectral scan blocks of 30 sec or less, with the beam running 24 h per day. The Synchrotron's beam is continuous which makes it unsuitable for time domain spectroscopy, thus novel approaches are needed to maximise its utility. The synchrotron ATR apparatus is equipped with a diamond crystal, which has a refractive index (\(n\)) of 2.40, and since \(n\) of water at THz frequencies is in the order of 1.95 to 2.15, the setup is ideal to determine the dielectric properties of water-based compounds by using the novel \"reflection/transmission\" technique [6]. In terms of THz dielectric parameters, biological tissues can be broadly classified into two types. The \"high bulk water content\" tissues (over 70%), are tissues in which a major part of the THz dielectric properties is dependent on the bulk (non-bound) water concentration. Examples are heart muscle (78% bulk water) and liver (78% bulk water), [7]. These tissues have an \(n\) in the 1.80 to 2.30 range at THz frequencies. The low water content tissues, such as adipose (15% to 40% bulk water, [8] and cortical bone (-30% bulk water, [9]), have a major part of the THz dielectric properties dependent on non-bulk water constituents. These tissues have an \(n\) in the 1.30 to 1.65 range at THz frequencies. Non-bulk-water constituents with important THz dielectric parameters include lipids, melanin, collagen, keratin and bound water. Bound water in biological tissues is either hydrogen bonded to proteins and other polar molecules or a loose envelope surrounding the central molecule. Bound water has fewer degrees of freedom, and behaves differently to bulk water, and this is reflected in THz dielectric parameters. The binding of water has the equivalent effect on dielectric parameters to reducing the temperature of bulk water, however bound water is also prevented from freezing at bulk water freezing points. The hypothesis central to the novel method is that THz dielectric parameters of the constituents of biological tissues will vary with temperature and with THz frequency, and that this variation can be clinically useful. Many disease states lead to changes in cellular components. In particular, cancers tend to have increased concentrations of bulk water. For example, the bulk water content of melanoma is in the region of 80-85%, compared to 70-72% in normal skin [10]. Temperature dependent variation of THz dielectric parameters in biological tissues has been reported by several authors [11; 12; 13]. The reported data are typically limited to frequencies of less than 1.2 to 1.5 THz.