Wim Verkruysse

Remote plethysmographic imaging using ambient light

Plethysmographic signals were measured remotely (> 1m) using ambient light and a simple consumer level digital camera in movie mode. Heart and respiration rates could be quantified up to several harmonics. Although the green channel featuring the strongest plethysmographic signal, corresponding to an absorption peak by (oxy-) hemoglobin, the red and blue channels also contained plethysmographic information. The results show that ambient light photo-plethysmography may be useful for medical purposes such as characterization of vascular skin lesions (e.g., port wine stains) and remote sensing of vital signs (e.g., heart and respiration rates) for triage or sports purposes.

Determination of human skin optical properties from spectrophotometric measurements based on optimization by genetic algorithms

We present an initial study on applying genetic algorithms (GA) to retrieve human skin optical properties using visual reflectance spectroscopy (VRS). A three-layered skin model consisting of 13 parameters is first used to simulate skin and, through an analytical model based on optical diffusion theory, we study their independent effects on the reflectance spectra. Based on a preliminary analysis, nine skin parameters are chosen to be fitted by GA. The fitting procedure is applied first on simulated reflectance spectra with added white noise, and then on measured spectra from normal and port wine stain (PWS) human skin. A normalized residue of less than 0.005 is achieved for simulated spectra. In the case of measured spectra from human skin, the normalized residue is less than 0.01. Comparisons between applying GA and manual iteration (MI) fitting show that GA performed much better than the MI fitting method and can easily distinguish melanin concentrations for different skin types. Furthermore, the GA approach can lead to a reasonable understanding of the blood volume fraction and other skin properties, provided that the applicability of the diffusion approximation is satisfied.

Optimal Cryogen Spray Cooling Parameters for Pulsed Laser Treatment of Port Wine Stains

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, τs, and optimal delay, τd, between the cryogen spurt and laser pulse when using CSC in treatment of port wine stain birthmarks.

Changes in optical properties of human whole blood in vitro due to slow heating

Optical properties of human whole blood were investigated in vitro at 633 nm using a double integrating sphere set‐up. The blood flow was maintained at a constant rate through a flow cell while continuously heating the blood at 0.2–1.lC/min from approximately 25 to 55°C in a heat exchanger. A small, but rather abrupt decrease in the scattering asymmetry factor (g‐factor) of 1.7 ± 0.6% and a similar increase in the scattering coefficient of 2.9 ± 0.6% were observed at approximately 45–46°C yielding an increase in the reduced scattering coefficient of 40 ± 10%. Furthermore, a continuous, manifest increase in the absorption coefficient was seen with increasing temperature, on average 80 ± 70% from 25 to 50°C. The effect of the heating on the blood cells was also studied under a white‐light transmission microscope. A sudden change in the shape of the red blood cells, from discshaped to spherical, was observed at approximately the same temperature at which the distinct changes in g‐factor and scattering coefficient were observed, i.e. at 45–46°C. The results indicate that this shape transformation could explain the sudden change in scattering properties.

Theoretical and experimental analysis of droplet diameter, temperature, and evaporation rate evolution in cryogenic sprays

Cryogenic sprays are used for cooling human skin during selected laser treatments in dermatology. In order to optimize their cooling efficiency, a detailed characterization and understanding of cryogen spray formation is required. Various instruments and procedures are used to obtain mean size (D), velocity (V), and temperature (T) of tetrafluoroethane spray droplets from straight-tube nozzles. A single-droplet evaporation model is used to predict droplet diameter and temperature as a function of distance from the nozzle, D(z) and T(z), from the values of D, V, and T at the nozzle exit, i.e., D0, V0, and T0. In the model, it is assumed that D and V decrease in accordance with the D2-law, and due to drag force, respectively. To compute T(z), the instantaneous D and V are incorporated into a phase-change heat transfer balance, which includes a heat convection term. The predicted evolutions of T(z) and D(z) are in reasonable agreement with experimental data.

Non-invasive determination of port wine stain anatomy and physiology for optimal laser treatment strategies

The treatment of port wine stains (PWSs) using a flashlamp-pumped pulsed dye laser is often performed using virtually identical irradiation parameters. Although encouraging clinical results have been reported, we propose that lasers will only reach their full potential provided treatment parameters match individual PWS anatomy and physiology. The purpose of this paper is to review the progress made on the technical development and clinical implementation of (i) infrared tomography (IRT), optical reflectance spectroscopy (ORS) and optical low-coherence reflectometry (OLCR) to obtain in vivo diagnostic data on individual PWS anatomy and physiology and (ii) models of light and heat propagation, predicting irreversible vascular injury in human skin, to select optimal laser wavelength, pulse duration, spot size and radiant exposure for complete PWS blanching in the fewest possible treatment sessions. Although non-invasive optical sensing techniques may provide significant diagnostic data, development of a realistic model will require a better understanding of relevant mechanisms for irreversible vascular injury.

Wavelengths for port wine stain laser treatment: influence of vessel radius and skin anatomy

Recent Monte Carlo computations in realistic port wine stain (PWS) models containing numerous uniformly distributed vessels suggest equal depth of vascular injury at wavelengths of 577 and 585 nm. This finding contradicts clinical experience and previous theory. From a skin model containing normal and PWS vessels in separate dermal layers, we estimate analytically the average volumetric heat production in the deepest targeted PWS vessel. The fluence rate distribution is approximated by Beers law, which depends upon the tissues effective attenuation coefficient, and includes a homogeneous fractional volumetric blood concentration corrected for finite-size blood vessels. The model predicts 585-587 nm wavelengths are optimal in adult PWSs containing at least one layer of small-radius blood vessels. In superficial PWSs, typically in young children with small-radius vessels, 577-580 nm wavelengths are optimal. Wavelength-independent results similar to those from Monte Carlo models are valid in single-layered PWSs of large-radius vessels. In conclusion, the volumetric heat production in the deepest targeted PWS blood vessel can be maximized on an individual patient basis. However, absorption of 585-587 nm wavelengths is sufficiently high in superficial lesions, so we hypothesize that these wavelengths may be considered adequate for the treatment of any PWS.

Measurement of heat flux and heat transfer coefficient during continuous cryogen spray cooling for laser dermatologic surgery

Cryogen spray cooling (CSC) has been used for selective epidermal cooling of human skin during laser therapy of patients with port wine stain (PWS) birthmarks. Unfortunately, current commercial CSC devices do not provide optimal cooling selectivity and, therefore, provide insufficient epidermal protection for some PWS patients. To assist in the development of improved atomizing nozzle designs, a reliable method to quantify the CSC heat flux is needed. We introduce a novel method to determine the heat flux (q/sub s/) and heat transfer coefficient (h) at the surface of a sprayed object, based on measurements of steady-state temperature gradients along a thin copper rod during continuous cryogen spraying. For an atomizing nozzle of inner diameter d/sub N/ = 0.7 mm, we found that q/sub s/ varies from 15 to 130 W/cm/sup 2/ and h increases nonlinearly from 15000 to 35000 W/m/sup 2/.K in the explored range of surface temperatures (T/sub s/, from -32 to -7/spl deg/C). Values of q/sub s/ obtained with a wider diameter nozzle (d/sub N/ = 1.4 mm) are approximately twice as large than those of the narrow nozzle. The corresponding values of h are significantly higher (32000-40000 W/m/sup 2/.K) and almost independent of T/sub s/ within the same temperature range. When combined with fast flashlamp photography (FFLP) of spray shapes and sprayed surfaces, the results demonstrate that the liquid cryogen layer, as deposited by finely atomized sprays from narrower nozzles, can significantly impair q/sub s/. In contrast, the higher-momentum impact of coarser sprays from wider nozzles reduces the thickness of the liquid layer in the impact area and/or enhances convection within it, yielding a larger q/sub s/.

Comparison of diffusion approximation and Monte Carlo based finite element models for simulating thermal responses to laser irradiation in discrete vessels

Both diffusion approximation (DA) and Monte Carlo (MC) models have been used to simulate light distribution in multilayered human skin with or without discrete blood vessels. However, no detailed comparison of the light distribution, heat generation and induced thermal damage between these two models has been done for discrete vessels. Three models were constructed: (1) MC-based finite element method (FEM) model, referred to as MC-FEM; (2) DA-based FEM with simple scaling factors according to chromophore concentrations (SFCC) in the epidermis and vessels, referred to as DA-FEM-SFCC; and (3) DA-FEM with improved scaling factors (ISF) obtained by equalizing the total light energy depositions that are solved from the DA and MC models in the epidermis and vessels, respectively, referred to as DA-FEM-ISF. The results show that DA-FEM-SFCC underestimates the light energy deposition in the epidermis and vessels when compared to MC-FEM. The difference is nonlinearly dependent on wavelength, dermal blood volume fraction, vessel size and depth, etc. Thus, the temperature and damage profiles are also dramatically different. DA-FEM-ISF achieves much better results in calculating heat generation and induced thermal damage when compared to MC-FEM, and has the advantages of both calculation speed and accuracy. The disadvantage is that a multidimensional ISF table is needed for DA-FEM-ISF to be a practical modelling tool.

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 (muIR). While assumed constant in reported reconstruction algorithms, muIR of human skin varies by two orders of magnitude in the commonly used 3-5 microm 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 muIR(lambda). Alternatively, narrowing the detection band to 4.5-5 microm reduces the spectral variation muIR(lambda) to a level that permits the use of the simpler, unaugmented algorithm. Implementation of the latter approach for depth profiling of port wine stain birthmarks in vivo is presented and discussed.