Imaging of Port Wine Stain Lesions Using a Multi-sensor Photoacoustic Probe

Successful treatment of port wine stain (PWS) birthmarks in human skin utilizes cryogen spray cooling (CSC) in conjunction with laser treatment. CSC pre-cools the epidermis to protect it from subsequent laser irradiation which raises the temperature of both the epidermis and the deeper PWS. As the epidermal temperature is depressed by cryogen, damage to the skin surface is minimized while the PWS reaches temperatures sufficient to permanently damage the lesion. In order to optimize cooling and laser heating dosages and to properly guide laser therapy, the spatial relationship of epidermal melanin and PWS must be known. Photoacoustic depth profiling of human skin uses low energy, nanosecond pulses of laser light to induce acoustic waves in optically absorbing media, such as blood and melanin. We used a 532 nm Nd:YAG laser to measure total epidermal melanin content in human skin, comparing the results with visible reflectance spectroscopy. Furthermore, we performed numerical simulations of photoacoustic generation in skin, showing that a hemisperical acoustic sensor array could be used to reconstruct the rete pattern of epidermal melanin in the basal layer. Finally, we built a hemispherical probe for use in future experiments for imaging of human skin. INTRODUCTION Epidermal melanin content has been measured noninvasively by Kollias, et al. [1, 2] and Svaasand et al. [3], among ddress all correspondence to this author. 1 others, using visible reflectance spectroscopy (VRS). As most laser dermatologic procedures result in absorption of laser light by melanin, intended or not, the total content and distribution of melanin are required to optimize a laser procedure, such as laser therapy of port wine stain (PWS) skin [4, 5]. In this procedure, laser energy is used to thermally coagulate the blood vessels in a PWS lesion. Cryogen spray is used to pre-cool the epidermis, to prevent scarring due to absorption of high energy laser light. In order to optimize the laser and cryogen parameters, the spatial relationship of epidermal melanin and the PWS vessels must be known. We propose to use photoacoustic generation and analysis in human skin to determine total epidermal melanin content and for possible depth profiling and imaging of melanin. Photoacoustic methods have been used for probing subsurface structures in tissue and tissue phantoms [6–9]. We previously performed depth profiling of PWS skin [10] and measured epidermal melanin content in human skin [11]. We continue those studies to measure photoacoustically the total melanin content in human skin and construct a multi-sensor photoacoustic probe for imaging of melanin in human skin. In this paper we measure epidermal melanin content in human subjects using a single sensor photoacoustic probe. We then simulate photoacoustic generation and detection in human skin for various geometries of melanin expected in human skin and show backprojection reconstructions using a multi-sensor hemispherical photoacoustic probe. Finally we construct a multisensor photoacoustic probe for use in future studies for image reconstruction of skin structure. Copyright c © 2004 by ASME