Spatially and spectrally resolved particle swarm optimization for precise optical property estimation using diffuse-reflectance spectroscopy.

This paper presents a new approach to estimate optical properties (absorption and scattering coefficients µa and µs) of biological tissues from spatially-resolved spectroscopy measurements. A Particle Swarm Optimization (PSO)-based algorithm was implemented and firstly modified to deal with spatial and spectral resolutions of the data, and to solve the corresponding inverse problem. Secondly, the optimization was improved by fitting exponential decays to the two best points among all clusters of the "particles" randomly distributed all over the parameter space (µs, µa) of possible solutions. The consequent acceleration of all the groups of particles to the "best" curve leads to significant error decrease in the optical property estimation. The study analyzes the estimated optical property error as a function of the various PSO parameter combinations, and several performance criteria such as the cost-function error and the number of iterations in the algorithms proposed. The final one led to error values between ground truth and estimated values of µs and µa less than 6%.

Spectral features selection and classification for bimodal optical spectroscopy applied to bladder cancer in vivo diagnosis.

This paper describes an experimental study combining spatially resolved autofluorescence (AF) and diffuse reflectance (DR) fibred spectroscopies to discriminate in vivo between healthy and pathological tissues in a preclinical model of bladder cancer. Then, a detailed step-by-step analysis scheme is presented for the extraction and the selection of discriminative spectral features (correlation, linear discriminant, and logistic regression analysis), and for the spectroscopic data final classification algorithms (regularized discriminant analysis and support vector machines). Significant differences between healthy, inflammatory, and tumoral tissues were obtained by selecting a reasonable number of discriminant spectral features from AF, DR, and intrinsic fluorescence spectra, leading to improved sensitivity (87%) and specificity (77%) compared to monomodality (AF or DR alone).

Monte Carlo modeling of multilayer phantoms with multiple fluorophores: simulation algorithm and experimental validation.

This work is first a description of a statistical simulation algorithm developed for simulating the spectral absorption and emission of several fluorophores in an absorbing and diffusing multilayer model. Second, a detailed experimental validation of the simulation program is conducted on two sets of liquid and solid multilayer phantoms, containing one, two, or three fluorophores, within absorbing and scattering media. Experimental spatially resolved reflectance spectra are acquired in the wavelength band 400 to 800 nm and compared to corresponding simulated spectra. The degree of similarity between experimentation and simulation data is quantified. The results obtained underline good correlations with mean errors varying from 2 to 10%, depending on the number of layers and on the complexity of the phantom's composition.

Classification of ultraviolet irradiated mouse skin histological stages by bimodal spectroscopy: multiple excitation autofluorescence and diffuse reflectance.

Histopathological analysis and in vivo optical spectroscopy were used to discriminate several histological stages of UV-irradiated mouse skin. At different times throughout the 30-week irradiation, autofluorescence (AF) and diffuse reflectance (DR) spectra were acquired in a bimodal approach. Then skin was sampled and processed to be classified, according to morphological criteria, into four histological categories: normal, and three types of hyperplasia (compensatory, atypical, and dysplastic). After extracting spectral characteristics, principal component analysis (data reduction) and the k-nearest neighbor classifying method were applied to compare diagnostic performances of monoexcitation AF (based on each of the seven excitation wavelengths: 360, 368, 390, 400, 410, 420, and 430 nm), multiexcitation AF (combining the seven excitation wavelengths), DR, and bimodal spectroscopies. Visible wavelengths are the most sensitive ones to discriminate compensatory from precancerous (atypical and dysplastic) states. Multiexcitation AF provides an average 6-percentage-point increased sensitivity compared to the best scores obtained with monoexcitation AF for all pairs of tissue categories. Bimodality results in a 4-percentage-point increase of specificity when discriminating the three types of hyperplasia. Thus, bimodal spectroscopy appears to be a promising tool to discriminate benign from precancerous stages; clinical investigations should be carried out to confirm these results.

Simultaneous characterization of optical and rheological properties of carotid arteries via bimodal spectroscopy: experimental and simulation results.

This study aimed at identifying potential correlations between rheological and optical properties of carotid artery rings before and after cryopreservation at different mechanical deformations using experimental and simulation results. Therefore, a uniaxial mechanical test bench was coupled to fibered optical spectroscopes measuring 410 nm excited autofluorescence and 650-850 nm elastically backscattered intensity spectra. Furthermore, we developed a statistical simulation program of light transport and fluorescence adapted to our specific experimental configuration. Both spectroscopies gave intensity spectra with higher amplitude for the cryopreserved samples. These observations are to be related to histological modifications affecting the arterial wall of postcryopreserved samples. We also observed significant spectral amplitude variations (increasing autofluorescence intensity and decreasing diffuse reflectance) as a function of the circumferential strains (0%-60%). Due to simulation, we identified values of absorption, diffusion, and anisotropy coefficients, and their variations as a function of state (fresh-cryopreserved), strains (0, 30%, 60%), and wavelengths (700, 740, 780 nm). The media and the adventice are, respectively, less and more absorbing for postcryopreserved rings, and it is the opposite for the fresh ones at higher wavelengths. Absorption and diffusion coefficients are slightly higher, whatever the wavelengths and strains, for the fresh than for the cryopreserved samples.

Mosaicing of bladder endoscopic image sequences: distortion calibration and registration algorithm.

Cancers located on the internal wall of bladders can be detected in image sequences acquired with endoscopes. The clinical diagnosis and follow-up can be facilitated by building a unique panoramic image of the bladder with the images acquired from different viewpoints. This process, called image mosaicing, consists of two steps. In the first step, consecutive images are pairwise registered to find the local transformation matrices linking geometrically consecutive images. In the second step, all images are placed in a common and global coordinate system. In this contribution, a mutual information-based similarity measure and a stochastic gradient optimization method were implemented in the registration process. However, the images have to be preprocessed in order to register the data in a robust way. Thus, a simple correction method of the distortions affecting endoscopic images is presented. After the placement of all images in the global coordinate system, the parameters of the local transformation matrices are all adjusted to improve the visual aspect of the panoramic images. Phantoms are used to evaluate the global mosaicing accuracy and the limits of the registration algorithm. The mean distances between ground truth positions in the mosaiced image range typically in 1-3 pixels. Results given for in vivo patient data illustrate the ability of the algorithm to give coherent panoramic images in the case of bladders.

Experimental comparison between autofluorescence spectra of constrained fresh and cryopreserved arteries.

The study of mechanical properties of the arterial wall is an important step in the comprehension of the vascular physiopathological functioning. However, cryopreserving biological tissues using very low temperatures can induce biological and structural modifications which may involve complications (dilatation, bursting, stenosis) after reimplantation. Many procedures of mechanical tests (traction, dilatation) developed in research allow us to comprehend and analyse rheological behaviour of the arterial wall. The study presented in this article offers a new perspective to detect changes of mechanical properties of cryopreserved arterial samples. In fact, the original idea is to couple a mechanical test bed (uniaxial traction of arterial rings) with spectroscopic measurements (autofluorescence) for the purpose of correlating mechanical modifications and spectral variations. Ultimately, this new approach could lead to develop a device allowing atraumatic and contactless optical examinations of arterial graft to determine its mechanical state before reimplantation.

Rheological properties of healthy and atherosclerotic human arteries.

The aim of this study was to determine the comparative three-dimensional mechanical properties of healthy and atherosclerotic muscular human arteries. Using a previously developed experimental system, in vitro inflation tests were performed on twelve segments of arteries, in static conditions. Two different initial states were used to carry the mechanical study through (large deformation, thick-walled). Main significant differences between healthy and atherosclerotic tested segments are observed for axial traction force whatever the initial state and radial and circumferencial strains referenced to longitudinally pre-stretched state. We showed that strain energy allows to differentiate between both types of arteries only when absolute values of transversal components were considered. Differential values of energy were not discriminating. Our results also show the potential interest of studying arteries in vivo at low transmural pressure.