Martin J. C. van Gemert

Optical-Thermal Response of Laser-Irradiated Tissue

Overview of Optical and Thermal Laser-Tissue Interaction and Nomenclature A.J. Welch, M.J.C. van Gemert. Tissue Optics: Overview of Tissue Optics A.J. Welch, et al. Onedimensional Transport Theory M.J.C. van Gemert, et al. Monte Carlo Modeling of Light Transport in Tissue S. Jacques, L. Wang. Adding-Doubling S. Prahl. Diffusion Theory of Light Transport W. Star. The Diffusion Approximation in Three Dimensions S. Prahl. Thermal Interactions: Overview of Bioheat Transfer C.S. Orr, R.C. Eberhart. Solution of Heat Conduction Equation J. Roider, R. Birngruber. Approximate Solutions for Heat Conduction: Time Constants M.J.C. van Gemert, A.J. Welch. Tissue Thermal Properties and Perfusion J. Valvano. Temperature Measurements J. Valvano, J. Pearce. Pulsed Photo Thermal Radiation A. Vitkin. Medical Applications: Introduction to Medical Applications A.J. Welch, M.J.C. van Gemert. Optics of Fibers and Fiber Probes R.M. Verdaasdonk, C. Borst. Fluorescence R. Richards-Kortum. Pulse Ablation of Soft Tissue T.G. van Leeuwen, et al. Laser-induced Hyperthermia L. Svaasand. Laser Treatment of Port Wine Stains M.J.C. van Gemert, et al. 7 additional articles. Index.

Determining the optical properties of turbid media by using the adding–doubling method

A method is described for finding the optical properties (scattering, absorption, and scattering anisotropy) of a slab of turbid material by using total reflection, unscattered transmission, and total transmission measurements. This method is applicable to homogeneous turbid slabs with any optical thickness,albedo, or phase function. The slab may have a different index of refraction from its surroundings and may or may not be bounded by glass. The optical properties are obtained by iterating an adding-doubling solution of the radiative transport equation until the calculated values of the reflection and transmission match the measured ones. Exhaustive numerical tests show that the intrinsic error in the method is < 3% when four quadrature points are used.

Noninvasive imaging of in vivo blood flow velocity using optical Doppler tomography

We report the development of an optical technique for noninvasive imaging of in vivo blood flow dynamics and tissue structures with high spatial resolution (2-10 microm) in biological systems. The technique is based on optical Doppler tomography (ODT), which combines Doppler velocimetry with optical coherence tomography to measure blood flow velocity at discrete spatial locations. The exceptionally high resolution of ODT permits noninvasive in vivo imaging of both blood microcirculation and tissue structures surrounding the vessel, which has significance for biomedical research and clinical applications. Tomographic imaging of in vivo blood flow velocity in the chick chorioallantoic membrane and in rodent skin is demonstrated.

Double-integrating-sphere system for measuring the optical properties of tissue

A system is described and evaluated for the simultaneous measurement of the intrinsic optical properties of tissue: the scattering coefficient, the absorption coefficient, and the anisotropy factor. This system synthesizes the theory of two integrating spheres and an intervening scattering sample with the inverse adding-doubling algorithm, which employs the adding-doubling solution of the radiative transfer equation to determine the optical properties from the measurement of the light flux within each sphere and of the unscattered transmission. The optical properties may be determined simultaneously, which allows for measurements to be made while the sample undergoes heating, chemical change, or some otherexternal stimulus. An experimental validation of the system with tissue phantoms resulted in the determination of the optical properties with a < 5% deviation when the optical density was between 1 and 10 and the albedo was between 0.4 and 0.95.

Modelling light distributions of homogeneous versus discrete absorbers in light irradiated turbid media

Laser treatment of port wine stains has often been modelled assuming that blood is distributed homogeneously over the dermal volume, instead of enclosed within discrete vessels. The purpose of this paper is to analyse the consequences of this assumption. Due to strong light absorption by blood, fluence rate near the centre of the vessel is much lower than at the periphery. Red blood cells near the centre of the vessel therefore absorb less light than those at the periphery. Effectively, when distributed homogeneously over the dermis, fewer red blood cells would produce the same absorption as the actual number of red blood cells distributed in discrete vessels. We quantified this effect by defining a correction factor for the effective absorbing blood volume of a single vessel. For a dermis with multiple vessels, we used this factor to define an effective homogeneous blood concentration. This was used in Monte Carlo computations of the fluence rate in a homogeneous skin model, and compared with fluence rate distributions using discrete blood vessels with equal dermal blood concentration. For realistic values of skin parameters the homogeneous model with corrected blood concentration accurately represents fluence rates in the model with discrete blood vessels. In conclusion, the correction procedure simplifies the calculation of fluence rate distributions in turbid media with discrete absorbers. This will allow future Monte Carlo computations of, for example, colour perception and optimization of vascular damage by laser treatment of port wine stain models with realistic vessel anatomy.

Light distributions in a port wine stain model containing multiple cylindrical and curved blood vessels

Knowledge of the light distribution in skin tissue is important for the understanding, prediction, and improvement of the clinical results in laser treatment of port wine stains (PWS). The objective of this study is to improve modelling of PWS treated by laser using an improved and more realistic PWS model.

Monoamniotic-versus diamniotic-monochorionic twin placentas: anastomoses and twin-twin transfusion syndrome.

OBJECTIVE The purpose of this study was to compare monoamniotic-monochorionic and diamniotic-monochorionic twin placentas and to estimate the incidence of twin-twin transfusion syndrome in monoamniotic-monochorionic twin pregnancies. STUDY DESIGN We analyzed the angioarchitecture and cord insertion distance in 24 monoamniotic-monochorionic and 200 diamniotic-monochorionic placentas. RESULTS Compared with diamniotic-monochorionic placentas, monoamniotic-monochorionic placentas had significantly more arterioarterial anastomoses (20/20 vs 159/200, respectively; P=.013), significantly less opposite arteriovenous anastomoses (10/20 vs 165/200, respectively; P=.002), similar venovenous anastomoses (6/20 vs 46/200, respectively; P=.323), and arteriovenous anastomoses (20/20 vs 187/200 respectively; P=.279) and significantly shorter umbilical cord distances (median [+/-SD], 5.0+/-6.9 cm vs 17.5+/-6.8 cm; P<.001). CONCLUSION Monoamniotic-monochorionic and diamniotic-monochorionic placentas have different anastomotic patterns. The (virtually) 100% incidence of arterioarterial anastomoses in monoamniotic-monochorionic placentas explains the reason that twin-twin transfusion syndrome rarely occurs in monoamniotic-monochorionic twin pregnancies and predicts that twin-twin transfusion syndrome manifestations are approximately 5 times less often recognized in monoamniotic-monochorionic pregnancies than in diamniotic-monochorionic pregnancies.

Two-dimensional birefringence imaging in biological tissue using phase and polarization sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) was used to obtain images of optical birefringence in biological tissues. Through simultaneous detection of two orthogonal polarization states of the signal formed by interference of light backscattered from the biological sample and a mirror in the reference arm of a Michelson interferometer, the optical phase delay between light propagating along the fast and slow axes of birefringence was measured. Simultaneous detection of both polarizations also permits reconstruction of the electro-magnetic wave backscattered from the sample. Inasmuch as any fibrous structure will influence the polarization of light, PS-OCT is a potentially powerful technique in the field of biomedical imaging. It allows rapid non-contact investigation of tissue structural properties through spatially resolved imaging of birefringence.

Changes in the optical properties (at 632.8 nm) of slowly heated myocardium

The three transport equation optical properties, the absorption coefficient, the scattering coefficient, and the average cosine of the scattering angle, or anisotropy factor have been measured (at 632.8 nm) for canine myocardium after it is heated in a water bath at room temperature and at 37-75 degrees C for 1000 s. The measurement system was a double integrating sphere with collimated light and utilized the adding-doubling solution to the equation of radiative transfer. The absorption coefficient (room temperature control, 2.0 +/- 0.4 cm(-1)) began to increase and the anisotropy factor (room temperature control, 0.93 +/- 0.02) to decrease at above 45 degrees C. At 75 degrees C the changes were significant at the p < 0.0005 level (absorption, 4.5 +/- 1.3 cm(-1); anisotropy, 0.78 +/- 0.05). There was no significant change in the scattering coefficient (room temperature controls, 161 +/- 33 cm(-1)).

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.