Sigrid Avrillier

Phase function simulation in tissue phantoms: a fractal approach

Artificial media are needed for the calibration of optical diagnostic methods in order to work on reproducible, stable and well known samples. Since the scattering and absorption coefficients can easily be adjusted by using appropriate concentrations of scattering and absorbing components, the most difficult part in the design of a good tissue phantom is to obtain an actual phase function. The most common way to create phantoms is to use scattering microspheres of equal size, but the Mie phase function of such a phantom does not match the tissues real phase function. Moreover, we show in this paper that the similarity relations often used for the analysis of the results obtained with this type of phantom may sometimes be very inaccurate. The use of a mixture of different sized scattering particles is then considered, in order to imitate the whole phase function. However, as the determination of adequate sizes and concentrations is a difficult mathematical task, we describe a simple method to solve this problem. We first demonstrate that the extreme optical complexity of real biological samples could be simulated by a mixture of spheres with a fractal diameter distribution. Then, we present a few simple rules based on the knowledge of this fractal distribution, which can be used to obtain a realistic phase function with a limited number of sphere diameters.

Laser Induced Autofluorescence Diagnosis of Bladder Tumors: Dependence on the Excitation Wavelength

PURPOSE We assessed the ability of laser induced autofluorescence spectroscopy to distinguish neoplastic urothelial bladder lesions from normal or nonspecific inflammatory mucosa. MATERIALS AND METHODS Three different pulsed laser excitation wavelengths were used successively: 308 nm. (xenium chloride excimer laser), 337 nm. (nitrogen laser) and 480 nm. (coumarin dye laser). The excitation light was delivered by a specially devised multifiber catheter connected to a 1 mm. core diameter silica monofiber introduced through the working channel of a standard cystoscope with saline irrigation. The captured fluorescence light was focused onto an optical multichannel analyzer detection system. Device performance was evaluated in 25 patients after obtaining consent and immediately before transurethral resection of a bladder tumor. Spectroscopic results were compared with histological findings. RESULTS At 337 and 480 nm. excitation wavelengths the overall fluorescence intensity of bladder tumors was clearly decreased compared to normal urothelial mucosa regardless of tumor stage and grade. At the 308 nm. excitation wavelength the shape of the tumor spectra, including carcinoma in situ, was markedly different from that of normal or nonspecific inflammatory mucosa. No absolute intensity determinations were required in this situation, since a definite diagnosis could be established based on the fluorescence intensity ratio at 360 and 440 nm. CONCLUSIONS This spectroscopic study could be particularly useful to design a simplified autofluorescence imaging device for detection of occult urothelial neoplasms.

FAST SEMIANALYTICAL MONTE CARLO SIMULATION FOR TIME-RESOLVED LIGHT PROPAGATION IN TURBID MEDIA

The statistical estimator concept, created in the nuclear engineering field, has been adapted to the elaboration of a new and fast semianalytical Monte Carlo numerical simulation for time-resolved light-scattering problems. This concept has also been generalized to the case of unmatched boundaries. The model, discussed in detail in this paper, contains two stages. The first stage is the information generator in which, for each scattering event, the contribution to the total reflectance and transmittance is evaluated and subtracted from the photon current energy. This procedure reduces the number of photons required to produce a given accuracy, which makes it possible to store all event positions and energies. In the second stage, called the information processor, the results of the first stage are used to calculate analytically any desired result. Examples are given for scattering slabs of isotropic or anisotropic scatterers when collimated-beam incidence is used. Reflections at the boundaries are taken into account. The results obtained either with this new method or with classical Monte Carlo methods are very similar. However, the convergence of our new model is much better and, because of the separation into two stages, any quantity related to the problem can be easily calculated afterward without recomputing the simulation.

Influence of the emission-reception geometry in laser-induced fluorescence spectra from turbid media.

Routine clinical detection of precancerous lesions by laser-inducedautofluorescence was recently demonstrated in several medicalfields. This technique is based on the analysis of complex spectrawith overlapping broad structures. However, in biological tissues, scattering and absorption are wavelength dependent, and the observedfluorescence signals are distorted when the illumination and detectiongeometry varies, making comparison of results from different groupsdifficult. We study this phenomenon experimentally in human tissuein a simple experiment: A fiber is used for the excitation and anidentical fiber is used for reception of the signal; both fibers aremaintained in contact with the tissue. We study the distortion ofthe spectra as a function of the distance between the twofibers. For correction of the spectra we show that it is possibleto use a fast and accurate ab initio Monte Carlo simulationwhen the spectral variations of the optical properties of the mediumare known. The main advantage of this simulation is itsapplicability even for complex boundary conditions or when the sampleconsists of several layers.

Real-space Green’s function calculation for the solution of the diffusion equation in stratified turbid media

We have derived the space-time Greens function for the diffusion equation in layered turbid media, starting from the case of a planar interface between two random scattering media. This new approach for working directly in real space permits highly efficient numerical processing, which is a decisive criterion for the feasibility of the inverse problem in biomedical optics. The results obtained by this method in the case of a two-layered medium are compared with Monte Carlo simulations.

Stimulation in the rat of a nerve fiber bundle by a short UV pulse from an excimer laser.

A bundle of central nervous fibers was excited in the rat with a short pulse (40 ns) of UV light produced by an excimer laser. Evoked responses were recorded in the thalamic ventralis posterior nucleus after stimulation of the medial lemniscus or the cuneate bundle in the spinal cord. The effects of electrical and optical fiber applied UV stimulation were compared in both cases. At threshold, the latency for the UV light stimulation was slightly longer than for electrical stimulation. The excitation threshold was 0.9 J/cm2, very close to the UV photoablation threshold (order of 1 J/cm2). The intermediary events mediating the light excitation are discussed.

The isometric force that induces maximal surface muscle deoxygenation.

Abstract To determine the external force that induces maximal deoxygenation of brachioradialis muscle 32 trained male subjects maintained isometric contractions using the elbow flexor muscles up to the limit time (isotonic part of the isometric contraction, IIC) and beyond that time for 120 s (anisotonic part of the isometric contraction). During IIC each subject maintained relative forces of either 25% and 70% maximal voluntary contraction (MVC), 50% and 100% MVC, or 40% and 60% MVC. Muscle oxygenation was assessed using a near infrared spectroscope, and expressed as a percentage of the reference value (ΔO2rest) which was the difference between the minimal oxygenation obtained after 6 min of ischaemia at rest and the maximal reoxygenation following the release of the tourniquet. During IIC at 25% MVC, muscle oxygenation decreased to 17 (SEM 3)% ΔO2rest, then it levelled off [25 (SEM 1)% ΔO2rest]. After the point at which target force could not be maintained, reoxygenation was very weak. During IIC at 40%, 50%, 60%, and 70% MVC, the lowest muscle oxygenation values were obtained after 15–20 s of contraction and corresponded to −18 (SEM 6), −59 (SEM 12) −31 (SEM 6), and −29 (SEM 6)% ΔO2rest, respectively. For the contraction at 100% MVC, the lowest oxygenation [−19 (SEM 9)% ΔO2rest] was obtained while force was decreasing (69% MVC). During the anisotonic part of the isometric contractions, the greatest reoxygenation rate was obtained after 50% MVC IIC (P < 0.001). Our results showed that during isometric elbow flexions between 25% and 100% MVC, there was no linear relationship between external force and muscle oxygenation, and that the maximal deoxygenation of the brachioradialis muscle was obtained at 50% MVC.

Coupling losses in laser resonators containing a hollow rectangular dielectric waveguide

This paper discusses the losses in coupling radiation from the guide into free space and back into the guide for a hollow rectangular dielectric waveguide laser resonator with external spherical mirrors. Computations of the coupling loss for the EH11 lowest‐order mode of a square guide, as a function of mirror curvature and position, are presented in the limit of large mirror aperture. It is shown that some mirror positions and radii provide low coupling losses.

Time-resolved diffusing wave spectroscopy beyond 300 transport mean free paths

We presented theoretical and experimental demonstrations of the possibilities of performing time-resolved diffusing wave spectroscopy: We successfully registered field fluctuations for selected photon path lengths that can surpass 300 transport mean free paths. Such performance opens new possibilities for biomedical optics applications.

Cerebral hemodynamics and brain oxygen changes related to gravity-induced loss of consciousness in rhesus monkeys

The aim of the present experiment was to study the influence of +Gz acceleration (head-to-foot inertial forces) onset on cerebral oxygenation changes (cerebral oxy- and deoxy-hemoglobin) and cerebral blood volume (CBV) in order to evaluate the role of cerebral hypoxemia and ischemia in the appearance of +Gz-induced loss of consciousness (G-LOC). We used five rhesus monkeys which were equipped with near infrared spectroscopy optodes fixed onto the parietooccipital cranial bone. G-LOC (isoelectric electrocorticogram) was detected with silver balls electrodes in contact with the dura matter. The animals were centrifuged up to +12 Gz with two onset rates (0.1 and 3 G/s). Cerebral deoxy-hemoglobin increased significantly (max: +30 +/- 6% of control, P < 0.01) only during the 0.1 G/s run. At G-LOC, CBV changes were not related to G-onset rate (P = 0.30; mean change: -32 +/- 6% of control). We conclude that cerebral ischemia is the main mechanism in the occurrence of G-LOC.