The THz/Far-IR Beamline at the Australian Synchrotron was used to demonstrate a novel method of estimating the dielectric properties of homogenous substances in the 1.0 THz to 4.0 THz region. Attenuated total reflection (ATR) coupled with synchrotron sources allow rapid evaluation of samples. The source is incoherent, thus normally only reflectance can be derived, as phase shift data cannot be obtained with this arrangement. A method is presented of deriving the full complex dielectric parameters by a modified frustrated internal reflection technique which reflects the evanescent wave using a gold plated mirror. Oil and alcohol samples were used in the study. With the mirror in situ, the reflectance changed from being enhanced at some frequencies to undergoing a frequency dependent destructive interference at other frequencies. The change in different samples was noted to vary according to the refractive index (but not the absorption coefficient) of the sample.
The THz/Far-IR Beamline at the Australian Synchrotron was used to demonstrate a novel method of estimating the
dielectric properties of water based substances in the 2.0 THz region. The method involves using an ATR apparatus with a
diamond crystal in a way that converts the apparatus to a reflection/transmission mode by using a mirror at a set distance
from the crystal/sample interface to reflect the transmitted portion of the incoming beam back to the detector. The method
can extract the full dielectric properties of a sample. The method is only limited by the frequency range of the source and
the sensitivity range of the detector. It is best suited to studies at 2.0 THz and above with a diamond crystal ATR apparatus,
since the combination of the refractive index of diamond (n=2.40) and water (n=1.95 at 2.0 THz) are in the optimal ratio
to produce excellent contrast. Water and water based gel were used to demonstrate the technique.
By employing a FTIR spectrometer aligned for THz beam in the Australian Synchrotron, we measured absorption coefficients of different toad skin patches. Skin samples were precisely excised from cane toad and prepared for experiments in dried and fresh states. The transmission and attenuated total reflection modes were designated to measure absorption coefficients through mathematical relations between transmittance and absorption. The transmission study shows that in the frequency range of 0.8 to around 3 THz the dark part of toad skin demonstrates higher absorption than the pale part in both dried and fresh states while it is reversed at higher frequencies. Both dark and pale skin patches perform much less absorption than distilled water. ATR data corroborates the same result in the frequency range of 0.75 to 0.95 THz.
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.
The Terahertz (THz) frequency region of the electromagnetic spectrum is defined as radiation of 0.1 to 10.0 x 1012 Hz. A unique feature of the 0.1 to 2.0 THz frequency band is that there is a high disparity between liquid water and ice absorption, with ice being 100 times more permeable to THz radiation. The high absorption by liquid water limits the deployment of 0.1 to 2.0 THz band for imaging and therapeutics to 0.2-0.3 mm in soft tissues. By freezing tissue, however, an imaging depth of 5.0 mm is achievable. Computational finite difference time domain (FDTD) modelling was undertaken using realistic tissue phantoms to explore this enhanced depth for imaging of frozen skin lesions such as melanomas. The computational modeling confirms that there is adequate contrast between normal frozen skin and pathological lesions. The imaging is enhanced by sampling the frozen tissue at both 0.45 and 1.00 THz. A method of analysing the data in a simplified, systematic way is introduced by dividing the returning signal into time regions and comparing their relative intensity. The concept will be developed into a “THz eye”, where the differences in THz absorption and refraction of tissues between individual THz frequencies are exploited for superior imaging.
The terahertz (THz) band lies between the infrared and microwave regions of the electromagnetic spectrum. The 0.1-2.0 THz band is unique in that the radiation is both highly absorbed by liquid water and has a relatively low coefficient of absorption in ice; less than 0.0001% of the radiation survives to a depth of 1.0 millimetre in liquid water, whereas 90% of the signal survives in ice at 0.45 THz. The liquid water absorption has limited the potential for deployment of THz radiation for imaging and therapeutics in human tissues to the level of the epidermis. By first freezing the skin in situ, THz penetration to a depth of 5.0 millimetres becomes viable. Computational modelling using tissue phantoms was used to explore the concept of in situ skin freezing. The modelling indicates that the border between frozen skin and underlying non-frozen tissue provides a reflective boundary, which is the main site for signal return to the surface. The non-frozen layer just under the frozen skin is also the site for most of the THz radiation absorption. The results show that the freezing method may be useful in estimating the depth of frozen skin tissue in cryotherapy, imaging skin lesions and as method of accurate, targeted thermal ablation of lesions within the dermis by delivering high energy THz pulses through a frozen “window”.
Finite-difference time-domain (FDTD) computational phantoms aid the analysis of THz radiation interaction with human skin. The presented computational phantoms have accurate anatomical layering and electromagnetic properties. A novel “large sheet” simulation technique is used allowing for a realistic representation of lateral absorption and reflection of in-vivo measurements. Simulations carried out to date have indicated that hair follicles act as THz propagation channels and confirms the possible role of melanin, both in nevi and skin pigmentation, to act as a significant absorber of THz radiation. A novel freezing technique has promise in increasing the depth of skin penetration of THz radiation to aid diagnostic imaging.
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