Selective vascular injury during cutaneous laser therapy
Tunnell, James William
Doctor of Philosophy
Pulsed laser irradiation in conjunction with cryogen spray cooling (CSC) can induce selective vascular injury to remove cutaneous hypervascular malformations such as port wine stains (PWS), hemangiomas, and facial veins. In this group of studies, we characterized the cryogen heat removal process and determined the effects of pulsed laser irradiation in conjunction with cryogen spray cooling of human skin. First, we employed an inverse heat conduction algorithm to measure the thermal boundary condition due to CSC in in vitro skin phantoms. Second, we developed a mathematical model of laser irradiation in conjunction with CSC in human skin. We determined tissue damage and temperature profiles due to varying combinations of laser pulse duration, radiant exposure, and CSC application times. Finally, we used ex vivo and in vivo human skin to determine the effects of high radiant exposures and CSC on epidermal and vascular injury. CSC induces a dynamic cooling effect, removing heat from the skin both during and following the spurt application time. Residual cryogen, deposited on the skin surface during the cryogen spurt, remains on the skin surface several times as long as the as cryogen spurt itself. The heat removal rate during the cryogen spurt is greatest; however, the total energy removed following the cryogen spurt is also substantial (approximately half as much as during the spurt application time). CSC was effective in protecting the human skin epidermis in light to moderately pigmented skin. Mathematical modeling, ex vivo, and in vivo studies showed that the epidermal damage threshold could be increased by a factor of approximately two. Increased radiant exposures increased the risk of non-selective vascular injury observed in histology as injury to the epidermis and perivascular collagen; however, proper choice of cryogen cooling durations resulted in the elimination of epidermal injury as well as perivascular tissue injury. In addition, higher radiant exposures induced vascular injury deeper within the dermis. Although cooling alone was not sufficient in protecting the epidermis in dark skin patients, the mathematical model suggests that selecting the proper pulse duration in conjunction with CSC may eliminate epidermal injury in darker skin patients.
Rehabilitation and therapy; Biomedical engineering; Optics