Optimization of cryogen spray cooling for port wine stain laser therapy using photoacoustic measurement of epidermal melanin

Treatment of vascular skin lesions, such as port wine stain (PWS) birthmarks, uses laser energy to thermally damage the hypervascular regions in the dermis. Delivery of sufficient laser energy to the dermis can be confounded by the more superficial epidermal melanin layer, a broadband optical absorber. Laser pulses directed at skin lesions may result in excessive heating and scarring of the epidermis, resulting also in limiting the energy available to treat the dermal lesion. Cryogen spray cooling is used to selectively cool the epidermis, allowing the use of higher laser fluences which cause thermal damage in the deeper lesion, while preventing hypertrophic scarring of the epidermis. In order to optimize cryogen cooling and laser pulse parameter for treatment, the depth profile of the epidermal melanin layer and vascular lesion must be known. We use a photoacoustic probe to determine this depth profile. A Q-switched Nd:YAG laser emitting 532 nm light pulses of 4 ns duration is used to excite acoustic waves in epidermal melanin and hemoglobin in the vascular lesions via a miniature probe incorporating a piezoelectric detector. We used acoustic propagation time to determine the spatial relationship of the absorbing structures, thereby giving the necessary means to determine cryogen spurt and laser pulse timing. We present a finite difference thermal model along with data from tissue phantoms showing the effectiveness of the photoacoustic method.