Combining two excitation wavelengths for pulsed photothermal profiling of hypervascular lesions in human skin.

When pulsed photothermal radiometry (PPTR) is used for depth profiling of hypervascular lesions in human skin, melanin absorption also heats the most superficial skin layer (epidermis). Determination of lesion depth may be difficult when it lies close to the epidermal dermal junction, due to PPTR's limited spatial resolution. To overcome this problem, we have developed an approximation technique, which uses two excitation wavelengths (585 and 600 nm) to separate the vascular and epidermal components of the PPTR signal. This technique permits a noninvasive determination of lesion depth and epidermal thickness in vivo, even when the two layers are in close physical proximity to each other. Such information provides the physician with guidance in selecting the optimal parameters for laser therapy on an individual patient basis.

Active skin cooling in conjunction with laser dermatologic surgery.

The clinical objective in the laser treatment of patients with specific dermatoses is to maximize thermal damage to the target chromophore while minimizing injury to the normal skin. Unfortunately, for some lesions, the threshold incident light dosage for epidermal injury can be very close to the threshold for permanent removal of the target chromophore, thus precluding the use of higher light dosages. An important method of overcoming the aforementioned problem is to selectively cool the most superficial layers of the skin. Although melanin absorption will result in heat production during laser exposure, cooling the epidermis can prevent its temperature elevation from exceeding the threshold for thermal injury. Spatially selective cooling can be achieved by active cooling using a cryogen spray or cold sapphire contact handpieces. These devices promote rapid and spatially selective epidermal cooling to low temperatures without affecting the target chromophore temperature before the laser pulse is delivered. Cooling has become an Integral part in the emerging discipline of laser dermatologic surgery. Attend almost any academic dermatology conference and you are likely to find many lectures that relate to cooling during dermatologic laser surgery. Although cooling in conjunction with laser therapy has become the clinical standard for many laser procedures, considerable controversy surrounds this methodology. We present herewith an overview of currently used techniques for active cooling of human skin and explore their advantages and disadvantages in relationship to specific dermatoses amenable to laser therapy.

Calculation of crater shape in pulsed laser ablation of hard tissues.

BACKGROUND AND OBJECTIVE: An expression describing the ablation crater shape as a function of lateral fluence distribution has been derived in a recent paper by Ostertag et al. [Lasers Surg Med 1997; 21:384-394]. STUDY DESIGN/MATERIALS AND METHODS: The ablation model presented therein is improved by taking into account the influence of the ablation front inclination on the ablation dynamics. RESULTS: The resulting crater profiles deviate from the previously predicted ones (which reflect the Gaussian fluence distribution of the impinging laser beam) progressively with increasing pulse fluence. CONCLUSION: The ablation front inclination must be taken into account to predict the ablation crater shapes and volumes correctly.

Deep coagulation of dermal collagen with repetitive Er:YAG laser irradiation.

BACKGROUND AND OBJECTIVE: Er:YAG lasers are known to effectively ablate human skin with minimal thermal damage to subjacent dermal tissue. We have investigated whether deep coagulation of dermal collagen, similar to that observed with the CO(2) laser, could be achieved with repetitive Er:YAG laser exposures. STUDY DESIGN/MATERIALS AND METHODS: Skin on the back of a Sprague-Dawley rat in vivo was irradiated with sequences of 1-10 Er:YAG laser pulses at a repetition rate of 10 or 33 Hz and single-pulse fluences from 0.8 to 1.4 J/cm(2). The resulting lesions were biopsied within 1 hour after laser exposure, and the histologic sections were examined by using optical microscopy. RESULTS: The depth of dermal collagen denaturation increases dramatically when 3-10 low-fluence Er:YAG laser pulses are stacked at a repetition rate of 10 or 33 Hz. CONCLUSION: Coagulation of dermal collagen deeper than 200 microm below the epidermal-dermal junction is feasible by using the appropriate settings of a repetitive Er:YAG laser.

Er:YAG laser modification of root canal dentine: influence of pulse duration, repetitive irradiation and water spray.

The aim of this study was to determine the effects of varying parameters of Er:YAG laser irradiation with and without water spray cooling on root canal dentine in vitro. After horizontally removing tooth crowns from extracted human teeth, roots were axially sectioned into thin slices, exposing the root canal surface. An Er:YAG laser delivered 10-30 J/cm(2) into a 0.4-mm diameter laser spot on the root canal surface. Single pulses of different lengths (80-280 micro s) were applied with and without water spray cooling/irrigation, and sequences of three pulses at a repetition rate of 30 Hz were applied at selected pulse parameters. The irradiated samples were investigated using both confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). At most irradiation conditions, the root canal dentine surface was ablated. Three-dimensional images from CLSM revealed that the cavity walls were not smooth. Depths of the cavities revealed significant differences between the cavities. No debris was observed at the surface of cavities at any irradiation condition. Strong melting and recrystallisation, or unusually flat surfaces with open dentinal tubules were obtained with sequences of three pulses without water cooling. CLSM is an effective tool for investigation of laser effects on root canal dentine. By varying the irradiation conditions, the Er:YAG laser can induce different modifications of root canal surface, which may be very interesting for root canal preparation.

Er:YAG laser skin resurfacing using repetitive long-pulse exposure and cryogen spray cooling: I. Histological study.

BACKGROUND AND OBJECTIVE: To evaluate histologically the characteristics of repetitive Er:YAG laser exposure of skin in combination with cryogen spray cooling (CSC), and its potential as a method of laser skin resurfacing. STUDY DESIGN/MATERIALS AND METHODS: Rat skin was irradiated in vivo with sequences of 10 Er:YAG laser pulses (repetition rate 20 Hz, pulse duration 150 or 550 micros, single-pulse fluence 1.3-5.2 J/cm(2)). In some examples, CSC was applied to reduce epidermal injury. Histologic evaluation was performed 1 hour, 1 day, 5 days, and 4 weeks post-irradiation. RESULTS: A sequence of ten 550-micros pulses with fluences around 2 J/cm(2) resulted in acute dermal collagen coagulation to a depth of approximately 250 microm, without complete epidermal ablation. CSC improved epidermal preservation, but also diminished the coagulation depth. Four weeks after irradiation, neo-collagen formation was observed to depths in excess of 100 microm. CONCLUSIONS: Dermal collagen coagulation and neo-collagen formation to depths similar to those observed after CO(2) laser resurfacing can be achieved without complete ablation of the epidermis by rapidly stacking long Er:YAG laser pulses. Application of CSC does not offer significant epidermal protection for a given dermal coagulation depth.

Er:YAG laser skin resurfacing using repetitive long-pulse exposure and cryogen spray cooling: II. Theoretical analysis.

BACKGROUND AND OBJECTIVE: To analyze the effects of laser pulse duration and cryogen spray cooling (CSC) on epidermal damage and depth of collagen coagulation in skin resurfacing with repetitive Er:YAG laser irradiation. STUDY DESIGN/MATERIALS AND METHODS: Evolution of temperature field in skin is calculated using a simple one-dimensional model of sub-ablative pulsed laser exposure and CSC. The model is solved numerically for laser pulse durations of 150 and 600 microsec, and 6 msec cryogen spurts delivered just prior to ("pre-cooling"), or during and after ("post-cooling") the 600 microsec laser pulse. RESULTS: The model indicates a minimal influence of pulse duration on the extent of thermal effect in dermis, but less epidermal damage with 600 microsec pulses as compared to 150 microsec at the same pulse fluence. Application of pre- or post-cooling reduces the peak surface temperature after laser exposure and accelerates its relaxation toward the base temperature to a different degree. However, the temperature profile in skin after 50 msec is in either example very similar to that after a lower-energy laser pulse without CSC. CONCLUSIONS: When applied in combination with repetitive Er:YAG laser exposure, CSC strongly affects the amount of heat available for dermal coagulation. As a result, CSC may not provide spatially selective epidermal protection in Er:YAG laser skin resurfacing.

Optimal cryogen spray cooling parameters for pulsed laser treatment of port wine stains.

BACKGROUND AND OBJECTIVE: In dermatologic laser therapy, cryogen spray cooling (CSC) is a means to protect the epidermis while leaving dermal structures susceptible to thermal damage. The purpose of this study was to determine optimal spurt duration, tau(s), and optimal delay, tau(d), between the cryogen spurt and laser pulse when using CSC in treatment of port wine stain birthmarks. STUDY DESIGN/MATERIALS AND METHODS: A finite difference method is used to compute temperature distributions in human skin in response to CSC. Optimal tau(s) and tau(d) are determined by maximizing the temperature difference between a modeled basal layer and an imaginary target chromophore. RESULTS: The model predicts an optimal tau(s) of 170-300 msec and approximately 400 msec for shallow (150 microm) and deeper (400 microm) targets, respectively. Spraying for longer than the optimal tau(s) does not critically impair cooling selectivity. For a spurt duration of 100 msec, optimal delays are 5-10 msec and 25-70 msec for a shallow and deep basal layer, respectively. CONCLUSION: In the absence of knowledge about the lesion anatomy, using a tau(s) of 100-200 msec and no delay is a good compromise. A delay is justified only when basal layer and target chromophore are relatively deep and the optimal spurt duration cannot be applied, e.g., to avoid frostbite.

Influence of nozzle-to-skin distance in cryogen spray cooling for dermatologic laser surgery.

BACKGROUND AND OBJECTIVE: Cryogen sprays are used for cooling human skin during various laser treatments. Since characteristics of such sprays have not been completely understood, the optimal atomizing nozzle design and operating conditions for cooling human skin remain to be determined. MATERIALS AND METHODS: Two commercial cryogenic spray nozzles are characterized by imaging the sprays and the resulting areas on a substrate, as well as by measurements of the average spray droplet diameters, velocities, temperatures, and heat transfer coefficients at the cryogen-substrate interface; all as a function of distance from the nozzle tip. RESULTS: Size of spray cones and sprayed areas vary with distance and nozzle. Average droplet diameter and velocity increase with distance in the vicinity of the nozzle, slowly decreasing after a certain maximum is reached. Spray temperature decreases with distance due to the extraction of latent heat of vaporization. At larger distances, temperature increases due to complete evaporation of spray droplets. These three variables combined determine the heat transfer coefficient, which may also initially increase with distance, but eventually decreases as nozzles are moved far from the target. CONCLUSIONS: Sprayed areas and heat extraction efficiencies produced by current commercial nozzles may be significantly modified by varying the distance between the nozzle and the sprayed surface.

Cooling efficiency of cryogen spray during laser therapy of skin.

BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) is used extensively for epidermal protection during laser-induced photothermolysis of port wine stains and other vascular skin lesions. The efficacy of CSC depends critically on the heat transfer coefficient (H) at the skin surface for which, however, no reliable values exist. Reported values for H, based on tissue phantoms, vary from 1,600 to 60,000 W/m(2) K. STUDY DESIGN/MATERIALS AND METHODS: A simple experimental model was designed and constructed, consisting of a pure silver-measuring disk (diameter 10 mm, thickness approximately 1 mm), embedded in a thermal insulator. The disk was covered with a 10 microm thick stratum corneum layer, detached from in vivo human skin. The heat transfer coefficient of the stratum corneum/cryogen interface was measured during CSC with short spurts of atomized tetrafluoroethane. RESULTS: H was found to be dependent on the specific design of the cryogen valve and nozzle. With nozzles used in typical clinical settings, H was 11,500 W/m(2) K, when averaged over a 100 ms spurt, and 8,000 W/m(2) K when averaged over a 200 ms spurt. CONCLUSIONS: The presented model enables accurate prediction of H and thus improve control over temperature depth profile and cooling efficiency during laser therapy. Thereby, it may contribute to improvement of therapeutic outcome.

Cryogen spray cooling in laser dermatology: effects of ambient humidity and frost formation.

BACKGROUND AND OBJECTIVE: Dynamics of cryogen spray deposition, water condensation and frost formation is studied in relationship to cooling rate and efficiency of cryogen spray cooling (CSC) in combination with laser dermatologic surgery. STUDY DESIGN/MATERIALS AND METHODS: A high-speed video camera was used to image the surface of human skin during and after CSC using a commercial device. The influence of ambient humidity on heat extraction dynamics was measured in an atmosphere-controlled chamber using an epoxy block with embedded thermocouples. RESULTS: A layer of liquid cryogen may remain on the skin after the spurt termination and prolong the cooling time well beyond that selected by the user. A layer of frost starts forming only after the liquid cryogen retracts. Condensation of ambient water vapor and subsequent frost formation deposit latent heat to the target site and may significantly impair the CSC cooling rate. CONCLUSIONS: Frost formation following CSC does not usually affect laser dosage delivered for therapy of subsurface targets. Moreover, frost formation may reduce the risk of cryo-injury associated with prolonged cooling. The epidermal protection during CSC assisted laser dermatologic surgery can be further improved by eliminating the adverse influence of ambient humidity.