Measurements of the optical properties of tissue in conjunction with photodynamic therapy

A simple optical dosimeter was used to measure the light intensity in rat liver and muscle in vivo with fibers positioned at different depths to investigate whether the light penetration changed during photodynamic therapy (PDT). The results were then correlated with measurements of the three optical-interaction coefficients μ(s), μ(a) and g for wavelengths in the range 500-800 nm for PDT-treated and nontreated rat liver and muscle tissue in vitro. Adistinct increase in the absorption coefficient was seen immediately after treatment, in agreement with the decreasing light intensity observed during the treatment, as measured with the optical dosimeter. The collimated transmittance was measured with a narrow-beam setup, and an optical integrating sphere was used to measure the diffuse reflectance and total transmittance of the samples. The corresponding optical properties were obtained by spline interpolation of Monte Carlo-simulated data. To ensure that the measured values were correct, we performed calibration easurements with suspensions of polystyrene microspheres and ink.     

Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress

We report on a technique utilizing time-resolved detection of laser-induced stress transients for the measurement of optical properties in turbid media specifically suitable for biological tissues. The method was tested initially in nonscattering absorbing media so that it could be compared with spectrophotometry. The basis of this method is provided by the conditions of temporal stress confinement in the irradiated volume where the pressure generated in tissues heated instantly by laser pulses is proportional to the absorbed laser energy density, and the exponential profile of the initial stress distribution in the irradiated volume corresponds to the z-axial distribution of the absorbed laser fluence. Planar thermoelastic waves can propagate in water-containing media with minimal distortion, and their axial profiles can be detected by an acoustic transducer with sufficient temporal resolution. The acoustic waves induced by 14-ns laser pulses in nonscattering media, turbid gels, and tissues were measured by a piezoelectric transducer with a 3-ns response time. Temporal profiles of stress transients yielded z-axial distributions of the absorbed laser energy in turbid and opaque media, provided that the speed of sound in these media was known. The absorption and effective scattering coefficients of beef liver, dog prostate, and human aortic atheroma at three wavelengths, 1064 nm (in near infrared), 532 nm (visible), and 355 nm (near UV), were deduced from laser-induced stress profiles with additional measurements of total diffuse reflectance.     

Precision of measurement of tissue optical properties with optical coherence tomography

Accurate and noninvasive measurement of tissue optical properties can be used for biomedical diagnostics and monitoring of tissue analytes. Noninvasive measurement of tissue optical properties (total attenuation and scattering coefficients, optical thickness, etc.) can be performed with the optical coherence tomography (OCT) technique. However, speckle noise substantially deteriorates the accuracy of the measurements with this technique. We studied suppression of speckle noise for accurate measurement of backscattering signal and scattering coefficient with the OCT technique. Our results demonstrate that the precision of measurement of backscattering signals with the OCT technique can be 0.2% for homogeneously scattering media and 0.7% for skin, if spatial averaging of speckle noise is applied. This averaging allows us to achieve the precision of tissue scattering coefficient measurements of approximately +/-0.8%. This precision can be further improved by a factor of 2-3, upon optimization of OCT operating parameters.     

Ultrafast laser-induced microstructure/nanostructure replication and optical properties

This paper demonstrates replication of ultrafast laser-induced micro/nano surface textures on poly(dimethylsiloxane) (PDMS). The surface texture replication process reduces the processing steps for microtexturing while improving light trapping. Two methods are demonstrated to replicate surface microtexture, a simple mold method and an embossing method. The laser microtextured silicon and titanium surfaces with micro to nanoscale features have been successfully replicated. Optical characterization of the replicated microtextured PDMS surfaces is performed and the results agree with model predictions. The replicated microtextured PDMS film is applied on a silicon surface and optical characterization shows that surface reflectance can be suppressed over 55% compared to the control value.     

Calculation of optical thicknesses of magnesium emission spectral lines for diagnostics of laser-induced plasmas

In this work, plasma characterization by laser-induced breakdown spectroscopy (LIBS) has been investigated. We propose a method based on the calculation of the optical thicknesses of emission spectral lines in the framework of a homogeneous optically thick plasma in local thermodynamic equilibrium (LTE). In this approach, self-absorption is taken into account to retrieve the optically thin intensities and plasma characterization is achieved. The developed procedure is applied to magnesium (Mg) lines measured from plasmas generated in air at atmospheric pressure from calcium hydroxide samples using an infrared Nd:YAG laser. The influence of laser irradiance on both plasma shape and emission intensity was studied to select the most suitable experimental conditions. Spectral lines of Mg I-II were measured and analyzed for different laser energies, delay times, and concentrations of the analyte. In each case, the plasma temperature, the electron density, and the parameters Nl were determined, without employing curves-of-growth. The results obtained showed the practical usefulness of the method to provide valuable information in LIBS experiments.     

Effect of source spectral shape on task-based assessment of detection and resolution in optical coherence tomography

We demonstrate the effect of the spectral shape of broadband light sources in a task-based approach for assessment of signal detection and resolution in optical coherence tomography. We define two binary tasks: The signal is either present or absent and the signal can be either resolved or not. In a transparent sample bounded by two uniform interfaces we study the minimum detectable change in the index of refraction as well as the minimum resolvable distance between the layers in correlation with the source spectral shape and power. Results show that the area under the receiver operating curve (AUC) for a signal-detection task is not affected by the shape of the spectrum but solely by its optical power, whereas spectral shaping has an effect, which we quantify, on the AUC for the resolution task. Moreover, the AUC is demonstrated in relation to the concept of system sensitivity for a signal-detection task.     

Measurement of tissue optical properties by the use of oblique-incidence optical fiber reflectometry

Fiber-optic-based oblique-incidence reflectometry is a simple and accurate method for measuring the absorption and reduced scattering coefficients mu(a) and mu?(s) of semi-infinite turbid media. Obliquely incident light produces a spatial distribution of diffuse reflectance that is not centered about the point of light entry. The amount of shift in the center of diffuse reflectance is directly related to the medium's diffusion length D. We developed a fiber-optic probe to deliver light obliquely and sample the relative profile of diffuse reflectance. Measurement in absolute units is not necessary. From the profile, it was possible to measure D, perform a curve fit for the effective attenuation coefficient mu(eff), and then calculate mu(a) and mu?(s). This method was verified with Monte Carlo simulations and tested on tissue phantoms. Our measurements of D and mu(eff) had an accuracy of approximately 5%, thus giving us 10% and 5% accuracy for mu(a) and mu?(s), respectively.     

Acoustic characterization of microbubble dynamics in laser-induced optical breakdown

A real-time acoustic technique to characterize microbubbles produced by laser-induced optical breakdown (LIOB) in water was developed. Femtosecond laser pulses are focused just inside the surface of a small liquid tank. A tightly focused, high frequency, single-element ultrasonic transducer is positioned so its focus coincides axially and laterally with this laser focus. When optical breakdown occurs, a bubble forms and a pressure wave is emitted (i.e., acoustic emission). In addition to this acoustic signal, the microbubble is actively probed with pulse-echo measurements from the same transducer. After the bubble forms, received pulse-echo signals have an extra pulse, describing the bubble location and providing a measure of axial bubble size. Wavefield plots of successive recordings illustrate the generation, growth, and collapse of cavitation bubbles due to optical breakdown. These same plots also can be used to quantify LIOB thresholds.     

Detection of trace Al in model biological tissue with laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS), which is an excellent tool for trace elemental analysis, was studied as a method of detecting sub-part-per-10(6) (ppm) concentrations of aluminum in surrogates of human tissue. Tissue was modeled using a 2% agarose gelatin doped with an Al(2)O(3) nanoparticle suspension. A calibration curve created with standard reference samples of known Al concentrations was used to determine the limit of detection, which was less than 1 ppm. Rates of false negative and false positive detection results for a much more realistic sampling methodology were also studied, suggesting that LIBS could be a candidate for the real-time in vivo detection of metal contamination in human soft tissue.     

Design of a wavelength-division multiplexing bandpass with quasi-chebyshev spectral shape

By proper selection of the radiant reflectance of the reflectors that are interleaved between the half-wave thickness spacers it is possible to design an all-dielectric bandpass for wavelength-division multiplexing. Its passband spectral shape approximates a Chebyshev polynomial.     

Wavelength tuning and spectral properties of distributed feedback diode lasers with a short external optical cavity

We report on the wavelength tuning and spectral properties of distributed feedback (DFB) diode lasers operated with a plane external cavity (XC) mirror positioned as close as possible to the diode-laser front facet. These lasers generate single-frequency near IR radiation at wavelengths of 1392, 1580, 1602, and 1653 nm. A piezoelectric variation of the XC length provided continuous single-frequency tuning to as high as 19 GHz. A further benefit of XC DFB lasers is a residual amplitude modulation per gigahertz tuning of less than 10(-3). The XC feedback also suppresses residual side-mode oscillations to less than 60 dB. The laser's total intensity noise is close to the shot noise limit. The laser linewidth (measured in a beat note experiment) is less than 90 kHz within an acquisition time of 40 ms. The advantageous properties of XC DFB lasers for molecular spectroscopy are demonstrated by recording R(3) 2nu(3) overtone spectra of methane by single-scan single-pass absorption or frequency-modulation spectroscopy.     

Multiscale analysis of the laser-induced damage threshold in optical coatings

We have investigated the influence of laser beam size on laser-induced damage threshold (LIDT) in the case of single- and multiple-shot irradiation. The study was performed on hafnia thin films deposited with various technologies (evaporation, sputtering, with or without ion assistance). LIDT measurements were carried out at 1064 nm and 12 ns with a spot size ranging from a few tens to a few hundreds of micrometers, in 1-on-1 and R-on-1 modes. These measurements were compared with simulations obtained with the statistical theory of laser-induced damage caused by initiating inclusions. We show how to obtain information on the initiating defect properties and the related physical damage mechanisms with a multiscale study. Under certain conditions, it is possible with this method to discriminate different defects, estimate their densities, and follow the evolution of the defects under multiple irradiation. The different metrology implications of our approach, particularly for obtaining a functional LIDT of optical components are discussed.     

Modelling optical properties of soft tissue by fractal distribution of scatterers

Abstract

A knowledge of the local refractive index variations and size distribution of scatterers in biological tissue is required to understand the physical processes involved in light-tissue interaction. This paper describes a method for modelling the complicated soft tissue, based on the fractal approach, permitting numerical evaluation of the phase functions and four optical properties of tissue—scattering coefficient, reduced scattering coefficient, backscatter-ing coefficient, and anisotropy factor—by the use of the Mie scattering theory. A key assumption of the model is that refractive index variations caused by microscopic tissue elements can be treated as particles with size distribution according to the power law. The model parameters, such as refractive index, incident wavelength, and fractal dimension, that are likely to affect the predictions of optical properties are investigated. The results suggest that the fractal dimension used to describe how biological tissue can be approximated by particle distribution is highly dependent on how the continuous distribution is discretized. The optical properties of the tissue significantly depend on the refractive index of tissue, implying that the refractive index of the particles should be carefully chosen in the model in order accurately to predict the optical properties of the tissue concerned.     

Effect of genipin crosslinking on the optical spectral properties and structures of collagen hydrogels

Genipin, a natural cross-linking reagent extracted from the fruits of Gardenia jasminoides, can be effectively employed in tissue engineering applications due to its low cytotoxicity and high biocompatibility. The cross-linking of collagen hydrogels with genipin was followed with one-photon fluorescence spectroscopy, second harmonic generation, fluorescence and transmission electron microscopy. The incubation with genipin induced strong auto-fluorescence within the collagen hydrogels. The fluorescence emission maximum of the fluorescent adducts formed by genipin exhibit a strong dependence on the excitation wavelength. The emission maximum is at 630 nm when we excite the cross-linked samples with 590 nm light and shifts to 462 nm when we use 400 nm light instead. The fluorescence imaging studies show that genipin induces formation of long aggregated fluorescent strands throughout the depth of samples. The second harmonic generation (SHG) imaging studies suggest that genipin partially disaggregates 10 μm "fiberlike" collagen structures because of the formation of these fluorescent cross-links. Transmission electron microscopy (TEM) studies reveal that genipin largely eliminates collagen's characteristic native fibrillar striations. Our study is the first one to nondestructively follow and identify the structure within collagen hydrogels in situ and to sample structures formed on both micro- and nanoscales. Our findings suggest that genipin cross-linking of collagen follows a complex mechanism and this compound modifies the structure within the collagen hydrogels in both micro- and nanoscale.     

Clinical determination of tissue optical properties by endoscopic spatially resolved reflectometry

A noninvasive method to measure the optical properties of a diffusing and absorbing medium is described. Based on the spatially resolved measurement of diffuse reflectance at the sample surface, this method is particularly suitable for investigating the in vivo optical properties of biological tissues endoscopically in a clinical context. The sensitivity of the measurement is discussed, and two optical probes for two different clinical applications are presented. Preliminary measurements are performed on a nonbiological medium, which illustrate the possibilities of the proposed method. Finally, we report on in vivo measurements of the optical properties of the human esophageal wall at 630 nm.     

Influence of nanowire density on the shape and optical properties of ternary InGaAs nanowires

We have synthesized ternary InGaAs nanowires on (111)B GaAs surfaces by metal-organic chemical vapor deposition. Au colloidal nanoparticles were employed to catalyze nanowire growth. We observed the strong influence of nanowire density on nanowire height, tapering, and base shape specific to the nanowires with high In composition. This dependency was attributed to the large difference of diffusion length on (111)B surfaces between In and Ga reaction species, with In being the more mobile species. Energy dispersive X-ray spectroscopy analysis together with high-resolution electron microscopy study of individual InGaAs nanowires shows large In/Ga compositional variation along the nanowire supporting the present diffusion model. Photoluminescence spectra exhibit a red shift with decreasing nanowire density due to the higher degree of In incorporation in more sparsely distributed InGaAs nanowires.     

Diffuse optical tomography with spectral constraints and wavelength optimization

We present an algorithm that explicitly utilizes the wavelength dependence of tissue optical properties for diffuse optical tomography. We have previously shown that the method gives superior separation of absorption and scattering. Here the technique is described and tested in detail, and optimum wavelength sets for a broad range of chromophore combinations are discovered and analyzed.     

Changes in optical properties of rat cerebral cortical slices during oxygen glucose deprivation

We simultaneously measured the diffuse reflectance spectra and transmittance spectra of in vitro rat cerebral cortical tissue slices perfused with artificial cerebrospinal fluid (aCSF) in the wavelength range from 500 to 900 nm. An ischemia-like condition in the cortical tissue was induced by oxygen/glucose deprivation (OGD) of the aCSF. Diffuse reflectance and transmittance of the cortical slices were decreased and increased, respectively, during OGD. Spectral data of reduced scattering coefficients and absorption coefficients were estimated by the inverse Monte Carlo simulation for light transport in tissue. As with OGD, significant decrease of the reduced scattering coefficients and alteration of the absorption coefficient spectrum were observed over the measured wavelength range. The mean maximum amplitudes of change in the absorption coefficient at 520, 550, 605, and 830 nm were 0.33 ± 0.14, 0.30 ± 0.12, 0.30 ± 0.14, and -0.04 ± 0.16, respectively, whereas those in the reduced scattering coefficient at 520, 550, 605, and 830 nm were -0.37 ± 0.08, -0.38 ± 0.08, -0.38 ± 0.08, and -0.39 ± 0.08. Variations in the reduced scattering coefficients implied cell deformation mainly due to cell swelling, whereas those in the absorption spectra indicated reductions in heme aa(3) and CuA in cytochrome c oxidase and cytochrome c.