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Deep subsurface thermal lesions in tissue using an Nd:YAG laser and cryogen spray cooling: preliminary in vitro results

Surface cooling methods are used in dermatology to protect the skin surface during cosmetic laser procedures. This study investigates cryogen spray cooling of the tissue surface during laser production of deep subsurface tissue legions. Nd:YAG laser radiation with a wavelength of 1.06 microns, output power of 30 Watts, a 7.5-mm-diameter spot, and a pulse duration of 1 s, was delivered to porcine liver and skin samples, maintained at 33 - 37°C and monitored with embedded micro-thermocouples. The cryogen spray was delivered intermittently between laser pulses, with a pulse duration of 60 milliseconds, reptition rate of 0.333 Hz, and a 2-cm-diameter spot. Gross and histologic pathology was used to measure tissue lesion dimensions. Liver lesions measured 4.9 ± 1.2 mm in diameter, while preserving the tissue surface to depth of 2.1 ± 0.4 mm (n = 5). Surface temperatures remained below 40°C, while lesion temperatures ranged between 60 - 70°C. Skin lesions appeared 1 mm below the skin surface and measured ~ 2-mm-diameter (n = 4). This technique may be used to target subsurface anatomical structures in the male reproductive tract for noninvasive male sterilization. In vivo experiments will be necessary to optimize the laser heating and cryogen cooling parameters.