Thorsten Spott

Collimated light sources in the diffusion approximation

Collimated light sources in turbid media are difficult to describe within the diffusion approximation, because they do not meet the requirement of near isotropy. For precise calculation of light intensities close to the source, alternative descriptions of the light source are necessary. In this paper the transition of collimated light into diffusivity is studied by Monte Carlo simulations. On the basis of these simulations and the diffusion approximation a hybrid approach is designed and used to analyze approaches based on analytic source terms. The influence of boundaries to air is studied. The benefits of increased approximation orders are investigated. It is shown that, even in the presence of strong absorption, the diffusion approach can give satisfactory results if only the source terms are suitably chosen.

Reflectance measurements of layered media with diffuse photon-density waves: a potential tool for evaluating deep burns and subcutaneous lesions

The basic principles of a non-contact, near-infrared technique for the mapping of layered tissues are discussed theoretically and verified experimentally. The propagation properties of diffuse photon-density waves in tissues depend on the optical properties of the tissue. When a layered medium is irradiated by amplitude modulated light, the difference in optical properties between the layers is evident in the phase and amplitude of the diffuse reflection coefficient, which is a result of the interference of the partial waves propagating in the different layers. Thus, diffuse photon-density waves are applicable to the analysis of the structure of layered tissue. The probing depth is determined by the modulation frequency of the incident light. For modulation frequencies between several hundred megahertz and a few gigahertz, this allows us to analyse the properties of muscle tissue of up to 4-8 mm below the surface. Experimental results based on chicken breast muscle are given. As an example, the technique might be of use for evaluating the depth of necrosis and the blood volume fraction in deep burns.

Application of optical diffusion theory to transcutaneous bilirubinometry

Neonatal hyperbilirubinemia affects more than half of the newborns and represents a potentially serious condition due to the toxicity of bilirubin to the central nervous system. A precise non-invasive technique for the monitoring of bilirubin concentration is desirable for the treatment of icteric babies. Transcutaneous bilirubinometry based on optical reflectance spectra is complicated by the superposition of the spectral absorption properties of melanin and haemoglobin with those of bilirubin. Diffusion theory forms a suitable model for the description of light propagation in tissue. In this treatment, an inverse diffusion approach is developed to measure bilirubin concentration in tissue by means of the reflectance spectrum. First results of its application to in vivo measurements are encouraging.

Non-invasive burn depth determination by diffuse reflectance measurement of intensity modulated light

The surface of a slab of chicken breast muscle was gradually heated with a CO/sub 2/ laser. The goal of the experiment was to detect the depth of the coagulated top layer by measuring the diffuse reflectance at the tissue surface.

Reflectance tomography of two-layered turbid media with diffuse photon-density waves

As approach to the examination of the structure of layered tissue can be found in the measurement of the diffuse reflectance of plane diffuse photon-density waves in the near-infrared range. Here, phase resolved reflectance measurements from phantom tissue, at modulation frequencies of up to 2 GHz, are presented and compared to calculations provided by a theoretical model. The examination of the phase shift reveals that the reflectance properties are characterized by photon-density wave interference phenomena. The proposed technique allows the investigation of the structure of tissue down to more than one penetration depth. A medical application may be found in improved examination techniques for deep burns, as the method allows the investigation of the tissue structure without physical contact to the surface.

Quantitative optical parameter determination in tissues using diffuse photon-density waves: the impact of measurement geometry and source modulation frequency

ABSTRACT The measurement of the propagation characteristics of Diffuse Photon-Density Waves (DPDW) is a viable path todetermine the optical properties of tissue, i. e. the absorption and the reduced scattering coefficient. The techniqueallows to take measurements from bulk tissue, either by interstitial placement (infinite medium measurement) orsuperficial placement (semi-infinite medium measurement) of the source and detection fibers. Treatment of thetissue prior to measurement is unnecessary, such that changes to the structure of the tissue, and thus the opticalproperties, can be avoided. The optical properties can then be recovered by application of a diffusion model. Whenusing a semi-infinite medium model, non-invasive in vivo measurements are feasible. While the theory is well established and verified by phantom measurements, the small amount of published data for biological tissue acquired by DPDW measurements suggests that there are still open practical problems, both concerning the measurementtechnique and the application of the diffusion model. In this study, we investigate the optimal choice of modulationfrequencies for extracting a maximum amount of information on the scattering and absorption properties, examinedifferent measurement setups, analyze how amplitude and phase data should be combined and compare the qualityof infinite and semi-infinite medium measurements. The considerations are supported by experimental data, basedon measurements from chicken and turkey breast muscle with modulation frequencies of up to 0.4 GHz.Keywords: diffuse photon-density waves, diffusion theory, modulation frequency, optical properties, chicken breastmuscle, turkey breast muscle

The role of the source terms in the diffusion approximation when applied to reflectance calculations of skin

In the presence of high scattering albedos, as are typical for biological tissues, light assumes a near-isotropic distribution.