Pulsed photothermal profiling of hypervascular lesions: Some recent advances

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 mu m 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 mu m 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.