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Biomedical Engineering, IEEE Transactions on

Issue 2 • Date Feb. 1989

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Displaying Results 1 - 17 of 17
  • Realistic conductivity geometry model of the human head for interpretation of neuromagnetic data

    Page(s): 165 - 171
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    The computational and practical aspects of a realistically shaped multilayer model for the conductivity geometry of the human head are discussed. A method to handle the numerical difficulties caused by the presence of poorly conducting skull is presented. Using the method, both the potential on the surface of the head and the magnetic field outside the head can be computed accurately. The procedure is tested with the multilayer sphere model, for which analytical expressions are available. The method is then applied to a realistically shaped head model, and it is shown numerically that for the computation of B produced by cerebral current sources, it is sufficient to consider a brain-shaped homogeneous conductor only, since the secondary currents on the outer interfaces give only a negligible contribution to the magnetic field outside the head. Comparisons with the sphere model are included to pinpoint areas where the homogeneous conductor model provides essential improvements in the calculation of the magnetic field outside the head.<> View full abstract»

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  • Optimal stroke volume in left-ventricular ejection

    Page(s): 172 - 182
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    Optimal value of stroke volume ejected against a given arterial load is studied, i.e. left-ventricular function is described by an optimization model. In the model, end-diastolic volume (V/sub ed/), the linear end-systolic pressure-volume relation, heart rate, and ejection time are given, and arterial load is described by the three-element windkessel model. The cost function of the model takes into account two optimality criteria for ventricular function: energy economy and efficient response to an increase in V/sub ed/. The observed stroke volumes of isolated canine hearts could be predicted quite accurately by the model. It is concluded that the left-ventricular response to a change in arterial load and V/sub ed/ can be explained by an optimization model when contractility and heart rate are kept constant. The results also strongly suggest that energy economy and efficiency are essential features of left-ventricular function.<> View full abstract»

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  • A mathematical model of cerebral blood flow chemical regulation. I. Diffusion processes

    Page(s): 183 - 191
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    A mathematical model which describes the production and diffusion of vasoactive chemical factors involved in oxygen-dependent cerebral blood flow (CBF) regulation in the rat is presented. Partial differential equations describing the relations between input and output variables have been replaced with simpler ordinary differential equations by using mathematical approximations of the hyperbolic functions in the Laplace transform domain. The model is composed of two submodels. In the first, oxygen transport from capillary blood to cerebral tissue is analyzed to link changes in mean tissue oxygen pressure with CBF and arterial oxygen concentration changes. The second submodel contains equations describing the production of vasoactive metabolites by cerebral parenchyma, due to a lack of oxygen and their diffusion towards pial perivascular space. The equations have been used to simulate the time dynamics of mean tissue P/sub O2/, perivascular adenosine concentration, and perivascular pH following changes in CBF. The simulation shows that the time delay introduced by diffusion processes is negligible compared with the other time constants of the system under study.<> View full abstract»

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  • A mathematical model of cerebral blood flow chemical regulation. II. Reactivity of cerebral vascular bed

    Page(s): 192 - 201
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    For pt.I see ibid., vol.36, no.2, p.183-91 (1989). A mathematical model of the chemical oxygen-dependent cerebral blood flow (CBF) regulation in the rat is proposed. The model assumes that oxygen acts on cerebral vessels through an indirect mechanism, mediated by the release of two metabolic substances (adenosine and H/sup +/) from tissue, and that any change in perivascular concentration of these substances affects the diameter of both the medium and small pial arteries as well as that of intracerebral arteriole. The model is composed of several submodels, each closely related to a different physiological event. Mathematical equations which describe the reaction of the vasoactive portion of the cerebral vascular bed are given. The model permits the simulation of the role played by chemical factors in the control of CBF under many different physiological and pathological conditions. Several events associated with an alteration in oxygen supply to tissue have been simulated. The results suggest that chemical factors, adenosine and H/sup +/ play a significant but not exclusive role in the regulation of the cerebral vascular bed. The action of other mechanisms must be hypothesized to explain completely the CBF changes occurring in vivo.<> View full abstract»

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  • The use of adaptive algorithms for obtaining optimal electrical shimming in magnetic resonance imaging (MRI)

    Page(s): 202 - 210
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    A method of determining the DC coil current values in order to shim electrically the static magnetic fields used in magnetic resonance imaging (MRI) using the modified steepest descent adaptive algorithm is described. Using a 32-cm-diameter by 40-cm-long water phantom as the test volume, the algorithm achieved field homogeneities of 0.2 parts per million (p.p.m.) peak-to-peak within a 20-cm-diameter spherical imaging volume, and 1.3 p.p.m. peak-to-peak within the entire phantom. The algorithm achieved an inhomogeneity variance of 0.18 p.p.m./sup 2/. The shim system was successfully modeled as a sum of adaptive linear combiners. The model, which contains 13 parameters that can be varied, 12 shim coil currents, and the receiver mixer frequency, has been used to predict key adaptive algorithm parameters. Experimental verification of these parameters lends support to the accuracy of the model.<> View full abstract»

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  • Local tissue attenuation images based on pulsed-echo ultrasound scans

    Page(s): 211 - 221
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    The original conventional B scan is traversed, one pixel at a time. In the vicinity of each pixel, a uniform area is determined and average attenuation is computed utilizing an extended modified Prony approach. B scans of tissue mimicking phantoms and in vivo human liver scans are used to illustrate the performance of the proposed algorithm. The results show good agreement between estimated and known attenuation values. Clinical studies demonstrate significant potential of the technique, in particular for diagnosis of diffuse liver disease. View full abstract»

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  • High resolution NMR spectroscopy using a recursive algorithm

    Page(s): 222 - 231
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    A nuclear magnetic resonance (NMR) spectrum estimation method is derived. Under various conditions, the proposed method is compared with the discrete Fourier transform (DFT) using a proton /sup 1/H NMR signal observed from mayonnaise in a 2 (T) static magnetic field. The results show that if spin-spin relaxation times are known previously, the method can provide a higher resolution NMR spectrum than the DFT. The method can also reduce the discrete approximation errors of the NMR spectrum. If frequency components of the sources of the NMR signal are present in a limited range in the frequency domain of interest, the method can magnify the NMR spectrum for a fixed sampling time and interval.<> View full abstract»

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  • A tracking FFT processor for pulsed Doppler analysis beyond the Nyquist limit (medical ultrasound)

    Page(s): 232 - 237
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    The processor is based on the observation that the frequency translation required to reconstruct properly an aliased spectrum can be achieved by means of a simple reordering of data provided by a digital fast Fourier transform (FFT) unit. The amount of reordering is automatically derived by the computed value of a spectral parameter, e.g. the mean frequency. The procedure has been tested by introducing some modifications at the output of an FFT unit included in a conventional pulsed Doppler system. The dynamic evolution of the full Doppler spectrum and related mean frequency can be followed in real time over an extended range. Results of in vitro and in vivo experiments, as well as quantitative measurements performed with test signals, are presented. View full abstract»

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  • Design optimization of interstitial antennas

    Page(s): 238 - 246
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    The radiation characteristics of multisection insulated antennas in conductive tissue are discussed. The effects of varying the diameters and lengths of the center conductors in the various sections of the antenna and the diameter and type of the insulation on the electromagnetic power deposition pattern and input impedance characteristics were examined. An approximate numerical model which calculates the current distribution and the radiation characteristics of multisection insulated antennas was developed. The numerical predictions were verified in a qualitative way experimentally by mapping the various near- and far-field components of the antennas. On the basis of these results, design tradeoffs are identified and quantified and guidelines for optimum designs are specified. In particular, it is shown that an insulation-to-center-conductor diameter ratio between 1.5 to 2.0 is optimum for uniform Teflon insulation, and that a multisection arrangement with the thinnest insulation near the antenna tip has superior performance compared with the uniform-insulation or other multisection designs.<> View full abstract»

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  • Procedure for quantitative comparison of ground reaction data

    Page(s): 247 - 255
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    An analytical method for quantitative and detailed comparison of different ground reaction force patterns is described. It is based on suitable processing of the 3-D components of the ground reaction force and is essentially composed of three main steps: verification of the regular recurrence of measurement conditions, normalization, and data comparison. The first step is to check that the acquired data are not affected by inertial predominance in the direction of progression. Normalization involves the amplitude of force, the application point displacements and the stance-phase duration. Data comparison utilizes a dedicated implementation of an appropriate statistical approach to estimate significant differences between two suitable and typical ground reaction force patterns. Two practical cases are presented in detail to demonstrate the entire analytical procedure. View full abstract»

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  • Four digital algorithms for activation detection from unipolar epicardial electrograms

    Page(s): 256 - 261
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    The reproducibility of activation detection by each of four algorithms used to calculate maximum derivatives was tested on two sequential paced beats of right ventricular unipolar epicardial electrograms, which represented either local activation of the right ventricle alone or synchronous activation of both ventricles. The methods were evaluated by comparing the shape of the two beats aligned on their selected activation times, i.e. the time at which the maximum negative deflection occurred, the differences in activation intervals for the two beats, and the effect on the activation time of superimposing distant events on local activation. The 17-point second-order data fit algorithm performed slightly better than the first-order difference, three-point Lagrange derivative, and five-point second-order data fit algorithms except that activation time selection by the 17-point technique was slightly, but significantly, delayed by the superposition of distant potentials. The 17-point second-order data fit technique is therefore recommended for use in detecting activation unless computation time is a major consideration.<> View full abstract»

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  • Impulsive noise suppression and background normalization of electrocardiogram signals using morphological operators

    Page(s): 262 - 273
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    An approach to impulsive noise suppression and background normalization of digitized electrocardiogram signals using mathematical morphological operators that incorporate the shape information for a signal is presented. A brief introduction to these nonlinear signal processing operators, as well as a detailed description of the algorithm, is presented. Empirical results show that the algorithm has good performance in impulsive noise suppression and background normalization.<> View full abstract»

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  • The design and use of a microcomputerized real-time muscle fatigue monitor based on the medical frequency shift in the electromyographic signal

    Page(s): 284 - 286
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    A real-time microcomputerized muscle fatigue monitor based on the median frequency shift of the electromyographic signal, computed via the fast Fourier transform, is discussed. For the ten subjects performing an isotonic and isometric trunk extension task on two separate days, preliminary results suggest a repeatable linear decrease in median frequency as a function of time.<> View full abstract»

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  • Separation of fine crackles from vesicular sounds by a nonlinear digital filter

    Page(s): 286 - 291
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    Crackles are discontinuous adventitious sounds, and their separation is an important process in the analysis of lung sounds. In order to separate the crackles from vesicular sounds automatically, a nonlinear digital filter which was designed to separate nonstationary signals was used. This filter was applied to the lung sounds recorded from six patients with pulmonary fibrosis. The separation was satisfactory enough to make this method useful in clinical medicine.<> View full abstract»

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  • Vector analysis of three-dimensional evoked potentials: eccentric dipoles

    Page(s): 291 - 295
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    The quantitative description of three-dimensional cerebral evoked potentials is extended to include eccentric dipolar sources. Eccentricity-related distortions in dipole orientation and magnitude are assessed. The use of nonstandard montages, the prediction of topographic surface maps, dynamic analysis, and theoretical mechanisms of planar segment formation are discussed.<> View full abstract»

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  • A practical electrode-array myoprocessor for surface electromyography

    Page(s): 295 - 299
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    A myoprocessor which produces a relatively smooth and accurate measure of a muscle's electrical activity during both slow and rapid movements over a mechanical frequency range from 0 to 20 Hz is described. This is achieved partly by spatial averaging of weakly correlated EMG signals from electrodes distributed over the surface of the muscle. With the cutoff frequency for the final smoothing filter set at 30 Hz, the microprocessor output has a SNR of about 6. View full abstract»

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  • Assessment of autonomic regulation of heart rate variability by the method of complex demodulation

    Page(s): 274 - 283
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    Complex demodulation was used to examine the effect of both divisions of the autonomic nervous system (sympathetic and parasympathetic) on heart rate. Data were analyzed from dogs during classical conditioning procedures which caused different changes in the autonomic regulation of heart rate. Two significant peaks in the heart rate variability spectrum were examined by this technique. The amplitude of the peak at the respiration frequency showed parasympathetic changes, while the amplitude of the low frequency peak (0-0.124?? Hz) showed both sympathetic and parasympathetic effects. Complex demodulation results at these frequencies clearly showed the activities of both branches of the autonomic nervous system in regulating heart rate. During the CS+ period, when trained dogs were presented with a tone predicting a subsequent shock, the observed tachycardia was due to decreased parasympathetic activity and a transient increase in sympathetic activity. During the CS- period where a different tone predicts no shock, parasympathetic and sympathetic activities were unchanged from the baseline condition. The use of complex demodulation enables us to examine autonomic contributions to heart rate regulation in conditioning and a variety of other physiological and environmental conditions where autonomic input can be expected to change rapidly. View full abstract»

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Aims & Scope

IEEE Transactions on Biomedical Engineering contains basic and applied papers dealing with biomedical engineering. Papers range from engineering development in methods and techniques with biomedical applications to experimental and clinical investigations with engineering contributions.

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Editor-in-Chief
Bin He
Department of Biomedical Engineering