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This paper presents a new, simple model to evaluate the instrumental random errors in kinematic analysis of human movements using stereophotogrammetry. By means of equations analogous to that relate linear or angular momentum with linear or angular velocities, a direct measurement of instantaneous motion can be made without previous finite displacement analysis. Single explicit expressions can be obtained to evaluate the influence of instrumental random errors in the accuracy of the kinematic variables. From these expressions, some conclusions about the effect of marker cluster design on the experimental errors are obtained. An experiment has been carried out in order to validate the proposed technique and to assess the experimental errors in linear and angular velocity measurement and its influence in instantaneous helical axis determination. [ABSTRACT FROM AUTHOR]

This paper considers the magnetic field arising from action currents associated with the propagation of an action potential along a cylindrical fibre. The equivalent current sources of the magnetic field are precisely the difference in axial current density on each side of the membrane. Fibre bundles are considered by superposition. The total magnetic field varies as 0(1/R) hence it is quadrupolar. The equivalent source is more strongly related to the discontinuity in conductivity between the membrane andsurrounding media than directly to the ionic (primary) currents. [ABSTRACT FROM AUTHOR]

Neural source localization using electroencephalographic data is usually performed using either dipolar models or minimum norm based techniques. While the former demands a priori information about the number of active sources and is particularly suitable for generators, which occupy small pieces of cortical tissue, the major drawbacks of the second approach are its dependence on the uncorrelated noise, and its tendency to localize the sources at the surface. In this paper, a simple mathematical procedure, based on the behavior of the dispersion of the minimum norm solutions, is introduced, in order to estimate the depth of the sources. The correct position of the active generators is obtained using successively deeper surfaces instead of the application of a regularization matrix, as is commonly described in the bibliography. The evaluation of this technique is performed using single and double dipolar simulated generators and two different models for the head: spherical and realistic. The results yield a mean accuracy of about 10 mm for the most disadvantageous situations studied and thus, this method seems to be very promising to handle the depth of the neural generators. [ABSTRACT FROM AUTHOR]

This paper presents an inertial based sensing system for real-time three-dimensional measurement of human spinal motion, in a portable and non-invasive manner. Applications of the proposed system range from diagnosis of spine injury to postural monitoring, on-field as well as in the lab setting. The system is comprised of three inertial measurement sensors, respectively attached and calibrated to the head, torso and hips, based on the subject's anatomical planes. Sensor output is transformed into meaningful clinical parameters of rotation (twist), flexion-extension and lateral bending of each body segment, with respect to calibrated global reference space. Modeling the spine as a compound flexible pole model allows dynamic measurement of three-dimensional spine motion, which can be animated and monitored in real-time using our interactive GUI. The accuracy of the proposed sensing system has been verified with subject trials using a VICON optical motion measurement system. Experimental results indicate an error of less than 3.1 degrees in segment orientation tracking. [ABSTRACT FROM AUTHOR]

Uniaxial stress-strain data were obtained from in vitro experiments on 20 porcine livers for compressions, elongations and cycles of compression and then elongation. There were about 70 cylindrical samples, with diameter 7mm and varying height (4-11 mm). The combined compression and elongation test provide a unified framework for both compression and elongation for applications such as computer-aided surgical simulation. It enable the zero stress state of the experimental liver sample to be precisely determined. A new equation that combined both logarithmic and polynomial strain energy forms was proposed in modelling these experimental data. The assumption of incompressibility was justified from a preliminary Poisson's ratio for elongation and compression at 0.43+/-0.16 and 0.47+/-0.15, respectively. This equation provided a good fit for the observed mechanical properties of liver during compression-elongation cycles and for separate compressions or elongations. The root mean square errors were 91.92+/-17.43 Pa, 57.55+/-13.23 Pa and 29.78+/-17.67 Pa, respectively. In comparison with existing strain energy functions, this combined model was the better constitutive equation. Application of this theoretical model to small liver samples and other tissues demonstrated its suitability as the material model of choice for soft tissue. [ABSTRACT FROM AUTHOR]

The cardiokymograph or displacement cardiograph (DCG) is a noncontacting device which senses movement of the heart throughout the cardiac cycle by the interaction of a radiofrequency (10-20 MHz) electromagnetic field, generated by a sensing coil, with the thorax. In the paper three different techniques of detecting this movement will be discussed: monitoring the changes in sensing resonant frequency (FM modulation), monitoring the changes in impedance of the coil at resonant frequency (AM modulation) and a new technique which monitors the changes in coil impedance at fixed frequency. The sensitivities of these three techniques will be compared. A simplified theory of the mode of coupling between the coil and the thorax will be studied in terms of a transformer model. Preliminary clinical measurements of anterior left and right ventricular motion will be given. The presence of higher-frequency features related to atrial motion and the opening and closing of the aortic and pulmonary values will also be described. [ABSTRACT FROM AUTHOR]

This paper develops a model of 2-dimensional anisotropic cardiac tissue which permits the examination of the electrical sources arising from excitation at a point. The specific results depend on the anisotropy parameters and two cases are considered in detail. For assumed equal anisotropicity ratios in the interstitial and intracellular space the isochrones are (asymptotically) confocal ellipses and the total double layer source is (asymptotically) uniform. Surface fields are calculated and plotted for this condition. (For nonequal anisotropy ratios the total double layer source is necessarily nonuniform). When the preparation is very thin so that all fibres are essentially in contact with the extracellular medium the isochrones are (asymptotically) confocal ellipses but the total double-layer sources are now nonuniform. The sources for this case are determined and the fields at the surface of the cardiac tissue evaluated and plotted. The aforementioned 'thin' and 'thick' tissue preparations are contrasted with each other and the effects of anisotropicity in cardiac tissue discussed. [ABSTRACT FROM AUTHOR]

Surface-recorded somatosensory-evoked potentials (SEPs) are neural signals elicited by an external stimulus. In the case of electrically induced SEPs, the artifact generated by the stimulation process can severely distort the signal. In some cases, the artifact tail often lasts well into the initiation of the SEP making the determination of absolute latency very difficult. In this work, a new approach was taken to identify factors that affect the tail of the artifact. The methodology adopted was the development of a lumped electrical circuit model of the artifact generation process. While the modeling of the instrumentation hardware is relatively simple, this is not the case with tissue and electrode/skin interface effects. Consequently, this paper describes a novel tissue modeling approach that uses an autoregressive moving average (ARMA) parametric technique and an artificial neural network (ANN) to estimate tissue parameters from experimental data. This coupled with an estimation of the stimulation electrode-skin impedance completes the lumped circuit model. Simulink (The Mathworks Inc.) was used to evaluate the model under several different conditions. These results show that both the stimulation electrode-skin interface impedance and nature of the body tissue directly under the recording electrodes have a profound effect on the appearance of the stimulus artifact tail. This was verified by experimentally recorded data obtained from the median nerve using surface electrodes. Conclusions drawn from this work include that stimulation electrodes with low series capacitance should be used whenever possible to minimize the duration of the artifact tail. [ABSTRACT FROM AUTHOR]

The paper presents a detailed model of the entire human cardiovascular system which aims to study the changes in flow distribution caused by external stimuli, changes in internal parameters, or other factors. The arterial-venous network is represented by 325 interconnected elastic segments. The mathematical description of each segment is based on equations of hydrodynamics and those of stress/strain relationships in elastic materials. Appropriate input functions provide for the pumping of blood by the heart through the system. The analysis employs the finite-element technique which can accommodate any prescribed boundary conditions. Values of model parameters are from available data on physical and rheological properties of blood and blood vessels. As a representative example, simulation results on changes in flow distribution with changes in the elastic properties of blood vessels are discussed. They indicate that the errors in the calculated overall flow rates are not significant even in the extreme case of arteries and veins behaving as rigid tubes. [ABSTRACT FROM AUTHOR]

In the paper the equation of motion of the small amplitude transverse forced vibration of a radially prestressed and circularly clamped thin membrane has been developed. The material of the membrane is considered to be homogeneous, isotropic, incompressible and viscoelastic. From the analytical solution of this equation the incremental mechanical impedance of the membrane was derived as a function of frequency, geometrical parameters and incremental viscoelastic coefficients of the material. The parameters of the model were fitted to experimental impedance data using a global optimisation procedure to obtain the incremental viscoelastic moduli of the canine diaphragm. The estimated quasi-static behaviour of the model is shown to be consistent with the results of experimental quasi-static measurements. It is concluded that the incremental viscoelastic moduli of a soft tissue and the stress dependence of these material coefficients can be determined by fitting the parameters of the model to the impedance data of that particular tissue. [ABSTRACT FROM AUTHOR]

Most reported methods dealing with the optimal filtering of haemodynamic data are based on computationally onerous procedures and pay little attention to execution speed. In the paper the problem of filtering noisy aortic flow and pressure waveforms is considered and the possibility of applying a Kalman filter technique, which does not require a prèliminary identification of the signal generation process, is discussed. The basic hypothesis is that, in a cardiac cycle, the waveforms considered can be described by functions which permit Taylor series expansion. Thus the signal model is easy to obtain and the filtering procedure is fast, even if implemented on a minicomputer. The filter was applied to flow and pressure signals simultaneously measured in the canine ascending aorta under different physiological conditions. The filter performance is described and discussed. [ABSTRACT FROM AUTHOR]

Indices of heart rate variability (HRV) based on fractal signal models have recently been shown to possess value as predictors of mortality in specific patient populations. To develop more powerful clinical indices of HRV based on a fractal signal model, the study investigated two HRV indices based on a monofractal signal model called fractional Brownian motion and an index based on a multifractal signal model called multifractional Brownian motion. The performance of the indices was compared with an HRV index in common clinical use. To compare the indices, 18 normal subjects were subjected to postural changes, and the indices were compared on their ability to respond to the resulting autonomic events in HRV recordings. The magnitude of the response to postural change (normalised by the measurement variability) was assessed by analysis of variance and multiple comparison testing. Four HRV indices were investigated for this study: the standard deviation of all normal R-R intervals; an HRV index commonly used in the clinic; detrended fluctuation analysis, an HRV index found to be the most powerful predictor of mortality in a study of patients with depressed left ventricular function; an HRV index developed using the maximum likelihood estimation (MLE) technique for a monofractal signal model; and an HRV index developed for the analysis of multifractional Brownian motion signals. The HRV index based on the MLE technique was found to respond most strongly to the induced postural changes (95% CI). The magnitude of its response (normalised by the measurement variability) was at least 25% greater than any of the other indices tested. [ABSTRACT FROM AUTHOR]

The aortic input impedance is simulated by an asymmetric T-tube model loaded with complex loads. A geometric tapering is incorporated to represent the vasculature, assuming a triangular distribution of the wave transmission paths. Parametric analyses using physiological data demonstrate that the predicted impedance and reflection coefficient spectrum (RCS) closely mimic the experimental data. The simulation also reveals several significant features. As diameter tapering can minimise the presence and influence of wave reflections, the impedance modulus stays relatively constant with two distinct minima. The frequency of first minimum of impedance modulus is evidence of the tube elasticity and load compliance in the lower extremity, and the frequency of second minimum is evidence of those in the upper extremity. The high-frequency portion of the impedance modulus is affected by the tube elasticity, but not by the load compliance. The impedance spectrum at higher frequencies shows no notable fluctuations corresponding to a decrease in blood or wall viscosity. Furthermore, the low-frequency range in RCS is dominated by the longer lower body tube, and the high-frequency range by the shorter upper body tube. This geometrically tapered T-tube is considered a more natural model for the description of the systemic arterial system. [ABSTRACT FROM AUTHOR]

A mathematical method is presented that allows fast and simple computation of the electric field and current density induced inside a homogeneous spherical volume conductor by current flowing in a coil. The total electric field inside the sphere is computed entirely from a set of line integrals performed along the coil current path. Coils of any closed shape are easily accommodated by the method. The technique can be applied to magnetic brain stimulation and to magnetoencephalography. For magnetic brain stimulation, the total electric field anywhere inside the head can be easily computed for any coil shape and placement. The reciprocity theorem may be applied so that the electric field represents the lead field of a magnetometer. The finite coil area and gradiometer loop spacing can be precisely accounted for without any surface integration by using this method. The theory shows that the steady-state, radially oriented induced electric field is zero everywhere inside the sphere for ramping coil current and highly attenuated for sinusoidal coil current. This allows the model to be extended to concentric spheres which have different electrical properties. [ABSTRACT FROM AUTHOR]

A system model based on the simultaneous recording and analysis of the intracardiac and thoracic phonocardiograms to estimate the time-varying properties of the heart/thorax acoustic system of the dog is described. The presence of instrumental noise in the recording of intracardiac phonocardiograms is characterised, and it is demonstrated that its effect on the estimate of the transfer and coherence functions of the system can be quantified and corrected. Application of the model to study the spectral characteristics and the acoustic transmission properties of the mitral component M1 of the first heart sound and of the aortic component A2 of the second heart sound in the dog shows that the heart/thorax acoustic system acts like a bandpass filter having a higher attenuation for A2 than for M1. Between 20 and 100 Hz, the mean attenuation of M1 is 30 dB while that of A2 is 46 dB. Above 100 Hz, the attenuation slope is -12 dB per octave for M1 and -6 dB per octave for A2. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate the role of the interaction between peripheral resistance regulation and arterial compliance in the overall short-term regulation of mean arterial pressure. A nonlinear model previously proposed was linearised about control values of mean arterial pressure and cardiac output so that it could be reformulated in terms of transfer functions. The resulting pressure to pressure open-loop transfer function H(s) consists of a complex conjugate pair of poles (pertaining to the resistance regulating system) and a real pole (pertaining to the arterial system, 1/(R C), R=control resistance). Such a structure suggests an interaction between the resistance regulation and the arterial compliance (C). Quantitative evaluation of this interaction was obtained by estimating the model parameters during partial vena cava occlusions in four cats. Using these parameters the time response of the open-loop control system to a pressure step was found to be underdamped and oscillatory in all four cats (damping factor ξ ranged from 0·20 to 0·66), the amplitude of oscillations depending on the value of ξ and on the relative amplitude of the arterial time constant (compliance and peripheral resistance) with respect to the time constant 1/(ξω). Bode diagrams of H(jω) showed that the resonance peak due to the resistance regulating system may not be detectable, either because of the relatively high value of ξ or because it is masked by the pole of the arterial system. [ABSTRACT FROM AUTHOR]

With a model based on Poisson's equation, computations employing published data and measurements made in glass cylinders filled with saline, the influence of conductivity and geometry on the determination of volume was analysed with an impedancimetric catheter. Two distal electrodes inject a constant current while a set of central electrodes sense by sections the potential along the cylindrical chamber. It was concluded that each section ought to be independently calibrated, the distribution of the electrodes along the catheter and its relationship to the chamber radius being of paramount importance for a linear calibration. The ratio σ, of the external medium conductivity σ to the internal conductivity σ influences drastically both the linearity and the calibration of each section. If a section volume is linearly related to the total chamber volume, that section admittance is a total volume estimator. [ABSTRACT FROM AUTHOR]

To understand the special flow conditions which may be produced by the presence of stenosis in arteries, an analytical solution is obtained for pulsatile laminar flow in an elliptic tube. Blood is approximated by a Newtonian model and the geometry of the stenosis is introduced by specifying the change in area of cross-section of the stenosed artery with axial distance. The results for velocity, pressure, shear stress and impedance are presented. These are compared with the steady flow results as well as with those of the flow in a stenosed tube of circular cross-section. The study indicates that the fluid dynamic characteristics of the flow are affected by the percentage of stenosis as well as the geometry of the stenosis. The frequency of oscillation is also found to influence shearing stress and the impedance. [ABSTRACT FROM AUTHOR]

A method is presented for analysing sounds produced in vitro by prosthetic aortic heart valves, and a one-dimensional harmonic model is described. Procedures for estimating physical parameters of the model are outlined for the case of transient and nontransient sounds, and a computational method is described for making comparisons between two general sounds. The fast Fourier transform provides a satisfactory means for the basic transformation to the frequency domain. Useful representations of the acoustical information that are considered are the original time and amplitude plots. power-density spectra, power-distribution functions, a three-dimensional surface of power-frequency-time, sections of these three-dimensional surfaces, and a three-dimensional power-distribution surfaces. Note is made that each representation is useful for indicating specific acoustical characteristics which may be important when either comparing or describing sounds. The spectra provide an accurate means for estimating the parameters of the model and provide clearer comparisons when compared to the time-amplitude plots. This fact is most clearly shown by the three-dimensional powerdifference surface. This surface provides a very convenient means for the overall comparison of two sounds. [ABSTRACT FROM AUTHOR]

Elastic mismatching is studied by an impulse technique. Elastic mismatches are created by joining two latex rubber tubes of the same internal diameter but different wall thickness. A short duration pressure impulse is generated at the entrance of the water-filled tube system and the transmission characteristics of the impulse are determined from a series of measurements along the tube system using a catheter-tip pressure manometer. This technique enables the magnitude and phase change of the reflection at the elastic discontinuity to be obtained. A number of tube combinations are examined and in all cases a good agreement is shown between the measured reflection coefficient and the value calculated theoretically. [ABSTRACT FROM AUTHOR]

Recently many studies have been devoted to the evaluation of the behaviour of the arterial system by calculating the left ventricular afterload, on the basis of the hydraulic impedance concept. Since the best known computation procedure, based on Fourier analysis, does not seem to be fully reliable, simple mathematical models have been set up by various investigators. One important problem is the choice of an identification procedure which points out the reliability of such models on the one hand, and which on the other hand allows us to compare them with each other. The behaviour of such a procedure can depend on operating conditions such as: (a) the choice of the input/output signals for the model (pressure or flow) and (b) the amount of noise and/or transient arterfacts added to these signals. Therefore, in the present work, the assessment of a parametric identification procedure has been studied relating to two different models previously proposed by the authors themselves, and its robustness has been evaluated in regard to the above mentioned conditions. [ABSTRACT FROM AUTHOR]