Scott A. Prahl

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. >

Light scattering in Intralipid-10% in the wavelength range of 400-1100 nm

The absorption, scattering, and anisotropy coefficients of the fat emulsion Intralipid-10% have been measured at 457.9, 514.5, 632.8, and 1064 nm. The size and shape distributions of the scattering particles in Intralipid-10% were determined by transmission electron microscopy. Mie theory calculations performed by using the particle size distribution yielded values for the scattering and anisotropy coefficients from 400 to 1100 nm. The agreement with experimental values is better than 6%.

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.

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.

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.

In vivo determination of optical properties of normal and tumor tissue with white light reflectance and an empirical light transport model during endoscopy.

Determination of tissue optical properties is fundamental for application of light in either therapeutical or diagnostics procedures. In the present work we implemented a spatially resolved steady-state diffuse reflectance method where only two fibers (one source and one detector) spaced 2.5 mm apart are used for the determination of the optical properties. The method relies on the spectral characteristics of the tissue chromophores (water, dry tissue, and blood) and the assumption of a simple wavelength dependent expression for the determination of the reduced scattering coefficient. Because of the probe dimensions the method is suited for endoscopic measurements. The method was validated against more traditional models, such as the diffusion theory combined with adding doubling for in vitro measurements of bovine muscle. Mean and standard deviation of the absorption coefficient and the reduced scattering coefficient at 630 nm for normal mucosa were 0.87+/-0.22 cm(-1) and 7.8+/-2.3 cm(-1), respectively. Cancerous mucosa had values 1.87+/-1.10 cm(-1) and 8.4+/-2.3 cm(-1), respectively. These values are similar to data presented by other authors. Blood perfusion was the main variable accounting for differences in the absorption coefficient between the studied tissues.

Influence of photoinitiator type on the rate of polymerization, degree of conversion, hardness and yellowing of dental resin composites

OBJECTIVES To evaluate the degree of conversion (DC), maximum rate of polymerization (Rpmax), Knoop hardness (KHN) and yellowing (b-value) of resin composites formulated with phenylpropanedione (PPD), camphorquinone (CQ), or CQ/PPD at different concentrations. The hypotheses tested were (i) PPD or CQ/PPD would produce less Rpmax and yellowing than CQ alone without affecting DC and KHN, and (ii) Rpmax, DC, and KHN would be directly related to the absorbed power density (PDabs). METHODS CQ/amine, PPD/amine and CQ/PPD/amine were used at low, intermediate and high concentrations in experimental composites. Photoinitiator absorption and halogen-light emission were measured using a spectrophotometer, Rp with differential scanning calorimetry (DSC), DC with DSC and FTIR, KHN with Knoop indentation; and color with a chromameter. The results were analyzed with two-way analysis of variance (ANOVA)/Student-Newman-Keuls test (p<0.05). Correlation tests were carried out between PDabs and each of DC, Rpmax and KHN. RESULTS The PDabs increased with photoinitiator concentration and PPD samples had the lowest values. In general, maximum DC was comparable at intermediate concentration, while Rpmax and KHN required higher concentrations. DC was similar for all photoinitiators, but Rpmax was lower with PPD and CQ/PPD. PPD produced the lowest KHN. Yellowing increased with photoinitiator concentration. PPD did not reduce yellowing at intermediate and/or high concentrations, compared to CQ-formulations. PDabs showed significant correlations with DC, Rpmax and KHN. CONCLUSION PPD or CQ/PPD reduced Rpmax in experimental composites without affecting the DC. The use of PPD did not reduce yellowing, but reduced KHN. DC, Rpmax and KHN were dependent on PDabs.

Accuracies of the diffusion approximation and its similarity relations for laser irradiated biological media

The accuracy of the diffusion approximation is compared with more accurate solutions for describing light interaction with biological tissues. Generally the diffusion approximation underestimates the light distribution in the surface region, and, for high albedos, it significantly underestimates the fluence rate. This difference is only a few percent for albedos of less than 0.5 due to the dominance of collimated light. As the anisotropy of scattering increases, deviations increase. In general, fluxes can be computed more accurately with the diffusion approximation than fluence rates. For anisotropic scattering, better results can be obtained by simple transforms of optical coefficients using the similarity relations. The similarity relations improve flux calculations, but computed fluence rates have substantial errors for high albedo and the large index of refraction differences at the surface.

Preparation and characterization of polyurethane optical phantoms

We describe a method for the preparation of a polyurethane phantom to simulate the optical properties of biologic tissues at two wavelengths in the visible and near-infrared spectral range. We characterize the addition of added molecular absorbers with relatively narrow absorption bands [full width at half maximum (FWHM) 32 and 76 nm for Epolight 6084 and 4148, respectively] for independent absorption at 690 nm for absorption up to 5 cm(-1), and 830 nm for absorptions up to 3 cm(-1). Absorption by both dyes is linear with concentration in these respective regions and is consistent in polyurethane both before and after curing. The dyes are stable over long durations with no more than 4% change. The absorption of visible light by polyurethane decreases with time and is stable by one year with a drop of 0.03+/-0.003 cm(-1) from 500 to 830 nm. The scattering properties are selected by the addition of TiO2 particles to the polyurethane, which we functionally describe for the 690- and 830-nm wavelengths as related to the weight per volume. We demonstrate that the variation in absorption and scattering properties for large batch fabrication (12 samples) is +/-3%. The optical properties of the phantoms have not significantly changed in a period of exceeding one year, which makes them suitable for use as a reference standard.

Measurements and calculations of the energy fluence rate in a scattering and absorbing phantom at 633 nm

We have studied the influence of absorption, scattering, and refractive index of a phantom medium in conjunction with various beam diameters on the penetration depth of light at 633 nm. We used mixtures of Intralipid 10% (scattering medium) and Evans blue (absorbing medium). Measurements were performed in media with a scattering coefficient of 1 mm(-1), an anisotropy factor of 0.71, absorption coefficients of 1.3 x 10(-3), 0.01, and 0.05 mm(-1), and a refractive index of 1.33. The experimental results were compared with an analytical solution of the fluence rate based on diffusion theory. We found good agreement (deviations of <10%) between theory and experiment for incident beam diameters between 10 and 60 mm.