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Biomedical subjects

A H Gandjbakhche

Publications and source records attributed to A H Gandjbakhche.

13 recordsLinked to original sources
PubMed2006

Comment on 'the use of the Henyey-Greenstein phase function in Monte Carlo simulations in biomedical optics'.

In this letter the authors highlight the presence of an error appearing in the discussion of the note 'The use of the Henyey-Greenstein phase function in Monte Carlo simulations in biomedical optics' previously published by them (Binzoni et al 2006 Phys. Med. Biol. 51 N313). In the light of this error, the discussion and conclusions in the original paper are revised in this letter and the role of the use of the phase functions in MC simulations, interpreted in probabilistic terms, is better clarified. The exact definition for the probability density function for the deflection angle, in the case of the Henyey-Greenstein model, is also given.

Algorithms↗
PubMed2006

The use of the Henyey-Greenstein phase function in Monte Carlo simulations in biomedical optics.

Monte Carlo (MC) simulations are often at the heart of the testing procedure in biomedical optics. One of the critical points in MC simulations is to define the new photon direction after each scattering event. One of the most popular solutions is to use the Henyey-Greenstein phase function or some linear combinations of it. In this note, we demonstrate that randomly generating the angle defining the new direction of a photon after a collision, by means of the Henyey-Greenstein phase function, is not equivalent to generating the cosine of this angle, as is classically done. In practice, it is demonstrated that for a nearly isotropic medium (asymmetry parameter g approximately 0) this discrepancy is not large, however for an anisotropic medium as is typically found in vivo (e.g. g = 0.98) the two methods give completely different results.

Algorithms↗
PubMed2004

Quantitative assessment of tumor vasculature and response to therapy in kaposi's sarcoma using functional noninvasive imaging.

Two noninvasive methods, thermography and laser Doppler imaging (LDI), were assessed for their ability to quantitatively assess parameters of vascularity in lesions of HIV-associated Kaposi's sarcoma (KS). Thermography and LDI images of a representative KS lesion were recorded in 16 patients and compared to normal skin either adjacent to the lesion or on the contralateral side. Eleven of the 16 patients had greater than 0.5 degrees C increased temperature and 12 of the 16 patients had increased flux (measured by LDI) as compared to normal skin. There was a strong correlation between these two parameters (R = 0.81, p < 0.001). In ten patients, measurements were obtained prior to therapy and after receiving a regimen of liposomal doxorubicin and interleukin-12. After 18 weeks of therapy, temperature and blood flow of the lesions were significantly reduced from the baseline (p = 0.004 and 0.002 respectively). These techniques hold promise to assess physiologic parameters in KS lesions and their changes with therapy.

Acquired Immunodeficiency Syndrome↗
PubMed2000

Descriptive parameter for photon trajectories in a turbid medium.

In many applications of laser techniques for diagnostic or therapeutic purposes it is necessary to be able to characterize photon trajectories to know which parts of the tissue are being interrogated. In this paper, we consider the cw reflectance experiment on a semi-infinite medium with uniform optical parameters and having a planar interface. The analysis is carried out in terms of a continuous-time random walk and the relation between the occupancy of a plane parallel to the surface to the maximum depth reached by the random walker is studied. The first moment of the ratio of average depth to the average maximum depth yields information about the volume of tissue interrogated as well as giving some indication of the region of tissue that gets the most light. We have also calculated the standard deviation of this random variable. It is not large enough to qualitatively affect information contained in the first moment.

Lasers↗
PubMed1999

Effect of lateral boundaries on contrast functions in time-resolved transillumination measurements.

The method of images is employed to insert the effects of the presence of a single lateral boundary on contrast functions previously derived for an infinite slab using a random walk model of photon transport. The predictions of the model for zero and extrapolated boundary conditions are compared with Monte Carlo (MC) simulations and with experimental results obtained using a homogenous phantom with tissue-like optical properties. As expected, the extrapolated boundary condition yields better agreement between the theoretical predictions and results obtained from MC and experiments. This indicates that the random walk model has potential as a forward model in iterative imaging schemes developed for optical tomography.

Biophysical Phenomena↗
PubMed1999

Myocardial oxygenation in vivo: optical spectroscopy of cytoplasmic myoglobin and mitochondrial cytochromes.

The oxygenation state of myoglobin and the redox state of cytochrome c provide information on the PO(2) in the cytosol and mitochondria, respectively. An optical "window" from approximately 540 to 585 nm was found in the pig heart in vivo that permitted the monitoring of myoglobin and cytochrome c without interference from Hb oxygenation or blood volume. Scanning reflectance spectroscopy was performed on the surgically exposed left ventricle of pigs. Difference spectra between control and a total left anterior descending coronary artery occlusion revealed maxima and minima in this spectral region consistent with myoglobin deoxygenation and cytochrome c and b reduction. Comparison of in vivo data with in vitro fractions of the heart, including Hb-free tissue whole heart and homogenates, mitochondria, myoglobin, and pig red blood cells, reveals minimal contributions of Hb in vivo. This conclusion was confirmed by expanding the blood volume of the myocardium and increasing mean Hb O(2) saturation with an intracoronary infusion of adenosine (20 microgram. kg(-1). min(-1)), which had no significant effect on the 540- to 585-nm region. These results also suggested that myoglobin O(2) saturation was not blood flow limited under these conditions in vivo. Work jump studies with phenylephrine also failed to change cytochrome c redox state or myoglobin oxygenation. Computer simulations using recent physical data are consistent with the notion that myoglobin O(2) saturation is >92% under basal conditions and does not change significantly with moderate workloads. These studies show that reflectance spectroscopy can assess myocardial oxygenation in vivo. Myoglobin O(2) saturation is very high and is not labile to moderate changes in cardiac workload in the open-chest pig model. These findings indicate that myoglobin does not contribute significantly to O(2) transport via facilitated diffusion under these conditions.

Animals↗
PubMed1999

Visible-light photon migration through myocardium in vivo.

Empirical data between 510 and 590 nm of diffuse reflected light from the pig heart in vivo have shown that myoglobin and cytochrome c absorption peaks with little apparent contribution of red blood cell (RBC) Hb. Monte Carlo simulations of photon migration in tissue were performed to compare the effects of myoglobin and cytochromes with those of blood Hb on photon pathlengths and diffuse reflectance of visible wavelengths (450-600 nm) from the pig heart in vivo. Wavelength dependence of the input parameters, including the transport-corrected scattering coefficients (1.1-1.2 mm(-1)) and the absorption coefficients of blood-free solubilized heart tissue (0.43-1.47 mm(-1)), as well as the absorption coefficients of Hb, were determined by an integrating sphere method and standard spectrophotometry, respectively. The Monte Carlo simulations indicate that in the 510- to 590-nm range the mean path length within the myocardium for diffusely reflected light varies from 1.4 to 1.2 mm, whereas their mean penetration depth within the epicardium is only 330-400 micrometer for blood-free heart tissue. Analysis shows that the blood Hb absorption extrema are only observable between 510 and 590 nm when RBC concentration in tissue is >0.5%. Blood within vessels much larger than capillaries does not contribute significantly to the spectral features, because virtually all light in this spectral range is absorbed during transit through large vessels (>100 micrometer). This analysis suggests that diffuse reflected light in the 510- to 590-nm region will show spectral features uniquely associated with myoglobin and cytochrome c oxygenation states within 400 micrometer of the surface of the heart in situ as long as the capillary RBC concentration remains <0.5%.

Absorption↗
PubMed1998

Optical simulations of a noninvasive technique for the diagnosis of diseased salivary glands in situ.

A simulation experiment for three-dimensional (3D) imaging of exogenous fluorescinated antibodies that specifically bind to infiltrating lymphocytes in minor salivary glands was carried out. Small (approximately 1 mm3 volume) rhodamine targets, which mimic diseased minor salivary glands labeled with fluorescent antibodies to infiltrating lymphocytes in Sjøgren's syndrome, were embedded in a highly scattering tissue phantom consisting of a thick Delrin disk covered by index matched Delrin slabs of various thickness. In this way the variation of fluorescence profiles on the surface of tissue could be examined corresponding to the range of depths of the salivary glands in vivo. Surface images were obtained for different target depths and radial distances from laser excitation to target fluorophore. These images were analyzed and compared to calculations based on random walk theory in turbid media, using previously determined scattering and absorption coefficients of the Delrin. Excellent agreement between the surface profiles experimentally measured and those predicted by our random walk theory was obtained. Derivation of these theoretical expressions is a necessary step toward devising an inverse algorithm which may have the potential expressions to perform 3D reconstruction of the concentration distribution of fluorescent labels within tissue.

Diagnostic Imaging↗
PubMed1996

Dynamics of temperature dependent optical properties of tissue: dependence on thermally induced alteration.

Thermal damage in heated bovine myocardial tissue is assessed from measured changes in total reflection and transmission of light. Mathematical expressions, based on random walk analysis of light propagation within tissue slabs, are used to relate the diffuse reflection and transmittance to the absorption coefficient, mu a, and effective scattering coefficient, mu's, for samples of myocardial tissue which were subjected to rapid step changes in temperature. Time-dependent changes in mu's, indicate two processes, one with a fast and temperature-dependent rate the other with a slow and apparently temperature-independent rate. For final temperatures above 56.8 degrees C and for the first 500 s after the temperature change, the optical parameters are well fit by exponential forms that exhibit temperature-dependent time constants as predicted by Arrhenius reaction rate theory of thermal damage. The scattering changes are associated with an apparent activation energy, delta E, of 162 kJ/mole and a frequency constant, A, of 3 x 10(23) s-1. This method provides a means for estimating optical coefficients which are needed to assess laser tissue dosimetry.

Absorption↗
PubMed1996

Point spread functions of photons in time-resolved transillumination experiments using simple scaling arguments.

Simple scaling arguments are used to determine spatial resolution achievable in time-resolved transillumination experiments involving highly diffuse media. These arguments allow us to obtain relationships linking target resolution at different planes inside an optically turbid slab to the gating times of the imaging system. We show that this approach yields the same results as those obtained previously from an approximate and rather complicated analytical derivation. In addition, we are now able to assess the effects of scattering anisotropy on spatial resolution attainable when gating times are so short that a constant scaling of photon transport scattering length is not appropriate. These results should enable one to devise more accurate and simpler image reconstruction algorithms.

Algorithms↗
PubMed1994

Resolution limits for optical transillumination of abnormalities deeply embedded in tissues.

Random walk theory is used to calculate the line spread function (LSF) of photons as they cross the midplane of a slab of finite thickness. The relationship between the LSF and the photon transit time in transillumination time-resolved experiments is investigated. It is found that the LSF is approximately Gaussian distributed, with a standard deviation, sigma, which can be used as a criterion of the spatial resolution of the imaging system. Results are substantiated by comparison with actual data in the literature. Any given resolution can be improved by reducing the excess transit time delta t, but heterogeneity of the scattering medium and low levels of detected light enormously complicate the achievement of subcentimeter spatial resolution. The latter point is discussed by using optical parameters of breast tissues for visible and near-infrared radiation (NIR) light.

Breast Diseases↗
PubMed1993

Rheological effects of the presence of hyaluronic acid in the extracellular media of differentiated 3T3-L1 preadipocyte cultures.

The viscoelastic properties of culture medium obtained from confluent 3T3-L1 preadipocytes, after differentiation with isobutyl-methylxanthine and dexamethasone, were studied with a rotational Couette viscometer. In close association with adipocyte differentiation, the culture medium showed gel-like properties, in concert with an increase in viscosity. This behavior vanishes after digestion by Streptomyces hyaluronidase or chondroitinase ABC, but not after application of collagenase, pronase, trypsin, DNase, or neuraminidase, or by treatment with EDTA or mercaptoethanol, indicating that the primary substance responsible for this behavior is hyaluronic acid. The material revealed a non-Newtonian behavior with an irreversible disruption of the network by shear force at high speeds. The viscosity of the medium, containing about 1 microgram/ml of hyaluronic acid, was calculated to be similar to that of a solution containing 1.7 mg high molecular weight hyaluronic acid per milliliter of stock culture medium. The comparison of rheological properties between the culture medium and solutions of hyaluronic acid indicated the possibility of a highly organized network in the culture medium that is more complicated than a simple interaction between homologous hyaluronic acid molecules. The non-Newtonian behavior depends on the hyaluronic acid concentration in the medium as well as on the length of exposure of the 3T3-L1 cells to the isobutyl-methylxanthine/dexamethasone mixture. The results point toward the possibility of interaction between hyaluronic acid and binding proteins.

1-Methyl-3-isobutylxanthine↗