Abstract
Pulsed photothermal radiometry can be used for non-invasive depth profiling of optically scattering samples, including biological tissues such as human skin. Computational reconstruction of the laser-induced temperature profile from recorded radiometric signals is sensitive to the value of the tissue absorption coefficient in the infrared detection band (μIR). While assumed constant in reported reconstruction algorithms, μIR of human skin varies by two orders of magnitude in the commonly used 3−5 μm detection band. We analyse the problem of selecting the effective absorption coefficient value to be used with such algorithms. In a numerical simulation of photothermal profiling we demonstrate that results can be markedly impaired, unless the reconstruction algorithm is augmented by accounting for spectral variation μIR(λ). Alternatively, narrowing the detection band to 4.5–5 μm reduces the spectral variation μIR(λ) to a level that permits the use of the simpler, un-augmented algorithm. Implementation of the latter approach for depth profiling of port wine stain birthmarks in vivo is presented and discussed.
Highlights
Pulsed photothermal radiometry (PPTR) is based on the time-resolved acquisition of infrared (IR) emission from a sample after pulsed laser exposure
To assess which value μeff is optimal to use in PPTR algorithms that disregard the spectral variation μIR(λ), we equate the radiometric signal expression (7a) with the approximate μ [mm-1] eff
Despite the poor convergence of the reconstruction algorithm, it is evident that the narrowing of the detection band has modified the obtained temperature profile
Summary
Pulsed photothermal radiometry (PPTR) is based on the time-resolved acquisition of infrared (IR) emission from a sample after pulsed laser exposure. While the above approaches may provide reliable structural information, PPTR measures epidermal temperature rise following a diagnostic laser pulse This enables assessment of the highest permissible radiant exposure (varying strongly with individual patient’s skin type), which is a key to the success and safety of PWS therapy. The experiment involves the recently introduced dual-wavelength excitation (DWE) technique, which explores spectral differences between melanin and haemoglobin to separate the epidermal and vascular contributions to the radiometric signals (Majaron et al 2000a, 2000b) In this way, PWS depth can be assessed even when it lies in close proximity to the epidermal–dermal junction, despite the inherently limited spatial resolution of the PPTR technique (Milner et al 1995, Sathyam and Prahl 1997, Smithies et al 1998)
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