Pulsed Photothermal Profiling Research Articles

Pulsed photothermal profiling of water-based samples using a spectrally composite reconstruction approach

Pulsed photothermal profiling involves reconstruction of temperature depth profile induced in a layered sample by single-pulse laser exposure, based on transient change in mid-infrared (IR) emission from its surface. Earlier studies have indicated that in watery tissues, featuring a pronounced spectral variation of mid-IR absorption coefficient, analysis of broadband radiometric signals within the customary monochromatic approximation adversely affects profiling accuracy. We present here an experimental comparison of pulsed photothermal profiling in layered agar gel samples utilizing a spectrally composite kernel matrix vs. the customary approach. By utilizing a custom reconstruction code, the augmented approach reduces broadening of individual temperature peaks to 14% of the absorber depth, in contrast to 21% obtained with the customary approach.

Performance evaluation of pulsed photothermal profiling of port wine stain in human skin

Treatment of port wine stain (PWS) birthmarks in human skin by pulsed laser irradiation requires the knowledge of the maximum epidermal temperature rise and PWS depth for an attending physician to select the optimal light dosage, irradiation wavelength, and cryogen spray cooling spurt duration on an individual patient basis. Pulsed photothermal radiometry (PPTR) is a promising technique to provide such information. In this article, computer simulations are performed to evaluate the performance of PPTR depth profiling of the laser-induced temperature rise in PWS. An iterative, non-negatively constrained conjugate gradient algorithm is used to reconstruct the laser-induced temperature profile from simulated PPTR signals. Human skin is assumed to contain an epidermal melanin layer and a single homogeneous PWS layer in the dermis. The influence of structural, experimental, and algorithm parameters on the temperature profile reconstruction are discussed. Accuracy of the maximum epidermal temperature rise and PWS depth determined from the reconstructed profiles is statistically analyzed. The simulations show that when the melanin and PWS layers are physically discrete, a good reconstruction can be obtained and the maximum epidermal temperature rise and PWS depth can be determined with accuracy sufficient for the intended clinical application. Measurements and reconstructions from PWS patients are performed and the results are in agreement with the simulations.

Determination of parameter β for dual-wavelength pulsed photothermal profiling of human skin

In pulsed photothermal profiling of human skin, subsurface blood vessels may be difficult to resolve due to nonselective absorption in the overlying epidermis. The dual-wavelength excitation (DWE) technique solves this problem by exploiting spectral differences between the epidermal melanin and blood hemoglobin. An improved approach to determination of DWE-specific parameter β for depth profiling of port wine stain birthmarks in vivo using the 585 and 600 nm wavelengths is discussed.

Spectral variation of the infrared absorption coefficient in pulsed photothermal profiling of biological samples

Pulsed photothermal radiometry can be used for non-invasive depth profiling of optically scattering samples, including biological tissues such as human skin. Computational reconstruction of the laser-induced temperature profile from recorded radiometric signals is sensitive to the value of the tissue absorption coefficient in the infrared detection band (μIR). While assumed constant in reported reconstruction algorithms, μIR of human skin varies by two orders of magnitude in the commonly used 3−5 μm detection band. We analyse the problem of selecting the effective absorption coefficient value to be used with such algorithms. In a numerical simulation of photothermal profiling we demonstrate that results can be markedly impaired, unless the reconstruction algorithm is augmented by accounting for spectral variation μIR(λ). Alternatively, narrowing the detection band to 4.5–5 μm reduces the spectral variation μIR(λ) to a level that permits the use of the simpler, un-augmented algorithm. Implementation of the latter approach for depth profiling of port wine stain birthmarks in vivo is presented and discussed.

Combining two excitation wavelengths for pulsed photothermal profiling ofhypervascular lesions in human skin

When pulsed photothermal radiometry (PPTR) is used for depth profiling ofhypervascular lesions in human skin, melanin absorption also heats the mostsuperficial skin layer (epidermis). Determination of lesion depth may bedifficult when it lies close to the epidermal-dermal junction, due to PPTR'slimited spatial resolution. To overcome this problem, we have developed anapproximation technique, which uses two excitation wavelengths (585 and 600nm) to separate the vascular and epidermal components of the PPTR signal. Thistechnique permits a noninvasive determination of lesion depth and epidermalthickness in vivo, even when the two layers are in close physicalproximity to each other. Such information provides the physician withguidance in selecting the optimal parameters for laser therapy on anindividual patient basis.

Nomograms for correction of blood Po2, Pco2, pH, and base excess for time and temperature.

Nomograms for correction of blood Po2, Pco2, pH, and base excess for time and temperature.