Photothermal tomography of subcutaneous chromophores

Photothermal tomography (PU) is applied to characterize diameter and depth of subcutaneous chromophores such as blood vessels that comprise port wine stain (PWS) birthmarks. PU uses a fast infrared detector array to measure temperature rises in a PWS induced by pulsed laser radiation. A FIT record of PWS in response to pulsed laser exposure is composed of a sequence of infrared emission frames, each consisting of elevated temperature regions indicative of subcutaneous blood vessel heating. An analytic expression for recorded infrared emission frames is derived as a convolution integral of a PTF point spread function and the three dimensional temperature distribution in the PWS immediately following laser exposure. Diameters of blood vessels comprising the PWS are best resolved in early infrared emission frames when radial heat diffusion is relatively small. Blood vessel images in subsequent frames increase in amplitude due to heat generated in the subsurface PWS diffusing to the skin surface indicative of a "delayed thermal wave". Influence of diameter and depth of blood vessels on the FVF record is analyzed using an in-vitro PWS model consisting of multilayered collagen films