Abstract
Thermal wave imaging of discrete subsurface chromophores in biological materials is reported using a phase sensitive coherent detection technique applied to recorded infrared (IR) images. We demonstrate that utilization of a periodically modulated laser source for thermal wave excitation and coherent detection applied to each pixel may be used to compute images of thermal wave amplitude and phase at the laser modulation frequency. In comparison to recorded IR images, the narrow-band detection technique significantly improves the quality of thermal wave amplitude images of subsurface chromophores in biological materials. Additionally, the technique provides phase information, which may be used to estimate chromophore depth in tissue. Application of the technique is demonstrated using tissue phantoms and in vivo biological models. We present a theoretical analysis and computer simulations that demonstrate the effect of tissue optical and thermal properties on thermal wave amplitude and phase. In comparison to the pulsed photothermal technique, coherent thermal wave imaging of subsurface chromophores in tissue for diagnostic applications allows reduction of peak incident laser fluence by as much as four orders of magnitude and is safer and more amenable to in vivo imaging.
Highlights
Subsurface chromophores in biological materials affect light penetration and are important in understanding mechanisms of laser–tissue interactions
The image depicted in figure 3(A) demonstrates one of 1024 frames recorded by the IR camera when the shutter was in the open position
The phase image shown in figure 3(C) reveals subsurface chromophores as dark linear features, because the grey scale assigns darker colour to smaller phase values
Summary
Subsurface chromophores in biological materials affect light penetration and are important in understanding mechanisms of laser–tissue interactions. Identification of light-absorbing discrete structures and homogeneous substances in tissue is important for a variety of medical applications utilizing laser sources. Non-invasive characterization of chromophores in tissue is a challenging problem due to the structural heterogeneity of biological tissues which often consist of multiple layers with different optical properties (e.g. skin) and may contain discrete chromophores (e.g., blood vessels, melanin, hair follicles) associated with particular tissue constituents. The basic principle of photothermal radiometry (PTR) is that areas of dominant laser absorption in the substrate under study may be detected remotely using an infrared (IR) sensor. This technique has found numerous applications ranging from material testing to determination of optical properties (Leung and Tam 1984, Prahl et al 1992)
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