Biomedical subjects

A G Yodh

Publications and source records attributed to A G Yodh.

4 recordsLinked to original sources

PubMed1998

Regional imager for low-resolution functional imaging of the brain with diffusing near-infrared light.

We have developed a near-infrared spectroscopy system for low-resolution regional imaging of the brain. Our regional imager employs two intensity-modulated (frequency-domain) diode lasers operating at 779 and 834 nm, respectively, in order to produce macroscopic waves of diffusing photons. The interaction of these diffusive waves with tissue depends on laser modulation frequency, laser wavelength and the optical properties of the sample tissue volume. The lasers can be modulated over a range of frequencies from 50 to 400 MHz. Light is coupled to and from the head using a pad that has 12 source and 4 detector positions within an area of approximately 40 cm2. The pad can be moved to different positions on the head. Measurements from different source-detector combinations enable reconstruction of low-resolution images of the tissue volume beneath the pad. For example, we have made two-dimensional back-projection images of model systems in order to demonstrate the capabilities of the regional imager. We also present preliminary results from initial clinical studies at the Children's Hospital of Philadelphia.

Brain↗

PubMed1997

Does the photon-diffusion coefficient depend on absorption?

We investigate the controversy over the precise form of the photon diffusion coefficient and suggest that it is largely independent of absorption, i.e., Do = v/3mu(s)'. After presentation of the general theoretical arguments underlying this assertion, Monte Carlo simulations are performed and explicitly reveal that the absorption independent diffusion coefficient gives better agreement with theory than the traditionally accepted photon diffusion coefficient, D(mu)a = v/3(mu(s) + mu(a)). The importance of resolving this controversy for the proper characterization of the material optical properties is discussed.

Absorption↗

PubMed1995

Determination of optical properties and blood oxygenation in tissue using continuous NIR light.

In this article, we introduce a simple method to characterize optical properties and blood oxygenation in tissue using spatially resolved, steady-state reflectance. The method considers multiple source-detector separations larger than 2 cm, i.e. 20 times the optical mean free path in tissue, and makes an approximation to linearize the relationship between the separation and reflectance. Simulation results show that errors of the algorithm due to the approximation are less than 10%. Using a calibration sample, we calculate from the slope and intercept the absorption and reduced scattering coefficients, mu a and mu's, of a tissue-like solution, and experimental results confirm the usefulness of the method for quantitation of haemoglobin saturation in tissue.

Algorithms↗

PubMed1994

Scattering of diffuse photon density waves by spherical inhomogeneities within turbid media: analytic solution and applications.

We present an analytic solution for the scattering of diffuse photon density waves by spherical inhomogeneities within turbid media. The analytic result is compared to experimental measurements. Close agreement between theory and experiment permits the use of the theory to determine the properties of unknown sphere-like objects embedded in turbid media. The analytic solution is extended to encompass several problems of practical interest in imaging, including the influence of multiple sources, multiple objects, and boundaries on the characterization of spherical inhomogeneities. We also extend the solution to encompass time-domain measurements.

Algorithms↗