Abstract
The penetration depth of an evanescent wave in Attenuated Total Reflection (ATR) is dependent on the wavelength of the radiation utilised. At THz frequencies, the penetration depth into biological tissues is in the order of 0.1 to 0.5 mm; rendered pig lard was used as a model sample in this study. A method for the direct measurement of the evanescent wave penetration depth is presented which allows for the estimation of the dispersion of the complex refractive index by using the reflection of the evanescent wave from varying sample depths. The method employs frustrated total internal reflection, and has been demonstrated by using the THz/Far-IR beamline at the Australian synchrotron, and modelled using finite difference time domain (FDTD) simulations.
Highlights
There are many established techniques used for clinical assessment of skin pathology of skin tissues, such as photo-acoustic imaging [1], optical coherence tomography [2], confocal reflectance microscopy [3], and high frequency ultrasound [4]
The high brilliance synchrotron radiation is of especial interest for the proof of the concept experiments for large penetration depth; the brilliance B is defined as the number of photons Nλ of energy hν (J) and direction within the solid angle Ω per unit of time t (s), area A, and 0.1% of the bandwidth, BW (J), as B = Nλ hν/(tAΩ0.1%BW )
The THz-skin freeze method of skin imaging and other applications based on the disparity between the THz dielectric properties of liquid and frozen water can provide the motivation for development of better THz technology
Summary
There are many established techniques used for clinical assessment of skin pathology of skin tissues, such as photo-acoustic imaging [1], optical coherence tomography [2], confocal reflectance microscopy [3], and high frequency ultrasound [4]. No imaging modality has been established for studying skin tissue depths beyond 1.5 mm from the surface. The precise depth of lesions is clinically important to assess the prognosis and to plan surgical treatment [8]. The current staging of melanomas includes estimation of thickness, with stage T1 is < 1.0 mm, T2 is 1.0–2.0 mm, and T3. 2.0–4.0 mm with melanomas greater than 4.0 mm thickness classified as stage T4 [8,9,10]. An accurate thickness measurement would reduce the chances of residual melanoma remaining and would help to avoid unnecessarily large surgical resection. Characterisation and imaging of up to several millimeters of skin is of paramount importance and can be only achieved by combination of different optical techniques over the UV-vis-IR-THz spectral ranges. The permittivity (complex refractive index) at this wide spectral range should be known for different materials encountered at the surface (0–4 mm) of skin
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