Matthias Essenpreis

Near-infrared optical properties of ex vivo human skin and subcutaneous tissues measured using the Monte Carlo inversion technique

The absorption and transport scattering coefficients of caucasian and negroid dermis, subdermal fat and muscle have been measured for all wavelengths between 620 and 1000 nm. Samples of tissue 2 mm thick were measured ex vivo to determine their reflectance and transmittance. A Monte Carlo model of the measurement system and light transport in tissue was then used to recover the optical coefficients. The sample reflectance and transmittance were measured using a single integrating sphere comparison method. This has the advantage over conventional double-sphere techniques in that no corrections are required for sphere properties, and so measurements sufficiently accurate to recover the absorption coefficient reliably could be made. The optical properties of caucasian dermis were found to be approximately twice those of the underlying fat layer. At 633 nm, the mean optical properties over 12 samples were 0.033 mm(-1) and 0.013 mm(-1) for absorption coefficient and 2.73 mm(-1) and 1.26 mm(-1) for transport scattering coefficient for caucasian dermis and the underlying fat layer respectively. The transport scattering coefficient for all biological samples showed a monotonic decrease with increasing wavelength. The method was calibrated using solid tissue phantoms and by comparison with a temporally resolved technique.

Correlation between blood glucose concentration in diabetics and noninvasively measured tissue optical scattering coefficient.

Diabetics would benefit greatly from a device capable of providing continuous noninvasive monitoring of their blood glucose levels. The optical scattering coefficient of tissue depends on the concentration of glucose in the extracellular fluid. A feasibility study was performed to evaluate the sensitivity of the tissue reduced scattering coefficient in response to step changes in the blood glucose levels of diabetic volunteers. Estimates of the scattering coefficient were based on measurements of the diffuse reflectance on the skin at distances of 1-10 mm from a point source. A correlation was observed between step changes in blood glucose concentration and tissue reduced scattering coefficient in 30 out of 41 subjects measured.

Influence of glucose concentration on light scattering in tissue-simulating phantoms.

The presence of glucose dissolved in an aqueous solution increases the refractive index of the solution and therefore has an influence on the scattering properties of any particles suspended within it. We present experimental data on the effect of glucose concentration on the scattering coefficient of a suspension of spherical polystyrene particles. The experimental results are in good agreement with Mie theory. The effect of glucose on light transport in highly scattering, tissue-simulating phantoms is demonstrated both experimentally and theoretically by application of diffusion theory. The possible application of this effect for noninvasive glucose monitoring of diabetic patients is discussed.

Effect of temperature on the optical properties of ex vivo human dermis and subdermis

The effect of temperature on the optical properties of human dermis and subdermis as a function of near-infrared wavelength has been studied between 25 degrees C and 40 degrees C. Measurements were performed ex vivo on a total of nine skin samples taken from the abdomen of three individuals. The results show a reproducible effect of temperature on the transport scattering coefficient of dermis and subdermis. The relative change of the transport scattering coefficient showed an increase for dermis ((4.7+/-0.5) x 10(-3) degrees C(-1)) and a decrease for subdermis ((-1.4+/-0.28) x 10(-3) degrees C(-1)). Note that the magnitude of the temperature coefficient of scattering was greater for dermis than subdermis. A reproducible effect of temperature on the absorption coefficient could not be found within experimental errors. System reproducibility in transport scattering coefficient with repeated removal and repositioning of the same tissue sample at the same temperature was excellent at +/-0.35% for all measurements. This reproducibility enabled such small changes in scattering coefficient to be detected.

Influence of layered tissue architecture on estimates of tissue optical properties obtained from spatially resolved diffuse reflectometry

Most instruments used to measure tissue optical properties noninvasively employ data-analysis algorithms that rely on the simplifying assumption that the tissue is semi-infinite and homogeneous. The influence of a layered tissue architecture on the determination of the scattering and absorption coefficients has been investigated in this study. Reflectance as a function of distance from a point source for a two-layered tissue architecture that simulates skin overlying fat was calculated by using a Monte Carlocode. These data were analyzed by using a diffusion theory modelfor a homogeneous semi-infinite medium to calculate the scatter and absorption coefficients. Depending on the algorithm and the radial distance, the estimated tissue optical properties were different from those of either layer, and under some circumstances, physically impossible. In addition, the sensitivity and cross talk of the estimated optical properties to changes in input optical properties were calculated for different layered geometries. For typical optical properties of skin, the sensitivity to changes in optical properties is highly dependent on the layered architecture, the measurement distance, and the fitting algorithm. Furthermore, a change in the input absorption coefficient may result in an apparent change in the measured scatter coefficient, and a change in the in put scatter coefficient may result in an apparent change in the measured absorption coefficient.

Optical properties of phantoms and tissue measured in vivo from 0.9 to 1.3 um using spatially resolved diffuse reflectance

A near infrared spectrometer has been constructed which is capable of performing spatially resolved diffuse reflectance measurements in the wavelength range of 0.9 - 1.6 micrometer. In this technique, broadband light is delivered by an optical fiber to a point on the tissue surface and diffusely reflected light is collected by 300 micrometer fibers located at 15 distances ranging between 1.0 to 10.0 mm from the source. The light from the detector fibers is imaged with a monochromator onto an InGaAs photodiode array. Wavelengths can be selected by automated scanning of the monochromator grating. A diffusion theory model fit to the reflectance versus distance data has been used to estimate the absorption and scattering coefficients ((mu) a and (mu) s) of phantoms and tissue under analysis. Reflectance measurements have been performed on tissue simulating water-based phantoms as well as in vivo on different skin locations. The absorption coefficient of skin was found to have a spectral structure similar to that of water. Unexpected spectral features in the scattering coefficient of skin were observed which may be a result of not considering the layered structure of skin in the current model. The temporal stability of the system has been demonstrated on tissue-simulating phantoms and human volunteers, indicating that the reflectance measurement may be suitable for in vivo monitoring of physiologically induced changes in the absorption and scattering coefficients.

Glucose-induced changes in scattering and light transport in tissue-simulating phantoms

The presence of glucose in an aqueous solution increases its refractive index and therefore has an influence upon the scattering properties of particles suspended in solution. The subsequent effect upon light transport in multiple scattering, tissue simulating phantoms is demonstrated experimentally in a slab geometry and theoretically by applying diffusion theory. As the glucose-induced scattering changes are small, any possible application for noninvasive glucose monitoring in diabetic parients has to rely on an accurate separation of scattering and absorption changes. In phangom studies, it is shown that optional measurements in the frequency domain allow this separation. Furthermore, preliminary experiments suggest that this method can be applied in vivo. It was found that changing the blood flow in tissue, the effect on the scattering coefficient is small.

Near-infrared optical properties of ex-vivo human skin and subcutaneous tissues using reflectance and transmittance measurements

The vast majority of non-invasive measurements of human tissues using near infrared spectroscopy rely on passing light through the dermis and subdermis of the skin. Accurate knowledge of the optical properties of these tissues is essential to put into models of light transport and predict the effects of skin perfusion on measurements of deep tissue. Additionally, the skin could be a useful accessible organ for non-invasively determining the constituents of blood flowing through it. Samples of abdominal human skin (including subdermal tissue) were obtained from either post mortem examinations or plastic surgery. The samples were separated into a dermal layer (epidermis and dermis, 1.5 to 2 mm tick), and a sub-cutaneous layer comprised largely of fat. They were enclosed between two glass coverslips and placed in an integrating sphere to measure their reflectance and transmittance over a range of wavelengths from 600 to 1000 nm. The reflectance and transmittance values were converted into average absorption and reduced scattering coefficients by comparison with a Monte Carlo model of light transport. Improvements to the Monte Carlo model and measurement technique removed some previous uncertainties. The results show excellent separation of reduced scattering and absorption coefficient, with clear absorption peaks of hemoglobin, water and lipid. The effect of tissue storage upon measured optical properties was investigated.

Influence of tissue inhomogeneities on estimates of tissue optical properties obtained from steady-state reflectometry

Most instrumentation used to measure tissue optical properties non-invasively employ data analysis algorithms which rely upon the simplifying assumption that the tissue is semi-infinite and homogeneous. The influence of a layered tissue architecture on the determination of the scattering and absorption coefficients has been investigated. Steady state reflectance data for a two-layered tissue architecture were generated using a Monte Carlo code. These were analyzed using a diffusion theory model to calculate the scatter and absorption coefficients. The estimated tissue optical properties were different from those for either layer, and for certain choices of input optical properties would result in physically impossible results. The sensitivity and specificity of the estimated optical properties to changes in input optical properties were calculated for different layered geometries. For typical optical properties of skin the results suggest that a change in the absorption coefficient of 100% will result in an apparent change in scatter coefficient of 15% and that a similar change only in scatter coefficient will result in an apparent change of 100% in absorption coefficient.

Determination of optical properties by variation of boundary conditions

Propagation of photons in multiple scattering media depends on absorbing and scattering properties as well as the boundary conditions of the semi-infinite medium. A new method is shown that makes use of differences in boundary conditions to determine the optical properties. Induced are these different conditions by varying the reflectivity of a sensor head. We describe the influence of the change in reflectivity with the common diffusion theory. By building a ratio between the spatially-resolved diffuse reflectance under different boundary conditions it is possible to calculate the optical properties of homogeneous phantoms. Due to optical heterogeneities in living tissue, limitations of the method was observed, which restricts the application to in vivo measurements.