Olaf Minet

The spatial variation of the refractive index in biological cells

With a phase microscope the phase shift of cells from type L 929 fibroblast and mitochondria from liver cells was measured. Compared to the total phase shift caused by the cell relative to vacuum (approximately 1400 nm) the single phase shift of the mitochondria (approximately 180 nm) is small. Only the nucleus and the membrane of the cell give a visibly different phase shift relative to the mean value of the cell. The Fraunhofer diffraction of the measured phase object is calculated. With a simplified scattering theory, i.e. Rayleigh-Gans Scattering, different phase objects are investigated and their differential cross section is discussed.

Determination of optical tissue properties with double integrating sphere technique and Monte Carlo simulations

The optical properties of white matter human brain, canine prostate and pig liver were measured in the wavelength range 330 - 1100 nm. The measurements were carried out in native as well as in coagulated tissues. We used the double integrating sphere technique to provide reflection and transmission measurements and a special homogenizing technique to prepare the tissue. The optical properties were evaluated using an inverse Monte-Carlo simulation, considering the geometry of the experimental set-up. All tissues show characteristic absorption bands at 420 nm and 550 nm, related to the strong absorption of haemoglobin. After coagulation the scattering increases drastically while absorption remains nearly unchanged. The anisotropy factor g increases with increasing wavelength and drops down slightly after coagulation. The wavelength behavior of tissue scattering has been compared with theoretical calculations (Mie-theory), showing that ideal spheres with an diameter between 0.6 and 0.8 micrometers fit best to the experimental results.

Diagnosis of inflammatory rheumatic diseases with photon density waves

Rheumatoid arthritis (RA) is a common inflammatory disease of interphalangeal joints. The utilization of conventional imaging systems (e.g. x-rays) for non invasive diagnostics at an early stage of the disease is difficult, since pathologically induced changes do not occur at this stage in hard tissue. Use of MR and ultrasound methods are both methodically problematic and expensive. Therefore investigations for optical diagnostics using photon density waves (PDW) were carried out. The PDW was realized with an intensity modulated laser diode (825 nm, fmod: 110 MHz) and an ac- and phase detection in a 2D transillumination scanner. Measurements of optical properties of synovia and synovialis of healthy and early RA stages were performed and indicated a significant pathological increase of (mu) s. The detected PDW-pictures provided corresponding results. Further investigations regarding the object- variation of the modulation transfer function provide a sufficient spatial resolution in order to assign functional changes to anatomical structures. The results are presented using photos.

Specific heat capacities of human and animal tissues

Successful laserinduced tumor therapy requires the knowledge of optical properties of tissue as well as of the thermal ones (specific heat capacity, thermal conductivity, thermal diffusivity). Therefore, a comprehensive review of one of those values, the specific heat capacity cp, of different human and animal tissues is given measured calorimetrically by us or collected from the literature. A Differential Scanning Calorimeter was used to determine the in vitro specific heat capacities of various healthy and tumorous human tissues. The influence of freezing in liquid nitrogen and of thermal coagulation on the specific heat capacity was investigated.

Experimental set-up and Monte-Carlo model for the determination of optical tissue properties in the wavelength range 330 to 1100 nm

The optical properties of different types of tissue were measured in the wavelength range 330 - 1100 nm. The measurements were carried out in native as well as in coagulated tissues. We used the double integrating sphere technique to provide reflection and transmission measurements and a special homogenizing technique to prepare the tissue. The optical properties were evaluated using an inverse Monte-Carlo simulation, considering the geometry of the experimental set-up. All tissues show characteristic absorption bands at 420 nm and 550 nm, related to the strong absorption of hemoglobin. After coagulation the scattering increases drastically while absorption remains nearly unchanged. The anisotropy factor g increases with increasing wavelength and drops down slightly after coagulation.

RA diagnostics applying optical tomography in frequency domain

Our aim is to reconstruct the optical parameters in a slice of a finger joint phantom for further investigations about rheumatoid arthritis (RA). Therefore, we have developed a flexible NIR scanning system in order to collect amplitude and phase delay of photon density waves in frequency-domain. A cylindrical finger joint phantom was embedded in a container of Intralipid solution due to the application of an inverse method for infinite geometry. The joint phantom was investigated by a laser beam obtaining several projections. The average optical parameters of each projection was calculated. Using different reconstruction techniques, e.g. ART and SIRT with a special projection operator, we reconstructed the optical parameters in a slice. The projection operator can be heuristically described by a photon path density function of a homogeneous media with infinite geometry. Applied to an object with an unknown distribution of optical parameters it calculates the expectation value of the investigated object. The potentials and limits of these fast reconstruction methods will be presented.

Observations of the fluorescence response of the coenzyme NADH in biological samples.

The disturbed fluorescence response of the coenzyme NADH in biological tissues has been observed. The oscillation features of the NADH fluorescence decay (excitation at 337.1 nm, fluorescence at 460 ± 20 nm) for isolated cell membrane structures are different from that for cytoplasm. An analysis of the experimental data with the method of the system theory proves the nonlinear behavior of the fluorescence decay.

Optical Molecular Imaging: Overview and Technological Aspects

Summary Optical Molecular Imaging (OMI) is an in vivo procedure for early detection of pathologically changed tissue, such as tumours and inflammatory rheumatoid finger joints. Combining the advantages of optical diagnostics with the potential of genetic research, it employs specific markers to enhance the contrast of diseased tissue areas. These so-called optical markers can be both fluorescent and absorbent. The purpose of OMI is to activate the markers exclusively in the pathologically changed areas by means of specific enzyme reactions thus obtaining high contrast. The detection system must be highly sensitive and tailored to the markers mechanism and field of application. Procedures such as diffuse optical tomography (DOT) and laser-induced fluorescence spectroscopy (LIF) are suitable for OMI systems. One out of many suppositions for the application of OMI is the accessibility of light penetration (cm range) in pathological changes, provided appropriate markers are found. The OMI principle is another challenging field of research, which offers a wide range of applications and a huge potential for the development of cost-favourable and efficient diagnostic methods.

The Medical Use of Rescaling Procedures in Optical Biopsy and Optical Molecular Imaging

Laser-induced autofluorescences show a strong intensity distortion for endogenous NADH in the UV and synthesized markers in the NIR range because of tissue optics. Rescaling taking into account bio-optical methods results in the chromophore profile in the observed tissue region. For the in vivo tests an experimental NIR imager was used. NIR fluorescence of the entire body of small animals can be imaged. For first experiments an undifferentiated superficial tumor of mouse thigh was used. Corrections that are due to tissue optics must take care of a more strongly scattering of the light in the NIR range in comparison to the UV fluorescence such as in optical biopsy. For example, the diameter of the fluorescent volume is apparently larger for the same reason. Therefore, the established rescaling from the UV adapted to the NIR range is important for the interpretation fluorescence pictures in biomedicine.

Investigations concerning the determination of NADH concentrations using optical biopsy

The intrinsic NADH autofluorescence intensity of biological tissue depends on the local, cellular concentration of this coenzyme. It plays a dominant role in the Krebs-Cycle and therefore serves as indicator for the vitality of the observed cells. Due to individually and locally varying boundary conditions and optical tissue properties, which are scattering coefficients, absorption coefficients and g-factors the fluorescence signal needs to be rescaled. One possible rescaling method is the theoretical derived Photon Migration Theory. Our new rescaling method is partly based on measurements and partly theoretical derived. By using the 4 information channels: LIF time-resolved signal, biochemical concentration measurements, Monte Carlo simulations with optical parameters and microscopic investigations we demonstrate that simultaneous detection of the fluorescence and the backscattering signal can easily and accurately provide rescaled, quantitative values for the NADH concentrations.