René A. Bolt

Temperature dependence of optical properties of in-vivo human skin

We examined the effect of temperature change on the diffuse reflectance of the skin. The optical probe consists of several optical fibers located at the center of a thermal electric device, which controls the temperature at the surface of the skin in contact. Measured light reflectance profile between 0.4-1.9 mm was fitted to a mathematical model obtained by Monte Carlo simulation, and absorption and scattering coefficients were estimated. The reduced scattering coefficient of the forearms consistently showed a positive relationship with temperature between 22 and 42 degree(s)C. This dependency was reversible without apparent delay. The same effect was observed on ex vivo pigskin. It is possible to explain the positive instantaneous dependency of scattering on temperature by the change of the refractive index of intercellular fluid. The scattering coefficient of the subcutaneous fat of pigskin showed a negative dependence on temperature. This negative dependency of scattering can be attributed to a phase change as a function of temperature. The absorption coefficient in vivo also increased with temperature from 22 to 42 degree(s)C. But the change was not immediately reversible after temperature reached 40 degree(s)C. This relationship was similar to the nonlinear increase in blood perfusion observed in laser Doppler measurements.

Goniometric instrument for light scattering measurement of biological tissues and phantoms

In this Note, we present a goniometric instrument for measuring the forward angular light scattering behavior of liquid or solid samples with high angular resolution and large dynamic range. The instrument is designed to work in the visible and near-infrared regions. Along with measuring the angular scattering behavior, the instrument functions as a refractometer to determine the refractive index of the sample.

Pulsed-laser Doppler flowmetry provides basis for deep perfusion probing

A setup for pulsed-laser Doppler flowmetry ~LDF! measurements has been built and tested. Measurements were carried out comparing continuous-wave and pulsed LDF. With pulsed LDF a higher peak power can be injected into the tissue without exceeding the safety limits. This enables a much larger spacing between the locations of illumination and detection. Thus, the penetration depth, and thus the measurement volume, can be enlarged using the pulsed-LDF method. This method will allow, e.g., monitoring of the cerebral perfusion.

Photoacoustic imaging of blood perfusion in tissue and phantoms

To localize and monitor the blood content in tissue we developed a very sensitive photo-acoustical detector. PVDF has been used as piezo-electric material. In this detector also fibers for the illumination of the sample are integrated. Resolution is about 20 (m in depth and about 50-100 m laterally). We use 532 nm light. We will show how photoacoustics can be used for measuring the thickness of tissue above bone. We will also report measurements on tissue phantoms: e.g. a vessel delta from the epigastric artery branching of a Wistar rat, filled with an artificial blood-resembling absorber. The measurements have been carried out on phantoms containing vessels at several depths. Signal processing was enhanced by Fourier processing of the data.

Non-invasive determination of concentration of compounds in strongly absorbing biological tissue

We describe a non-invasive method for the determination of optical parameters of highly scattering media, such as biological tissue. An advantage of this method is that it does not rely on diffusion theory, thus it is applicable to strongly absorbing media and at small source-detector separations. Monte Carlo simulations and phantom measurements are used to illustrate the achievable accuracy of the system. The method was applied to non-invasive in- vivo tracking of haemoglobin concentration in biological tissue. The results correlated well to clinically determined Hb concentrations.

Pulsed laser-Doppler flowmetry for monitoring deep perfusion

Measurements have been carried out using a pulsed laser-Doppler setup. The main advantage of pulsing a laser-diode is that much higher peak powers can be used, allowing a larger source-detector separation, resulting in a larger penetration depth. The method enables e.g. monitoring of cerebral perfusion as well as monitoring perfusion through organs (e.g. kidney).

Radiative transfer through multilayered medium: high-frequency experiments and theory

Development of efficient analytical and numerical techniques for the determination of optical parameters of biotissues on the base of measured data is a crucial part of successful implementation of non- invasive diagnostic techniques in clinical conditions. Widely used approximations, like diffusion approximation (DA), were shown to fail in most real-life circumstances due to simplifications of modeling or neglect of various involve phenomena, like boundary effects, tissue inhomogeneity, skin roughness and deviations of optical properties of skin in time due to physiological effects. In this work we compare experimental results with results of numerical simulations. For our measurements, we used both spatial-resolved and frequency-domain techniques. To describe propagation of photons we numerically solved the radiatve transfer equation (RTE). We found that the Monte-Carlo method (MC) is too time-consuming for large source- detector separations. We achieved flexibility in preparation of experimental medium with tissue simulating sample containing several homogeneous layers. Our objective is the investigation of accuracy in determining unknown structures and optical coefficients from measured data, based on the realistic model of the tissue described in the RTE.

Application of efficient numerical technique in transport theory for inhomogeneous turbid medium: comparison with experiments

In this work we compare experimental results with results of numerical simulations. For our measurements, we used both spatial-resolved and frequency-domain techniques. To describe propagation of photons we solved the rigorous radiative transfer equation (RTE). We found that the Monte- Carlo method (MC) is too time-consuming for large source- detector separations. We achieved flexibility in preparation of experimental medium with tissue-simulating sample containing of several homogeneous layers. Our objective is the investigation of accuracy in determining unknown structures and optical coefficients from measured data, based on the realistic model of the tissue described in the RTE. We have shown that, by comparing the use of the RTE to the diffusion approximation or MC, we achieve better accuracy or universality in source-detector distances.