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Biomedical Engineering, IEEE Transactions on

Issue 1 • Date Jan. 1994

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Displaying Results 1 - 12 of 12
  • Uniqueness of the generators of brain evoked potential maps

    Page(s): 1 - 11
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1139 KB)  

    This study considers the uniqueness of neuronal generators of human brain evoked potentials measured on the scalp using the physical and mathematical properties of the volume conductor model. The results are applicable to a realistic, nonhomogeneous head shape where the potential map is known on a continuous set of points on the scalp. It is shown that sources which occupy "zero volume" in space such as point dipoles or sources distributed on an open surface or a line are uniquely defined by the potential maps. Finite volume nonoverlapping sources are also uniquely defined by their potential map. However, there are infinitely many different but overlapping sources which can create the same map. Several examples of such sources are provided. It is shown that there is a unique, minimum volume source which can be defined in this case. Results suggest that if a reconstruction of the sources starts from a continuous scalp map (obtained by interpolation of the data between electrode sites), one can obtain unique results concerning the source parameters that are not available in a search for a source whose potential map fits only at a discrete set of points. View full abstract»

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  • Binaural hearing in the presence of a low frequency magnetic field

    Page(s): 12 - 16
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (516 KB)  

    The authors propose the binaural auditory system as a candidate neural system that may be disrupted by exposure to relatively weak LF magnetic fields. Extracellular currents, induced by time-varying magnetic fields, may change the timing of action potentials in the auditory nerve, thereby disrupting sound localization when interaural time differences are very small. Three subjects were exposed to a 1,000 Hz magnetic field - with a maximum rate of change of 2.3 T/s at the location of the cochlea - while presenting two identical 1,000 Hz tones randomly delayed to the left or right ear by less than 10 μs. The subjects were asked whether the signal was perceived to be displaced to the left or right side of midline. After a total of over 20,000 trials, conducted at different phase angles between the field signal and the tones, there was no clear evidence for a consistent change in performance when the magnetic field was present. This, however, does not rule out an effect at other combinations of magnetic and acoustic frequencies. View full abstract»

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  • Practical induction heating coil designs for clinical hyperthermia with ferromagnetic implants

    Page(s): 17 - 28
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    Interstitial techniques for hyperthermia therapy of cancer continue to evolve in response to requirements for better localization and control over heating of deep seated tissues. Magnetic induction heating of ferromagnetic implants is one of several available techniques for producing interstitial hyperthermia, using thermal conduction to redistribute heat within an array of controlled temperature "hot sources." This report describes seven induction heating coil designs that can be used for producing strong magnetic fields around ferromagnetic seed implants located in different sites in the body. The effect of coil design on the extent and uniformity of the magnetic field is characterized, and appropriate electrostatic shield designs for minimizing electric field coupling to the patient are described. Advantages and disadvantages of each coil type are discussed in terms of the radiated fields, coil efficiency, and ease of use, and appropriate applications are given for each design. This armamentarium of induction coils provides the ability to customize magnetic field distributions for improved coupling of energy into ferromagnetic implant arrays located at any depth or orientation in the body. Proper selection of heating coil configuration should simplify patient setup, improve the safety of patient treatments, and pave the way for future applications of interstitial heating in sites that were previously untreatable. View full abstract»

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  • High frequency ultrasonic backscatter from erythrocyte suspension

    Page(s): 29 - 34
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    Previous studies have shown that ultrasonic backscattering from red blood cells suspended in saline is proportional to the fourth power of frequency for frequencies below 15 MHz, as predicted by Rayleigh scattering theory. Recently, the authors have extended the measurements up to 30 MHz, because scattering of ultrasound by red blood cells may no longer be negligible at these frequencies and can affect, to a great degree, the operation of intravascular imaging devices. The experimental results show that the fourth power dependence on frequency of the backscattering coefficient for porcine erythrocytes suspended in saline solution appears to be valid up to 30 MHz. To confirm this, backscattering cross-section of porcine red cells was computed as a function of frequency using the T-matrix method. Since at higher frequencies the shape of the scatterers may also play a significant role, its effect was investigated by treating the red cell as a sphere, a disc, and a biconcave disc of the same volume. Good agreement was obtained between the experimental and theoretical results. View full abstract»

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  • Pulse Doppler ultrasound detection, characterization and size estimation of emboli in flowing blood

    Page(s): 35 - 44
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    A theory describing pulse Doppler ultrasound signals due to backscattering due to emboli in flowing blood is presented. From this theory, the minimum detectable size of a formed-element embolus can be established as a function of carrier frequency and vessel size. Emboli can be sized and characterized, based on the ratio of the amplitude of the Doppler signal during embolus passage through the sample volume to background bloodflow Doppler signal when no embolus is present. This ratio is defined as the "embolus to blood ratio" (EBR). Size estimation of emboli can be done by insonating an embolus with a single frequency and measuring the EBR, only if the embolus does not exceed a certain size, and if the vessel diameter and per cent hematocrit are known. Using two different frequencies, the vessel geometry (diameter and sample volume length) and per cent hematocrit can be eliminated from calculation of embolus size. Sources of uncertainty in the EBR and their effect on embolus size estimation are discussed. Discrimination between gas and formed-element emboli is described, given a detector with sufficient dynamic range, and use of three carrier frequencies. The theory presented here is in agreement with experimental findings of other investigators. View full abstract»

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  • A fiber optic system for measuring dynamic mechanical properties of embryonic tissues

    Page(s): 45 - 50
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    Dynamic mechanical properties of embryonic tissues, including viscoelasticity and active motility, play a major role in morphogenesis. Mechanical abnormalities resulting from altered cytoskeletal chemistry presumably contribute to spina bifida and other birth defects, but research in this area has been limited by inadequate mechanical measurement techniques. The author discusses aspects of embryo tissues that make them difficult to measure and describes a versatile new system developed and used to study the biomechanics of morphogenesis in amphibian embryos. It incorporates a flexible optical-fiber probe driven by piezoceramic elements to impose and measure feedback-controlled, two-dimensional, time-varying patterns of stress or strain in the tissue. Compression, tension, and shear geometries are possible. Capacitive displacement sensors provide the precision and low drift rates required for these measurements. Software control permits the system to perform both standard and customized rheological tests, including interactive ones in which test parameters change in response to measured tissue behavior. Sample size range is 50 μm to 5 mm. Displacement range is 60 μm with a resolution of 0.1 μm. Force range is 10 μN with a resolution of 0.02 μN. Frequency range is DC to 20 Hz. View full abstract»

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  • An adaptive lung ventilation controller

    Page(s): 51 - 59
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    Closed loop control of ventilation is traditionally based on end-tidal or mean expired CO 2. The controlled variables are the respiratory rate RR and the tidal volume V T. Neither patient size or lung mechanics were considered in previous approaches. Also the modes were not suitable for spontaneously breathing subjects. This report presents a new approach to closed loop controlled ventilation, called adaptive lung ventilation (ALV). ALV is based on a pressure controlled ventilation mode suitable for paralyzed, as well as spontaneously breathing, subjects. The clinician enters a desired gross alveolar ventilation (V gA' in l/min), and the ALV controller tries to achieve this goal by automatic adjustment of mechanical rate and inspiratory pressure level. The adjustments are based on measurements of the patient's lung mechanics and series dead space. The ALV controller was tested on a physical lung model with adjustable mechanical properties. Three different lung pathologies were simulated on the lung model to test the controller for rise time (T 90), overshoot (Y m), and steady state performance (Δ max). The pathologies corresponded to restrictive lung disease (similar to ARDS), a "normal" lung, and obstructive lung disease (such as asthma). Furthermore, feasibility tests were done in 6 patients undergoing surgical procedures in total intravenous anesthesia. In the model studies, the controller responded to step changes between 48 seconds and 81 seconds. It did exhibit an overshoot between 5.5% and 7.9% of the setpoint after the step change. The maximal variation of V gA' in steady-state was between ±4.4% and ±5.6% of the setpoint value after the step change. In the patient study, the controller maintained the set V gA' and adapted the breathing pattern to the respiratory mechanics of each individual patient. View full abstract»

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  • Refractory period prolongation by biphasic defibrillator waveforms is associated with enhanced sodium current in a computer model of the ventricular action potential

    Page(s): 60 - 68
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (995 KB)  

    Mechanisms through which biphasic waveforms lower defibrillation threshold are unknown. Previous work showed that low-intensity biphasic shocks (BS2), delivered during the refractory period of a control action potential (S1), produced significantly longer responses than monophasic shocks (MS2). To test the hypothesis that longer responses are due to hyperpolarization-induced excitation channel recovery during the first portion of the biphasic waveform, the authors used the Beeler-Reuter ventricular action potential computer model with the Drouhard-Roberge (BRDR) modification to study refractory period stimulation with MS2 (10 msec) and symmetrical BS2 (10 msec each pulse). At 1.5 times diastolic threshold, BS2 prolonged action potential duration when delivered 50 msec into the S1 refractory period, and produced a maximum BS2 versus MS2 response duration difference of 62 msec. Longer BS2 responses corresponded to enhanced BS2-induced sodium current compared to MS2. Maximum BS2 vs MS2 sodium current difference was 400 uA/cm 2. These results show that, in a computer model of the ventricular action potential, hyperpolarization by the first phase of a biphasic waveform enhances S2 sodium current and prolongs duration of refractory-period responses. This effectively shortens the cellular refractory period. Prolonged refractory period responses, produced by biphasic defibrillator waveforms, may underlie enhanced defibrillating efficacy at low shock intensities. View full abstract»

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  • Transfer-function analysis of UFCT myocardial time-density curves by time-varying recursive least squares analysis

    Page(s): 69 - 76
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    Techniques which assume linear, time-invariant systems have been used to characterize indicator dilution pairs. As a basis for fully describing the relation between left ventricular (LV) and myocardial (MYC) time-density curves, produced by an intravenous contrast medium as measured by ultrafast CT, the assumption of time invariance was tested using recursive least squares regression and CUSUM, a test for time variability of regression parameters. Using data from anaesthetized dogs with concomitant microsphere information, constant and time-varying regression models, MYC(t)=b(t)LV(t−1), were generated from time-density curves of flows from two groups: Group 1 (MBF<2 ml/min/gm, n=11) and Group 2 (MBF>2 ml/min/gm, n=10). The time-varying regression models had reduced root mean square error: 0.6±1.1 and 0.5±0.8 versus 7.3±3.5 and 4.1±1.6 for Groups 1 and 2, respectively. Significant time variability (p<0.05) by CUSUM was found in 9/11 Group 1 models and 7/10 Group 2 models. Myocardial blood volume was estimated as the average value of b(t) over the rising portion of the LV curve. Myocardial blood flow was then calculated as myocardial blood volume divided by coronary transit time, determined from gamma variate fits of the LV and scaled, shifted LV curve, with excellent results over a wide range of flows (r=0.93, y=0.92x+0.28, range of 0.4 to 6.7 ml/min/gm). These results show that measurements of increased myocardial blood flow are possible with an intravenous contrast media, and that movement of contrast medium from intravascular space to extravascular space occurs during the course of the contrast medium's first pass. View full abstract»

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  • Time-frequency digital filtering based on an invertible wavelet transform: an application to evoked potentials

    Page(s): 77 - 88
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    Presents a method to analyze and filter digital signals of finite duration by means of a time-frequency representation. This is done by defining a purely invertible discrete transform, representing a signal either in the time or in the time-frequency domain, as simply as possible with the conventional discrete Fourier transform between the time and the frequency domains. The wavelet concept has been used to build this transform. To get a correct invertibility of this procedure, the authors have proposed orthogonal and periodic basic discrete wavelets. The properties of such a transform are described, and examples on brain-evoked potential signals are given to illustrate the time-frequency filtering possibilities. View full abstract»

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  • Classification of action potentials in multi-unit intrafascicular recordings using neural network pattern-recognition techniques

    Page(s): 89 - 91
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    Neural network pattern-recognition techniques were applied to the problem of identifying the sources of action potentials in multi-unit neural recordings made from intrafascicular electrodes implanted in cats. The network was a three-layer connectionist machine that used digitized action potentials as input. On average, the network was able to reliably separate 6 or 7 units per recording. As the number of units present in the recording increased beyond this limit, the number separable by the network remained roughly constant. The results demonstrate the utility of neural networks for classifying neural activity in multi-unit recordings. View full abstract»

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  • Ultrasound scattering from blood with hematocrits up to 100%

    Page(s): 91 - 95
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (605 KB)  

    The backscattering coefficient of saline suspensions of porcine red blood cells was measured for hematocrits up to about 90%. It was found that the coefficient peaks at approximately 15%, but then, contrary to what a simple "gap theory" might suggest, it decays smoothly to zero, without showing another peak at high hematocrits. A one-dimensional (1D) slab scattering model, in which the number of slabs per unit length represents the hematocrit and whose thickness and acoustical properties are similar to red cells/plasma, was also used to investigate the relation between the backscattered power and hematocrit. Monte-Carlo simulations performed for randomized boundary conditions show a similar relation to that of the 3D system. The experimental data is compared to the Percus-Yevick theory for the packing of hard spheres, and the simulated data is compared to the Percus-Yevick theory for infinite slabs. 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