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Biomedical Engineering, IEEE Transactions on

Issue 6 • Date June 1995

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Displaying Results 1 - 12 of 12
  • Intracranial pressure dynamics in patients with acute brain damage: a critical analysis with the aid of a mathematical model

    Page(s): 529 - 540
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    The time pattern of intracranial pressure (ICP) in response to typical clinical tests (i.e., bolus injection and bolus withdrawal of 1 to 4 mL of saline in the craniospinal space) was studied in 18 patients with acute brain damage by means of a mathematical model. The model includes the main biomechanical factors assumed to affect intracranial pressure, particularly cerebrospinal fluid (CSF) dynamics, intracranial compliance, and cerebral hemodynamics. Best fitting between model simulation curves and clinical tracings was achieved using the Powell minimization algorithm and a least-square criterion function. The simulation results demonstrate that, in most patients, the ICP time pattern cannot be explained merely on the basis of CSF dynamics but also requires consideration of the contribution of cerebral hemodynamics and blood volume alterations. In particular, only in a few patients (about 40% of total) the ICP monotonically returns toward baseline following the clinical maneuver. In most of the examined cases (about 60%), ICP exhibits an anomalous response to the same maneuver, characterized by a delayed increase after bolus injection and a delayed decrease after withdrawal. The model is able to explain these responses, imputing them to active intracranial blood volume changes induced by mechanisms controlling cerebral blood flow. Finally, the role of the main intracranial biomechanical parameters in the genesis of the ICP time pattern is discussed and a comparison with previous theoretical studies performed. View full abstract»

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  • Machine learning in control of functional electrical stimulation systems for locomotion

    Page(s): 541 - 551
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    Two machine learning techniques were evaluated for automatic design of a rule-based control of functional electrical stimulation (FES) for locomotion of spinal cord injured humans. The task was to learn the invariant characteristics of the relationship between sensory information and the FES-control signal by using off-line supervised training. Sensory signals were recorded using pressure sensors installed in the insoles of a subject's shoes and goniometers attached across the joints of the affected leg. The FES-control consisted of pulses corresponding to time intervals when the subject pressed on the manual push-button to deliver the stimulation during FES-assisted ambulation. The machine learning techniques used were the adaptive logic network (ALN) and the inductive learning algorithm (IL). Results to date suggest that, given the same training data, the IL learned faster than the ALN while both performed the test rapidly. The generalization was estimated by measuring the test errors and it was better with an ALN, especially if past points were used to reflect the time dimension. Both techniques were able to predict future stimulation events. An advantage of the ALN over the IL was that ALN's can be retrained with new data without losing previously collected knowledge. The advantages of the IL over the ALN were that the IL produces small, explicit, comprehensible trees and that the relative importance of each sensory contribution can be quantified. View full abstract»

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  • Epicardial cardiac source-field behavior

    Page(s): 552 - 558
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    The accurate determination of the spatial distribution of cardiac electrophysiological state is essential for the mechanistic assessment of cardiac arrhythmias in both clinical and experimental cardiac electrophysiological laboratories. The authors describe three fundamental cardiac source-field relationships: 1) activation fields; 2) electrotonic fields; and 3) volume conductor fields. The three cases are described analytically and illustrated with experimentally obtained canine cardiac recordings that capitalize on a recently formulated technique for in vivo cardiac transmembrane current estimation. View full abstract»

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  • Predicting cardiothoracic voltages during high energy shocks: methodology and comparison of experimental to finite element model data

    Page(s): 559 - 571
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    Finite element modeling has been used as a method to investigate the voltage distribution within the thorax during high energy shocks. However, there have been few quantitative methods developed to assess how well the calculations derived from the models correspond to measured voltages. Here, the authors present a methodology for recording thoracic voltages and the results of comparisons of these voltages to those predicted by finite element models. The authors constructed detailed 3D subject-specific thorax models of 6 pigs based on their individual CT images. The models were correlated with the results of experiments conducted on the animals to measure the voltage distribution in the thorax at 52 locations during synchronized high energy shocks. One transthoracic and two transvenous electrode configurations were used in the study. The measured voltage values were compared to the model predictions resulting in a correlation coefficient of 0.927±0.036 (average±standard deviation) and a relative rms error of 22.13±5.99%. The model predictions of voltage gradient within the myocardium were also examined revealing differences in the percent of the myocardium above a threshold value for various electrode configurations and variability between individual animals. This variability reinforces the potential benefit of patient-specific modeling. View full abstract»

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  • A three-dimensional finite element model of human transthoracic defibrillation: paddle placement and size

    Page(s): 572 - 578
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    A detailed 3-D finite element model of the conductive anatomy of the human thorax has been constructed to quantitatively assess the current density distribution produced in the heart and thorax during transthoracic defibrillation. The model is based on a series of cross-sectional CT scans and incorporates isotropic conductivities for eight tissues and an approximation of the anisotropic conductivity of skeletal muscle. Current density distributions were determined and compared for four paddle pairs and two paddle sizes. The authors' results show that the myocardial current density distributions resulting from a defibrillation shock were fairly uniform for the paddle pairs and sizes examined in this study. Specific details of the spatial distribution of the current density magnitudes in the heart were found to depend on paddle placement and size. When the minimum current necessary to defibrillate was delivered, the maximum myocardial current density produced with any of the paddle sizes and positions examined was less than four times the minimum current density necessary to render a myocyte in a fibrillating heart inexcitable, and less than 40% of the damage threshold. These results suggest that common clinically used defibrillation paddle positions have a safety margin as large as 2.5 for current and ∼6 for energy. View full abstract»

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  • A technique for measurement of the extent of spatial organization of atrial activation during atrial fibrillation in the intact human heart

    Page(s): 579 - 586
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    Atrial fibrillation (AF) is a common clinical problem, associated with considerable morbidity and motility, for which effective management strategies have yet to be devised. The absence of objective measures to guide selection of antiarrhythmic drug therapy for maintenance of sinus rhythm leaves only clinical endpoints (either beneficial or detrimental) for assessment of drug action, with occasional catastrophic consequences. As part of an attempt to provide an objective framework for the assessment of antiarrhythmic drug action on the electrophysiologic determinants of atrial fibrillation, the authors have developed a measure of the spatial organization of atrial activation processes during atrial fibrillation. By recording activation sequences at multiple equally spaced locations on the endocardial surface of the atrial during atrial fibrillation in humans and determining the degree of correlation between these activation sequences as a function of distance, the authors have been able to construct spatial correlation functions for atrial activation. They have found that atrial activation remains well-correlated, independent of distance during normal sinus rhythm and atrial flutter. During atrial fibrillation, correlation decays monotonically with distance and the space-constant for this decay may be used to describe the relative spatial organization of atrial fibrillation. The authors provide examples of the impact of antiarrhythmic agents on the space-constant and suggest that assessment of the relative spatial organization of atrial activation using this methodology may potentially provide an objective framework to guide therapy in patients with AF. View full abstract»

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  • Use of sonomicrometry and multidimensional scaling to determine the three-dimensional coordinates of multiple cardiac locations: feasibility and initial implementation

    Page(s): 587 - 598
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    The authors describe a new method which uses sonomicrometry and the statistical technique of multidimensional scaling (MDS) to measure the three-dimensional (3D) coordinates of multiple cardiac locations. The authors refer to this new method as sonomicrometry array localization (SAL). The new method differs from standard sonomicrometry in that each piezoelectric transducer element is used as both transmitter and receiver and the set of intertransducer element distances is measured. MDS calculates the 3D coordinates of each sonomicrometry transducer element from the set of intertransducer element distances. The feasibility of this new method was tested with mathematical simulations which demonstrated the ability of MDS to compensate for signal error and missing intertransducer element distances. The authors describe the design elements of a modified digitally controlled sonomicrometer in which a single transducer element can sequentially broadcast to as many as 8 receiver elements. That design is used to validate SAL in a water bath and in ex vivo and living hearts. Correlation with caliper measurement in the water bath (y int.=3.91±3.36 min, slope=1.04±0.05, r 2=0.969±0.027) and with radiography in ex vivo (y int.=-0.87±0.92 mm, slope=0.97±0.02, r 2=0.960±0.023) and in vivo hearts (y int.=2.98±2.59 mm, slope=1.01±0.06, r 2=0.953±0.031) was excellent. Sonomicrometry array localization is able to accurately measure the 3D coordinates of multiple cardiac locations. It can potentially measure myocardial deformation and remodeling after ischemic or valvular injury. View full abstract»

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  • Action potentials of curved nerves in finite limbs

    Page(s): 599 - 607
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    Previous simulations of volume-conducted nerve-fiber action-potentials have modeled the limb as semi-infinite or circularly cylindrical, and the fibers as straight lines parallel to the limb surface. The geometry of actual nerves and limbs, however, can be considerably more complicated. Here, the authors present a general method for computing the potentials of fibers with arbitrary paths in arbitrary finite limbs. It involves computing the propagating point-source response (PPSR), which is the potential arising from a single point source (dipole or tripole) travelling along the fiber. The PPSR can be applied to fibers of different conduction velocities by simple dilation or compression. The method is illustrated for oblique and spiralling nerve fibers. Potentials from oblique fibers are shown to be different for orthodromic and antidromic propagation. Such results show that the straight-line models are not always adequate for nerves with anatomical amounts of curvature. View full abstract»

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  • Selective minimum-norm solution of the biomagnetic inverse problem

    Page(s): 608 - 615
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    A new multidipole estimation method which gives a sparse solution of the biomagnetic inverse problem is proposed. This solution is extracted from the basic feasible solutions of linearly independent data equations. These feasible solutions are obtained by selecting exactly as many dipole-moments as the number of magnetic sensors. By changing the selection, the authors search for the minimum-norm vector of selected moments. As a result, a practically sparse solution is obtained; computer-simulated solutions for L p-norm (p=2, 1, 0.5, 0.2) have a small number of significant moments around the real source-dipoles. In particular, the solution for L 1-norm is equivalent to the minimum-L 1-norm solution of the original inverse problem. This solution can be uniquely computed by using linear programming. View full abstract»

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  • Evaluation of MUAP shape irregularity-a new concept of quantification

    Page(s): 616 - 620
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    A coefficient for quantifying the shape irregularities of the motor unit action potential (MUAP) is introduced. This coefficient is defined as the "length" of action potential curve normalized by the signal's amplitude in such a way that it is independent on duration and amplitude. It characterizes only the MUAP shape. The irregularity coefficient may be used to measure the deviations of the potential from the normal MUAP. The properties of this coefficient and its relation to the conventional parameters describing MUAP shape, viz. The number of phases and turns is discussed. The examples of classification of real signals according to this coefficient are presented. View full abstract»

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  • Performance of above elbow body-powered prostheses in visually guided unconstrained motion tasks

    Page(s): 621 - 631
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    The "classical" body-powered above elbow arm prosthesis continues to be used by a large majority of arm prosthesis users, even though many more modern devices are available. The authors present a set of experiments designed to compare performance of unimpaired arms and body-powered prostheses of 6 unilateral amputees. The experiments were designed to measure qualitatively how well the body-powered prosthesis can be used to perform free-motion tasks, as well as to study the qualitative features of movement common to both the prosthesis and unimpaired arm. It was found that regular peaks in velocity were common to both the unimpaired arm and prosthesis movements, suggesting that movements were composed of a sequence of successive actions. In addition, it was found that the body-powered prosthesis generally required more movements than the unimpaired arm to meet an accuracy constraint and could not keep up with the unimpaired arm when a speed constraint was imposed, even though the body-powered prosthesis was able to match the unimpaired arm in a simple nondynamic task. View full abstract»

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  • A 32-electrode data collection system for electrical impedance tomography

    Page(s): 632 - 636
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    A 32-electrode data collection system for electrical impedance tomography (ETT) is presented. In this system, the demodulator is a multiplexed sample and hold (S&H) circuit followed by a voltage difference stage. This configuration provides high CMRR due to the low (almost DC) operating frequency of the signals the difference stage is required to process. 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