We determine experimentally the accuracy of pulsed photothermal radiometric (PPTR) temperature depth profiling in water-based samples. We use custom tissue phantoms composed of agar gel layers separated by very thin absorbing layers. Two configurations of the acquisition system are compared, one using the customary spectral band of the InSb radiation detector (3.0-5.5 microm) and the other with a spectrally narrowed acquisition band (4.5-5.5 microm). The laser-induced temperature depth profiles are reconstructed from measured radiometric signals using a custom minimization algorithm. The results correlate very well with phantom geometry as determined by optical coherence tomography (OCT) and histology in all evaluated samples. Determination of the absorbing layer depth shows good repeatability with spatial resolution decreasing with depth. Spectral filtering improves the accuracy and resolution, especially for shallow absorption layers (~120 microm) and more complex structures (e.g., with two absorbing layers). The average full width at half maximum (FWHM) of the temperature peaks equals 23% of the layer depth.

We report on development of an optical-thermal model to evaluate the use of pulsed photothermal radiometry (PPTR) for depth profiling of port wine stain (PWS) skin. In the model, digitized histology sections of a PWS biopsy were used as the input skin geometry. Laser induced temperature profiles were reconstructed from simulated PPTR signals by applying an iterative, non-negatively constrained conjugate gradient algorithm. Accuracy of the following PWS skin characteristics extracted from the reconstructed profiles was determined: (1) average epidermal thickness (z(epi)), (2) maximum epidermal temperature rise (DeltaT(epi,max)), (3) depth of PWS upper boundary (z(PWS)), and (4) depth of maximum PWS temperature rise (z(PWS,max)). Comparison of the actual and reconstructed profiles from PPTR data revealed a good match for all four PWS skin characteristics. Results of this study indicate that PPTR is a viable approach for depth profiling of PWS skin.

We present a systematic experimental comparison of pulsed photothermal temperature profiling utilizing the customary spectral band of the InSb radiation detector (lambda=3.0 to 5.6 microm) and a narrowed acquisition band (4.5 to 5.6 microm). We use custom tissue phantoms composed of agar gel layers separated by thin absorbing layers. The laser-induced temperature profiles are reconstructed within the customary monochromatic approximation, using a custom minimization algorithm. In a detailed numerical simulation of the experimental procedure, we consider several acquisition spectral bands with the lower wavelength limit varied between 3.0 and 5.0 microm (imitating application of different long-pass filters). The simulated PPTR signals contain noise with amplitude and spectral characteristics consistent with our experimental system. Both experimental and numerical results indicate that spectral filtering reduces reconstruction error and broadening of temperature peaks, especially for shallower and more complex absorbing structures. For the simulated PPTR system and watery tissues, numerical results indicate an optimal lower wavelength limit of 3.8 to 4.2 microm.

Background and objectives

In order to optimize photorejuvenation of human skin, a method must be developed to reliably compare the potential for epidermal preservation and dermal fibroblast stimulation of different laser devices and irradiation parameters. We describe a novel human skin tissue culture model developed for this purpose.

Materials and methods

An artificial skin model, consisting of human keratinocytes in the epidermis and human fibroblasts and rat-tail collagen in the dermis, was cultured using the floating collagen gel (RAFT) method. Repetitive low-fluence Er:YAG laser irradiation was applied to test the applicability of our RAFT model for characterization of epidermal preservation and dermal fibroblast stimulation post-laser treatment.

Results

Histopathologic evaluation revealed a thin layer of epidermal keratinocyte preservation immediately after low fluence sub-ablative Er:YAG laser irradiation. One-week post-laser irradiation, the average increase in number of dermal fibroblasts as compared to control was statistically significant (P < 0.01).

Conclusions

The RAFT model can be used to assess the potential for epidermal preservation and dermal fibroblast stimulation of different photorejuvenation devices and irradiation parameters and offers several advantages over traditional animal and human skin models.

Nonablative photorejuvenation has become an integral procedure in the emerging discipline of laser dermatologic surgery. The objective is to confine selectively, without any epidermal damage, thermal injury to the papillary, and upper reticular dermis leading to fibroblast activation and synthesis of new collagen and extracellular matrix material. The procedure results in minimal patient morbidity, no interference with lifestyle, and a low risk of complications, while providing a satisfying degree of rhytides reduction. Multiple devices have been studied and marketed for nonablative photorejuvenation of human skin. However, currently, nonablative photorejuvenation should not be considered an alternative to laser skin resurfacing. The skin surface is not removed or modified. What really occurs may be more accurately referred to as dermal "remodeling" or "toning" as a wound healing response is initiated and collagen regenerated. The narrow "therapeutic window" of laser-induced dermal heating and epidermal cooling must still be optimized so that effective treatments can be obtained routinely. Clinical verification of effective treatment parameters (irradiation wavelength, pulse structure, radiant exposure, cooling time) will be obtained through further human studies. Most importantly, understanding the relationship between the degree of dermal thermal injury and synthesis of new collagen and extracellular matrix material will be fundamental to predicting the clinical efficacy and limitations of nonablative photorejuvenation.

Background and objective

Despite application of cryogen spray (CS) precooling, customary treatment of port wine stain (PWS) birthmarks with a single laser pulse does not result in complete lesion blanching for a majority of patients. One obvious reason is nonselective absorption by epidermal melanin, which limits the maximal safe radiant exposure. Another possible reason for treatment failure is screening of laser light within large PWS vessels, which prevents uniform heating of the entire vessel lumen. Our aim is to identify the parameters of sequential CS cooling and laser irradiation that will allow optimal photocoagulation of various PWS blood vessels with minimal risk of epidermal thermal damage.

Study design and methods

Light and heat transport in laser treatment of PWS are simulated using a custom 3D Monte Carlo model and 2D finite element method, respectively. Protein denaturation in blood and skin are calculated using the Arrhenius kinetic model with tissue-specific coefficients. Simulated PWS vessels with diameters of 30-150 µm are located at depths of 200-600 µm, and shading by nearby vessels is accounted for according to PWS histology data from the literature. For moderately pigmented and dark skin phototypes, PWS blood vessel coagulation and epidermal thermal damage are assessed for various parameters of sequential CS cooling and 532-nm laser irradiation, i.e. the number of pulses in a sequence (1-5), repetition rate (7-30 Hz), and radiant exposure.

Results

Simulations of PWS treatment in darker skin phototypes indicate specific cooling/irradiation sequences that provide significantly higher efficacy and safety as compared to the customary single-pulse approach across a wide range of PWS blood vessel diameters and depths. The optimal sequences involve three to five laser pulses at repetition rates of 10-15 Hz.

Conclusions

Application of the identified cooling/irradiation sequences may offer improved therapeutic outcome for patients with resistant PWS, especially in darker skin phototypes.

Our working hypothesis is that a dual-wavelength Nd:YAG laser, emitting simultaneously at 1064 and 532 nm, may induce stronger heating of PWS blood vessels relative to the epidermis than the customary KTP laser, due to conversion of hemoglobin to met-hemoglobin in the target blood vessels and the associated increase in NIR absorption. We apply pulsed photothermal radiometry to determine temperature depth profiles induced in PWS lesions by a dual-wavelength laser at sub-therapeutic radiant exposures. The results indicate no effect at 1 ms pulse duration and low radiant exposures (1-2 J/cm 2). Increased radiant exposure (3-4 J/cm2) and extended pulse duration (20-25 ms) result in increased energy deposition. In addition, two PWS lesions and one healthy skin site were irradiated at incrementally increasing radiant exposures, up to 9 J/cm2. Analysis of the laser-induced temperature profiles clearly revealed irreversible changes of tissue properties. Formation of met-hemoglobin and consequent increase of IR absorption was however not reliably detected. ©2007 SPIE-OSA.

Pulsed photothermal profiling of human skin using dual-wavelength excitation (DWE) was performed. The DWE technique exploited spectral differences between the epidermal melanin and blood hemoglobin. The parameter for depth profiling of port wine strain birthmarks (PWS) in vivo using the 585 and 600 nm wavelengths was discussed. Results showed improved convergence of the iteration, allowing a more reliable assessment of the optimal solution.

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.