The Forward Problem in Electroneurography I: A Generalized Volume Conductor Model

We describe a generalized volume conductor model for the compound action potential (CAP) of a peripheral nerve in situ. The extracellular single fiber action potentials (SFAP's), the constituting elements of the CAP, are expressed in terms of the intracellular action potentials and of the effect of volume conduction using a convolution formulation. The model incorporates variations in the intracellular action potential duration over the fiber population. Volume conduction is described in a generalized formalism for a class of cylinder symmetrical configurations. The CAP is finally formulated as a linear sumation of the SFAP's, with incorporation of the distribution of propagaytion velocities over the fiber population. We show that the final expressions for SFAP's and CAP can be given in a mathematically transparent form, which gives a clear insight into the mechanisms involved in the genesis of different potential waveshapes.     

Volume conduction in an anatomically based surface EMG model

A finite-element model to simulate surface electromyography (EMG) in a realistic human upper arm is presented. The model is used to explore the effect of limb geometry on surface-detected muscle fiber action potentials. The model was based on magnetic resonance images of the subject's upper arm and includes both resistive and capacitive material properties. To validate the model geometry, experimental and simulated potentials were compared at different electrode sites during the application of a subthreshold sinusoidal current source to the skin surface. Of the material properties examined, the closest approximation to the experimental data yielded a mean root-mean-square (rms) error of the normalized surface potential of 18% or 27%, depending on the site of the applied source. Surface-detected action potentials simulated using the realistic volume conductor model and an idealized cylindrical model based on the same limb geometry were then compared. Variation in the simulated limb geometry had a considerable effect on action potential shape. However, the rate of decay of the action potential amplitude with increasing distance from the fiber was similar in both models. Inclusion of capacitive material properties resulted in temporal low-pass filtering of the surface action potentials. This effect was most pronounced in the end-effect components of action potentials detected at locations far from the active fiber. It is concluded that accurate modeling of the limb geometry, asymmetry, tissue capacitance and fiber curvature is important when the specific action potential shapes are of interest. However, if the objective is to examine more qualitative features of the surface EMG signal, then an idealized volume conductor model with appropriate tissue thicknesses provides a close approximation.     

薄膜型光纤温度传感器的膜系设计

纳米薄膜与光纤的结合为新型感测提供了各种潜在可能.为了分析温度敏感薄膜的膜系设计及其对光纤温度传感器传感特性的影响,根据光学薄膜理论和光纤传感器的温度感测原理,探讨了光纤温度传感器中敏感薄膜的膜系设计,并构建了薄膜型光纤传感器的温度传感特性模型.以测试系统的参数、性能以及其对干涉光谱的要求为基础,设计了对称性较好的法布...     

A finite element model for describing the effect of muscle shortening on surface EMG

A finite-element model for the generation of single fiber action potentials in a muscle undergoing various degrees of fiber shortening is developed. The muscle is assumed fusiform with muscle fibers following a curvilinear path described by a Gaussian function. Different degrees of fiber shortening are simulated by changing the parameters of the fiber path and maintaining the volume of the muscle constant. The conductivity tensor is adapted to the muscle fiber orientation. In each point of the volume conductor, the conductivity of the muscle tissue in the direction of the fiber is larger than that in the transversal direction. Thus, the conductivity tensor changes point-by-point with fiber shortening, adapting to the fiber paths. An analytical derivation of the conductivity tensor is provided. The volume conductor is then studied with a finite-element approach using the analytically derived conductivity tensor. Representative simulations of single fiber action potentials with the muscle at different degrees of shortening are presented. It is shown that the geometrical changes in the muscle, which imply changes in the conductivity tensor, determine important variations in action potential shape, thus affecting its amplitude and frequency content. The model provides a new tool for interpreting surface EMG signal features with changes in muscle geometry, as it happens during dynamic contractions.     

A Simulation Study of the Ventricular Myocardial Action Potential

A mathematical model based on the formalism of the Hodgkin?Huxley equations was implemented on a microcomputer system and used to simulate the membrane action potential of ventricular myocardial fibers. The complete model is constituted in part from the representation used by Beeler and Reuter [1] and from a simplified version of a model used by us to simulate the Purkinje fiber action potential [3]. The experimental results from the frog ventricular myocardial preparation were reconstructed successfully in the present study. It was also shown that the Purkinje fiber and ventricular myocardial action potentials could be simulated by means of qualitatively similar models. The major differences are a simpler representation of the potassium current and a more prominent role of the calcium current in the representation used for the ventricular myocardial fiber.     

An assessment of variable thickness and fiber orientation of theskeletal muscle layer on electrocardiographic calculations

This paper assesses the effectiveness of including variable thickness and fiber orientation characteristics of the skeletal muscle layer in calculations relating epicardial and torso potentials. A realistic model of a canine torso which includes extensive detail about skeletal muscle layer thickness and fiber orientation is compared with two other uniformly anisotropic models: one of constant thickness and the other of variable thickness. First, transfer coefficients are calculated from the model data. Then torso potentials for each model are calculated from the transfer coefficients and measured epicardial potentials. The comparison of calculated and observed torso potentials indicates that a simple model consisting of a uniformly anisotropic skeletal muscle layer of 1.0-1.5 cm constant thickness significantly improves the model. However, if photographic slices of the canine torso are used to introduce more detailed data about the variation in skeletal muscle thickness and fiber orientation into the model, the agreement and between calculated and measured torso potentials decreased, although a finite element mesh of over 5000 nodes was used to describe the skeletal muscle in the more detailed model. One source of error increase was considered to be due to numerical discretization and could be reduced with a much finer mesh or by utilizing higher order polynomials to represent the potential distribution within each finite element. However, the results presented in this paper show that high precision computation (64-bit word length) on the mainframe IBM 3081 with an attached FPS-164 gives a slow rate of improvement with reduced discretization intervals and that utilizing higher order polynomials within each finite element gives an even slower rate of improvement.(ABSTRACT TRUNCATED AT 250 WORDS)     

Determination of nerve conduction velocity distribution fromsampled compound action potential signals

The sampled compound action potential (CAP) data sequence was expressed as the circular convolution of the delay sequence and the sampled single fiber action potential (SFAP) data sequence. An algorithm, based on Hirose's method (1986) was then developed to separate the delay sequence from the sampled CAP data sequence, and the nerve conduction velocity distribution (NCVD) was consequently calculated from the delay sequence. The NCVD was found to be the product of the amplitude of the SFAP and the number of fibers. Simulations show that the estimated results were in good agreement with the calculated results. Experiments were performed on ten sciatic nerves from five bullfrogs (Rana pipens) using two independent variables: interelectrode distance and stimulus current strength. The results estimated from CAP's recorded under each condition reflect the corresponding feature of NCVD of the condition. The advantage of the technique is to provide detailed information about both slow and fast conducting fibers. This technique also offers the possibility to directly calculate the nerve fiber diameter distribution from the sampled CAP data sequences     

On Modeling the Single Motor Unit Action Potential

A model has been proposed for the generation of single motor unit potentials routinely observed in the clinical EMG examination of the normal biceps brachii muscle. A dipole representation was chosen for the single fiber activity. The motor unit was constructed from a uniform random array of single fibers. Motor unit potentials generated by this array have been observed at various distances both inside and outside the array. The effects of single fiber dipole axial dispersions on the potentials observed at increasing distances from the array have also been investigated. Motor unit potentials generated by the model have been compared with existing data from multielectrode studies in the biceps brachii.     

The importance of anisotropy in modeling ST segment shift insubendocardial ischaemia

In this paper, a simple mathematical model of a slab of cardiac tissue is presented in an attempt to better understand the relationship between subendocardial ischaemia and the resulting epicardial potential distributions. The cardiac tissue is represented by the bidomain model where tissue anisotropy and fiber rotation have been incorporated with a view to predicting the epicardial surface potential distribution. The source of electric potential in this steady-state problem is the difference between plateau potentials in normal and ischaemic tissue, where it is assumed that ischaemic tissue has a lower plateau potential. Simulations with tissue anisotropy and no fiber rotation are also considered. Simulations are performed for various thicknesses of the transition region between normal and ischaemic tissue and for various sizes of the ischaemic region. The simulated epicardial potential distributions, based on an anisotropic model of the cardiac tissue, show that there are large potential gradients above the border of the ischaemic region and that there are dips in the potential distribution above the region of ischaemia. It could be concluded from the simulations that it would be possible to predict the region of subendocardial ischaemia from the epicardial potential distribution, a conclusion contrary to observed experimental data. Possible reasons for this discrepancy are discussed. In the interests of mathematical simplicity, isotropic models of the cardiac tissue are also considered, but results from these simulations predict epicardial potential distributions vastly different from experimental observations. A major conclusion from this work is that tissue anisotropy and fiber rotation must be included to obtain meaningful and realistic epicardial potential distributions.     

A comprehensive analytical model for III-V compound MISFET's

A one-dimensional analytical model for III-V compound deep-depletion-mode MISFET's is developed. The model calculates transconductance, drain resistance, and gate capacitance beyond current saturation where these devices are normally operated-a regime not treated by other MISFET models. It is shown that insulator thicknesses less than 50 nm and surface state densities less than 1 × 10¹²eV^-1. cm^-2will be required for optimum MISFET devices. In a comparison of the expected performance differences between GaAs, InP, and InGaAs FET devices with similar geometries, it is shown that InP and InGaAs MISFET's will have lower gate capacitance, a greater cut-off frequency, and up to 2-dB improvement in minimum noise figure compared with a GaAs MESFET. Device characteristics predicted by this model agree with measured values to an accuracy of ±20 percent, which is well within the accuracy with which the modeled input parameters can be measured. This represents a factor of two improvement in accuracy when compared to other MISFET models. The model predicts the characteristics expected for a MESFET device in the limit of zero insulator thickness.     

Analyzing carbon-fiber composite materials with equivalent-Layer models

The purpose of this paper is to investigate the use of equivalent-layer models for the analysis of carbon-fiber composite materials. In this paper, we present three different models for the electromagnetic characterization (effective material properties) of fiber composites that are commonly used in aircraft and EMC/EMI shielding materials. These three models represent various orders (or levels) of detail in the fiber composite structure and, hence, capture various physical aspects of the composite. These models can be used to efficiently calculate the reflection and transmission coefficients, as well as the shielding effectiveness, of these fiber composites. We compare results of the reflection coefficient and shielding effectiveness obtained from these effective-property models to results obtained from a full numerical solution based on the finite-element (FE) method of the actual periodic fiber composite. We show that, as expected, as more of the geometric detail of the fiber composite is captured with the different models, the upper frequency limit of validity increases.     

The single nerve fiber action potential and the filter bank--a modeling approach

Single fiber action potentials (SFAPs) from peripheral nerves, such as recorded with cuff electrodes, can be modelled as the convolution of a source current and a weight function that describes the recording electrodes and the surrounding medium. It is shown that for cuff electrodes, the weight function is linearly scaled with the action potential (AP) velocity and that it is, therefore, possible to implement a model of the recorded SFAPs based on a wavelet multiresolution technique (filterbank), where the wavelet scale is proportional to the AP velocity. The model resulted in single fiber action potentials matching the results from other models with a goodness of fit exceeding 0.99. This formulation of the SFAP may serve as a basis for model-based wavelet analysis and for advanced cuff design.     

A method to improve the estimation of conduction velocitydistributions over a short segment of nerve

Accurate, noninvasive determination of the distribution of conduction velocities (DCV) among fibers of a peripheral nerve has the potential to improve both clinical diagnoses of pathology and longitudinal studies of the progress of disease or the efficacy of treatments. Current techniques rely on long distances of propagation to increase the amount of temporal dispersion in the compound signals and reduce the relative effect of errors in the forward model. The method described in this paper attempts to reduce errors in DCV estimation through transfer function normalization and, thereby, eliminate the need for long segments of nerve. Compound action potential (CAP) signals are recorded from several, equally spaced electrodes in an array spanning only a 10-cm length of nerve. Relative nerve-to-electrode transfer functions (NETF's) between the nerve and each of the array electrodes are estimated by comparing discrete Fourier transforms of the array signals. NETF's are normalized along the array so that waveform differences can be attributed to the effects of temporal dispersion between recordings, and more accurate DCV estimates can be calculated from the short nerve segment. The method is tested using simulated and real CAP data. DCV estimates are improved for simulated signals. The normalization procedure results in DCV's that qualitatively match those from the literature when used on actual CAP recordings.     

Polarization properties of interferometrically interrogated fiber Bragg grating and tandem-interferometer strain sensors

Lead sensitivity in low-coherence interferometric fiber-optic sensors is a well-known problem. It can lead to a severe degradation in the sensor resolution and accuracy through its effect on the fringe visibility and interferometric phase. These sensitivities have been attributed to birefringence in the various components. In the current work, an analysis of the polarization properties of fiber Bragg grating and tandem-interferometer strain sensors, using Stokes calculus and the Poincare/spl acute/ sphere, is presented. The responses of these sensors as a function of the birefringence properties of the various components under different illuminating conditions are derived. The predicted responses demonstrate very good agreement with experimentally measured responses. These models provide a clear insight into the evolution of the polarization states through the sensor networks. Methods to overcome the lead sensitivity are discussed and demonstrated, which yield a differential strain measurement accuracy of 18 n/spl epsiv//spl middot/rms for a fiber Bragg grating sensor.     

Early Index Growth in Germanosilicate Fiber Upon Exposure to Continuous-Wave Ultraviolet Light

In this paper, high-accuracy measurements of ultraviolet (UV)-induced refractive-index changes (plusmn3times10^-7) in germanosilicate optical fiber as a function of intensity and exposure time are presented. To examine the early growth characteristics of the fiber, samples are irradiated with 244-nm light for 100 s at relatively low intensities (0.007-2.7 W/cm²). The combined growth data is then interpolated to generate a 3-D "index growth surface" of photo-induced index. An empirically derived mathematical expression relates the index growth to the exposure time and intensity. Evidence is presented that, after exposing the fiber at one intensity, additional growth at a different intensity is dictated by the final index change of the first exposure and the intensity of the second exposure. This "compound growth rule" permits the complete calculation of induced-grating structures produced by such a complex exposure history. Using the index-growth surface and the compound-growth rule, the growth and UV erasure of a fiber Bragg grating is successfully predicted using a modified F-matrix algorithm