In vivo measurement of the brain and skull resistivities using an EIT-based method and the combined analysis of SEF/SEP data

Results of "in vivo" measurements of the skull and brain resistivities are presented for six subjects. Results are obtained using two different methods, based on spherical head models. The first method uses the principles of electrical impedance tomography (EIT) to estimate the equivalent electrical resistivities of brain (rhobrain), skull (rhoskull) and skin (rhoskin) according to. The second one estimates the same parameters through a combined analysis of the evoked somatosensory cortical response, recorded simultaneously using magnetoencephalography (MEG) and electroencephalography (EEG). The EIT results, obtained with the same relative skull thickness (0.05) for all subjects, show a wide variation of the ratio rhoskull/rhobrain among subjects (average = 72, SD = 48%). However, the rhoskull/rhobrain ratios of the individual subjects are well reproduced by combined analysis of somatosensory evoked fields (SEF) and somatosensory evoked potentials (SEP). These preliminary results suggest that the rhoskull/rhobrain variations over subjects cannot be disregarded in the EEG inverse problem (IP) when a spherical model is used. The agreement between EIT and SEF/SEP points to the fact that whatever the source of variability, the proposed EIT-based method 相似文献   

A method for localizing EEG sources in realistic head models

A computationally practical method for performing moving dipole calculations to localize EEG sources in realistic, boundary element (integral equation) type of head models is presented. This method makes use of a rapid method of solving the forward problem of calculating the EEG's produced by a dipole in a realistic head model. This rapid forward calculation method allows the use of standard Simplex search techniques to solve the inverse problem of localizing electrical sources in the brain from EEG's measured on the scalp     

Influence of head tissue conductivity in forward and inverse magnetoencephalographic simulations using realistic head models

The influence of head tissue conductivity on magnetoencephalography (MEG) was investigated by comparing the normal component of the magnetic field calculated at 61 detectors and the localization accuracy of realistic head finite element method (FEM) models using dipolar sources and containing altered scalp, skull, cerebrospinal fluid, gray, and white matter conductivities to the results obtained using a FEM realistic head model with the same dipolar sources but containing published baseline conductivity values. In the models containing altered conductivity values, the tissue conductivity values were varied, one at a time, between 10% and 200% of their baseline values, and then varied simultaneously. Although changes in conductivity values for a single tissue layer often altered the calculated magnetic field and source localization accuracy only slightly, varying multiple conductivity layers simultaneously caused significant discrepancies in calculated results. The conductivity of scalp, and to a lesser extent that of white and gray matter, appears especially influential in determining the magnetic field. Comparing the results obtained from models containing the baseline conductivity values to the results obtained using other published conductivity values suggests that inaccuracies can occur depending upon which tissue conductivity values are employed. We show the importance of accurate head tissue conductivities for MEG source localization in human brain, especially for deep dipole sources or when an accuracy greater than 1.4 cm is needed.     

Improved surface laplacian estimates of cortical potential using realistic models of head geometry

Surface Laplacian of scalp EEG can be used to estimate the potential distribution on the cortical surface as an alternative to invasive approaches. However, the accuracy of surface Laplacian estimation depends critically on the geometric shape of the head model. This paper presents a new method for computing the surface Laplacian of scalp potential directly on realistic scalp surfaces in the form of a triangular mesh reconstructed from MRI scans. Unlike previous methods, this algorithm does not resort to any surface fitting proxy and can improve the surface Laplacian estimation of cortical potential patterns by as much as 34% on realistically shaped head models. Simulations and experimental data are presented to demonstrate the advantage of the proposed method over the conventional spherical approximation and the utility of a more accurate surface Laplacian method for estimating cortical potentials from scalp electrodes.     

In vitro and in vivo measurement for biological applications using micromachined probe

We developed a small-sized micromachined probe for the measurement of biological properties using microelectromechanical systems (MEMS) technology. We also experimentally showed the suitability of the micromachined probe for biological applications through in vivo, as well as in vitro measurements of various types of tissue. We measured the permittivities of 0.9% saline and the muscle and fat of pork using the micromachined probe after liquid calibration. The measured permittivities of 0.9% saline and pork agreed well with both the expected values of the Cole-Cole equation along with the measured values obtained through the use of a 1-mm-diameter open-ended coaxial probe. We also performed in vivo measurements of breast cancer tissue implanted in an athymic nude mouse to show the suitability of the small-sized micromachined probe for practical biological applications. Through the obtained data, the capability of the micromachined probe of distinguishing different tissue types from one another was shown. The actual aperture size of the micromachined probe is only 240 /spl mu/m /spl times/ 70 /spl mu/m and, therefore, we can extract the biological information from very small biological tissues and drastically decrease the invasiveness of this method through the implementation of the small probe created through the use of MEMS technology.     

In vivo measurement of tumor conductiveness with the magnetic bioimpedance method

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Reliable prediction of mobile phone performance for realistic in-use conditions using the FDTD method

The objective of this study was to analyze whether advanced simulation platforms provide the effectiveness, accuracy, reliability, and efficiency to predict impairment of mobile-phone RF performance under various usage patterns. The investigation was based on the mechanical CAD data of a commercial phone with two alternative antennas. Three significant hand positions were modeled and evaluated with the device against the SAM head. The results demonstrated high reliability and suitability for providing decision rationale for the design of complex high-end multi-band mobile phones.     

New simple one-end splice loss measurement method using an OTDR for single-mode optical fibre

The letter presents a new simple method using an optical time-domain reflectometer for measuring the real splice loss of single-mode optical fibres from one end of the fibre. Importantly, the method eliminates the difference in backscattering properties of the spliced fibres by subtracting two discontinuities obtained at different wavelengths in the received backscattered power.     

Hierarchical techniques for the development of realistic models of complex computer systems

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Electric potential and current distribution in a rectangular sample of anisotropic material with application to the measurement of the principal resistivities by an extension of van der Pauw's method

A procedure is described for measuring the principal resistivities in anisotropic samples of rectangular shape, the theory involving the calculation of the electric potential and flux distribution in the sample when the electric current enters and leaves respectively at line contact electrodes attached to adjacent corners of the sample.It was previously shown that van der Pauw's method of measuring electrical resistivity in flat samples of arbitrary shape gives the geometric mean of the principal components of the resistivity tensor in the plane of the sample when applied to anisotropic material. In this paper a method of determining the ratio of the principal components in a sample of rectangular shape is described, and from the two measurements the individual values of the principal components may be obtained.     

In vivo measurement of the oxygen saturation of retinal vessels in healthy volunteers

A new method for the spatially resolved measurement of the oxygen saturation of retinal vessels is described. Imaging spectrometry was used for both measurements of transmission and reflectance spectra of whole blood in cuvettes as well as for fundus reflectance spectra. A model was developed for the calculation of the oxygen saturation, valid in the wavelength range between 510 nm and 586 nm, in that the internal reflectance is constant and only the transmitted light depends on layer thickness and hematocrit. Altogether 265 measurements were performed in different number at 30 eyes. In each measurement, the oxygen saturation was simultaneously determined for 193 locations along a line of 1.5 mm at the fundus. The mean oxygen saturation in retinal arteries was (92.2 +/- 4.1)% and (57.9 +/- 9.9)% in retinal veins. The mean retinal arterio-venous difference of the oxygen saturation was (35.1 +/- 9.5)%. The venous oxygen saturation depended on distance from the optic disc. The measured mean of the arterio-venous difference of the oxygen saturation corresponded well to the value of the brain (34%). The utilization of oxygen in the temporal quadrants (inferior: 39.4 +/- 10.4%) is significantly (p = 0.05) higher than in the nasal quadrants (inferior: 31.3 +/- 6.7%).     

Simulation method of reflectance measurement error using the OTDR

This letter presents an algorithm relating two different optical time-domain reflectometer (OTDR) reflectance measurement methods. From the impulse response of an OTDR, and the attenuation and reflectance values of a reflective event supplied by the OTDR operating according to a first classical implemented method, the algorithm allows the determination of the reflectance that would be measured according to a second more accurate method. Over a 10-dB comparison range the deviations between experimental results and those obtained with the proposed algorithm do not exceed 0.5 dB     

The construction of a simulation-based system for the developmentof powerful and realistic models and practicals for healthcareprofessionals

The aim of this paper is to examine the importance of computer-based simulation by the demonstration and study of complex systems and the presentation of essential tools and applications that can help health professionals deliver good quality practicals, which is now impeded by cost and/or technical constrains. The tools that have been developed in the framework of the Courseware Authoring for Scientific Training (COAST) project are the “modeler environment”, which is used to describe the different tools and mathematical functions available for building models, and the “simulation author environment”, which is used for building simulation sequences and providing the required tools and functions. This effort provides scientists with new technological and cost-effective means, specifically based on multimedia simulation, for preparing educational material, so as to gradually replace laboratory practicals that are gradually becoming more expensive, and improves student's understanding of complex systems     

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Annular phase arrays (APAs) of aperture and dipole antennas used for hyperthermia are simulated in three dimensions by using the finite-difference time-domain (FDTD) method. A 17363 cell, 1.31 cm resolution, anatomically based model of the human torso surrounded by a bolus of deionized water is used for calculations of specific absorption rates (SARs). Test runs on the calculation of fields in the water-filled interaction space and with homogeneous circular- and elliptical-cylinder phantoms correlate well with the experimental data in the literature, lending support to the accuracy of the FDTD method for near-field exposure conditions. Results are given for APAs using different sizes of aperture and dipole antennas and for a subannular array to obtain higher SARs in the liver. It is concluded that, because of its flexibility, the proposed procedure may be useful for a variety of realistic radiofrequency applicators for hyperthermia and other biomedical applications     

In vivo quantification of a homogeneous brain deformation model for updating preoperative images during surgery

Clinicians using image-guidance for neurosurgical procedures have recently recognized that intraoperative deformation from surgical loading can compromise the accuracy of patient registration in the operating room. While whole brain intraoperative imaging is conceptually appealing it presents significant practical limitations. Alternatively, a promising approach may be to combine incomplete intraoperatively acquired data with a computational model of brain deformation to update high resolution preoperative images during surgery. The success of such an approach is critically dependent on identifying a valid model of brain deformation physics. Towards this end, we evaluate a three-dimensional finite element consolidation theory model for predicting brain deformation in vivo through a series of controlled repeat-experiments. This database is used to construct an interstitial pressure boundary condition calibration curve which is prospectively tested in a fourth validation experiment. The computational model is found to recover 75%-85% of brain motion occurring under loads comparable to clinical conditions. Additionally, the updating of preoperative images using the model calculations is presented and demonstrates that model-updated image-guided neurosurgery may be a viable option for addressing registration errors related to intraoperative tissue motion.     

Three stochastic measurement schemes for direction-of-arrival estimation using compressed sensing method

We focus on the design of the measurement schemes in the compressed sensing (CS) method for direction-of-arrival estimation, and three stochastic measurement schemes are considered. In the perspectives of average apertures and incoherences, we give a detailed mathematical analysis for these schemes. The superiorities in incoherences for these schemes are illustrated, compared with the popularly used random Gaussian measurement scheme. Then we demonstrate that the newly used Poisson disk sampling scheme and uniform jittered sampling scheme can obtain relatively large average apertures by using a proposed computational method. Finally, several simulations are implemented to evaluate the performances of the CS methods with these stochastic measurement schemes.     

In vivo and real-time measurement of magnetic nanoparticles distribution in animals by scanning SQUID biosusceptometry for biomedicine study

Magnetic nanoparticles have been widely applied to biomagnetism, such as drug deliver, magnetic labeling, and contrast agent for in vivo image, etc. To localize the distribution of these magnetic particles in living organism is the first important issue to confirm the effects of magnetic nanoparticles and also evaluate the possible untoward effects. In this study, a scanning high T(c) rf-SQUID superconducting quantum interference devices (SQUIDs) biosusceptometry, composed of static SQUID unit and scanning coil sets, is developed for biomedicine study with the advantages of easy operation and unshielded environment. The characteristics tests showed that the system had the low noise of 8 pT/Hz at 400 Hz and the high sensitivity with the minimum detectable magnetization around 4.5 × 10(-3) EMU at distance of 13 mm. A magnetic nanoparticle detection test, performed by ex vivo scanning of the magnetic fluids filled capillary under swine skin for simulation of blood vessels in living bodies, confirmed that the system is feasible for dynamic tracking of magnetic nanoparticles. Based on this result, we performed further studies in rats to clarify the dynamic distribution of magnetic nanoparticle in living organism for the pharmacokinetics analysis like drug delivers, and propose the possible physiological metabolism of intravenous magnetic nanoparticles.     

球面补偿法测量透镜焦距研究

基于球面补偿原理和自参考单板剪切干涉技术,提出了一种测量焦距的新方法。该方法装置简单,测量方便,精度高,信息丰富,不仅可用于焦距测量,而且还可给出光学球面的曲率半径等参数。文中详细分析了测量原理,并给出了不同透镜的测量结果。     

Body effects on SAR distributions for microwave exposures in a realistic model of the human head.

The finite-difference time-domain (FDTD) simulations of specific absorption rate (SAR) in a human head exposed to microwaves have, to date, been carried out on models of the head only. This was because it was believed that the body effects on the average SAR of the head could be ignored at high frequencies of around 1 GHz. That opinion, however, was based on inappropriate calculation conditions and is therefore unreliable. In this paper, we have re-examined the body effects on the SAR distributions in a realistic homogeneous model of the adult head exposed to microwaves. We found that the SAR on the eye surface of the head-only model exposed to E-polarized waves was 31% smaller than that of the whole-body model at 900 MHz, and 43% larger at 1.5 GHz. For a size that can practically be considered whole-body, it is necessary to have the top of the head to the belly for 900 MHz and to the chest area for 1.5 GHz. The previously unclear body effects of H-polarized waves were assumed to be less than those of E-polarized waves, suggesting that the chest area would be sufficient for both frequencies.