Abstract
Pulsed photothermal radiometry (PPTR) can be used for non-invasive depth profiling of port wine stain (PWS) birthmarks, aimed towards optimizing laser therapy on an individual patient basis. Reconstruction of laser-induced temperature profile from the experimentally obtained radiometric signal involves the skin absorption coefficient in the infrared detection band. In the commonly used 3-5 μm detection band (InSb), the absorption coefficient varies by two orders of magnitude, while assumed to be constant in the reconstruction algorithms used thus far. We discuss the problem of choosing the effective absorption coefficient value to be used under such conditions. Next, we show how to account explicitly for the strong spectral variation of the infrared absorption coefficient in the image reconstruction algorithm. Performance of such improved algorithm is compared to that of the unaugmented version in a numerical simulation of photothermal profiling. Finally, we analyze implementation of a bandpass filter which limits the detection band to 4.5-5 μm. This reduces the absorption coefficient variation to a level that permits the use of unaugmented algorithm. An experimental test of the latter approach for in vivo characterization of the depth of PWS lesion and epidermal thickness will be presented, including a novel technique that uses two laser excitation wavelengths in order to separate the epidermal and vascular components of the radiometric signal.
Highlights
Port-wine stain birthmarks (PWS) are hypervascular lesions in human skin, which consist of an excess of ectatic blood vessels
In order to optimize therapy on an individual patient basis, determination ofboth PWS depth and epidermal thickness is required, especially when cryogen spray cooling is applied.2'3'4 Pulsed photothennal radiometry (PPTR), which is based on time-resolved acquisition of infrared (IR) radiant emission following pulsed laser exposure, was recently introduced for assessment of laser-induced temperature profiles in PWS.5'6 The ability ofPPTR to determine the depth of sub-surface chromophores has been demonstrated by profiling layered tissue phantoms, and by comparison with histological assessment ofPWS depth.[7]
Note that in lesions with PWS in close proximity to the epidermal-dermal junction, epidermal heating due to broad melanin absorption may prevent determination of PWS depth, owing to limited spatial resolution of PPTR.6'9 In order to overcome this problem, we use an approximation technique, which utilizes two laser excitation wavelengths to separate the epidermal and vascular components of the radiometric signal.1° In this way, the epidermal thickness and PWS depth can be assessed with adequate accuracy and reliability
Summary
Port-wine stain birthmarks (PWS) are hypervascular lesions in human skin, which consist of an excess of ectatic blood vessels. These are usually fully contained within the most superficial millimeter of the skin. Reconstruction of laser-induced temperature profiles from experimental radiometric signals involves the value of the tissue absorption coefficient at the infrared detection wavelength. We show how to account explicitly for such strong spectral variation of the absorption coefficient in the image reconstruction algorithm. Performance of such an improved algorithm is compared to that ofthe unaugmented version in a numerical simulation ofphotothermal profiling of PWS. Note that in lesions with PWS in close proximity to the epidermal-dermal junction, epidermal heating due to broad melanin absorption may prevent determination of PWS depth, owing to limited spatial resolution of PPTR.6'9 In order to overcome this problem, we use an approximation technique, which utilizes two laser excitation wavelengths to separate the epidermal and vascular components of the radiometric signal.1° In this way, the epidermal thickness and PWS depth can be assessed with adequate accuracy and reliability
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