Ashley J. Welch

A review of the optical properties of biological tissues

The known optical properties (absorption, scattering, total attenuation, effective attenuation, and/or anisotropy coefficients) of various biological tissues at a variety of wavelengths are reviewed. The theoretical foundations for most experimental approaches are outlined. Relations between Kubelka-Munk parameters and transport coefficients are listed. The optical properties of aorta, liver, and muscle at 633 nm are discussed in detail. An extensive bibliography is provided. >

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.

IN VIVO BIDIRECTIONAL COLOR DOPPLER FLOW IMAGING OF PICOLITER BLOOD VOLUMES USING OPTICAL COHERENCE TOMOGRAPHY

We describe a novel optical system for bidirectional color Doppler imaging of flow in biological tissues with micrometer-scale resolution and demonstrate its use for in vivo imaging of blood flow in an animal model. Our technique, color Doppler optical coherence tomography (CDOCT), performs spatially localized optical Doppler velocimetry by use of scanning low-coherence interferometry. CDOCT is an extension of optical coherence tomography (OCT), employing coherent signal-acquisition electronics and joint time-frequency analysis algorithms to perform flow imaging simultaneous with conventional OCT imaging. Cross-sectional maps of blood flow velocity with <50-microm spatial resolution and <0.6-mm/s velocity precision were obtained through intact skin in living hamster subdermal tissue. This technology has several potential medical applications.

A Monte Carlo model of light propagation in tissue

The Monte Carlo method is rapidly becoming the model of choice for simulating light transport in tissue. This paper provides all the details necessary for implementation of a Monte Carlo program. Variance reduction schemes that improve the effiency of the Monte Carlo method are discussed. Analytic expressions facilitating convolution calculations for flat and Gaussian beams are included. Useful validation benchmarks are presented.

Use of an agent to reduce scattering in skin.

A method to increase light transport deeply into target areas of tissue would enhance both therapeutic and diagnostic laser applications. The effects of a hyperosmotic agent on the scattering properties of rat and hamster skin were investigated.

Effects of compression on soft tissue optical properties

Tissue optical properties are necessary parameters for prescribing light dosimetry in photomedicine. In many diagnostic or therapeutic applications where optical fiber probes are used, pressure is often applied to the tissue to reduce index mismatch and increase light transmittance. In this paper, we have measured in vitro optical properties as a function of pressure with a visible-IR spectrophotometer. A spectral range of 400-1800 mm with a spectral resolution of 5 nm was used for all measurements. Skin specimens of a Hispanic donor and two Caucasian donors were obtained from the tissue bank. Bovine aorta and sclera, and porcine sclera came from a local slaughter house. Each specimen, sandwiched between microscope slides, was compressed by a spring-loaded apparatus. Then diffuse reflectance and transmittance of each sample were measured at no load and at approximately 0.1, 1, and 2 kgf/cm/sup 2/. Under compression, tissue thicknesses were reduced up to 78%. Generally speaking, the reflectance decreased while the overall transmittance increased under compression. The absorption and reduced scattering coefficients were calculated using the inverse adding doubling method. Compared with the no-load controls, there was an increase in absorption and scattering coefficients among most of the compressed specimens.

Development and application of three-dimensional light distribution model for laser irradiated tissue

A three-dimensional model for estimating light distribution in laser irradiated tissue is presented. Multiple scattering and absorption of the laser beam are modeled using seven fluxes. One-, two-, and three-dimensional solutions are discussed and light distributions computed from the seven flux model are compared to those computed with the diffusion approximation. Methods for obtaining the phase function, absorption coefficient, and scattering coefficient for tissue are discussed and illustrated with measurements for human aortic vessel wall at the wavelength of 632.8 nm. Measured values are used in the seven flux model to estimate the rate of heat generation in the vessel wall.

Three-dimensional reconstruction of blood vessels from in vivo color Doppler optical coherence tomography images.

Purpose: Current laser treatment for vascular disorders such as port wine stains can have incomplete or unacceptable results. A customized treatment strategy based on knowledge of the patient’s blood vessel structure may effect an improved clinical outcome. Procedure: We tested the feasibility of using color Doppler optical coherence tomography (OCT) and image processing techniques to locate, measure and reconstruct cutaneous blood vessels in rat and hamster skin. OCT is a recent, potentially noninvasive technique for imaging subsurface tissue structures with micrometer scale resolution. Results: Blood vessels were identified in a series of cross-sectional images, then a three-dimensional reconstruction was made. Parameters that can affect optimum laser treatment parameters, such as average blood vessel depth and luminal diameter, were found from the images. Conclusion: This study shows that color Doppler OCT is a potential tool for improving laser treatment of vascular disorders.

Fluorescence spectroscopy of tissue: recovery of intrinsic fluorescence from measured fluorescence

We present a method for recovering the intrinsic fluorescence coefficient, defined as the product of the fluorophore absorption coefficient and the fluorescence energy yield, of an optically thick, homogeneous, turbid medium from a surface measurement of fluorescence and from knowledge of medium optical properties. The measured fluorescence signal is related to the intrinsic fluorescence coefficient by an optical property dependent path-length factor. A simple expression was developed for the path-length factor, which characterizes the penetration of excitation light and the escape of fluorescence from the medium. Experiments with fluorescent tissue phantoms demonstrated that intrinsic fluorescence line shape could be recovered and that fluorophore concentration could be estimated within ±15%, over a wide range of optical properties.