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Biomedical Engineering, IEEE Transactions on

Issue 2 • Date Feb. 1995

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Displaying Results 1 - 13 of 13
  • Combined ultrasound and fluorescence spectroscopy for physico-chemical imaging of atherosclerosis

    Page(s): 121 - 132
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    Describes a combined ultrasonic and spectroscopic system for remotely obtaining physico-chemical images of normal arterial tissue and atherosclerotic plaque. Despite variations in detector-tissue separation, R, fluorescence powers corresponding to pixels in the image are converted to the same set of calibrated units using distance estimations from A-mode ultrasound reflection times. An empirical model, validated by Monte Carlo simulations of light propagation in tissue, is used to describe changes in fluorescence power as a function of R. Fluorescence spectra of normal and atherosclerotic human aorta obtained with this system are presented as a function of R. To compensate for changes in fluorescence power with R, the empirical model was used in each case to calculate the fluorescence power at a constant reference value of R(R ref=1.67 mm). Prior to compensation, tissue fluorescence power decreased more than a factor of two as R was increased from 2.5 to 5 mm. Following compensation. The fluorescence power varied less than ±10% of the average compensated peak. The chemical composition of each sample was determined by fitting its fluorescence spectrum (in calibrated units) to a model of tissue fluorescence incorporating structural protein and ceroid fluorescence, as well as structural protein and hemoglobin attenuation. Parameters of the fit were used to classify tissue type. Without compensation for distance variation, classification of tissue type was frequently incorrect; however, with compensation, predictive value was high. A 1D chemical image of a section of human aorta containing both normal and atherosclerotic regions obtained with this system is also presented. After compensation for detector-sample separation, tissue classifications along the cross-section closely resemble those obtained from histology. Regions of elevated ceroid concentration and intimal thickening are clearly observable in the resultant chemical image. The potential value - - of this type of system in the diagnosis and treatment of coronary artery disease is discussed. View full abstract»

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  • A real-time electrical impedance tomography system for clinical use-design and preliminary results

    Page(s): 133 - 140
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    An instrument is described which produces images of the electrical impedance distribution within the body at a rate of 25 frames per second, allowing lung ventilation and lung perfusion to be observed in real time. The instrument makes impedance measurements using an array of 16 electrodes on the surface of the body, and reconstructs the images using a weighted backprojection technique. The design of the data acquisition electronics and the reconstruction and display processor are described. Some preliminary in vitro and in vivo results from the system are presented. View full abstract»

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  • Multiple sources of the impedance cardiogram based on 3-D finite difference human thorax models

    Page(s): 141 - 148
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    Two 3D electrical models of the human thorax, each consisting of 216,000 control volumes, were constructed based upon MR images taken at end diastole and end systole. Using the finite difference method, the contributions of various sources to the impedance cardiogram were studied for the traditional band electrode configuration. The contributions were categorized into three areas: 1) the structural changes between end diastole and end systole, 2) the flow-induced blood resistivity changes in major arteries and veins, and 3) the lung resistivity variation due to the lung blood volume change. Based on the models, Z 0 and ΔZ between end diastole and end systole were 24.4 Ω and -0.132 Ω, as compared with the measurements of 21.8 Ω and -0.123 Ω made on the same subject from whom the images were taken. Arterial and venous blood resistivity changes caused approximately 57% of the total impedance change. The lung resistivity change and the structural changes contributed 39% and 4%, respectively. The structural changes inside the thorax included the dimensional changes of blood vessels, the blood volume changes of the heart chambers, and heart movement. Although the net impedance change due to the structural changes was relatively small, the individual variation of various factors was large, with significant cancellation occurring. The results suggest that the thoracic impedance cardiographic signal is a mixed representation of many inseparable factors and may not be reliable for the stroke volume calculation. Also, the O-wave, which is clinically observed in various cardiac conditions, may be linked to the diastolic blood flow in the central veins. View full abstract»

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  • Average-intensity reconstruction and Wiener reconstruction of bioelectric current distribution based on its estimated covariance matrix

    Page(s): 149 - 157
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    Proposes two methods for reconstructing current distributions from biomagnetic measurements. Both of these methods are based on estimating the source-current covariance matrix from the measured-data covariance matrix. One method is the reconstruction of average current intensity distributions. This method first estimates the source-current covariance matrix and, using its diagonal terms, it reconstructs current intensity distributions averaged over a certain time. Although the method does not reconstruct the orientation of each current element at each time instant, it can retrieve information regarding the current time-averaged intensity at each voxel location using extremely low SNR data. The second method is Wiener reconstruction using the estimated source-current covariance matrix. Unlike the first method, this Wiener reconstruction can provide a current distribution with its orientation at each time instant. Computer simulation shows that the Wiener method is less affected by the choice of the regularization parameter, resulting in a method that is more effective than the conventional minimum-norm method when the SNR of the measurement is low. View full abstract»

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  • An analytical model to predict the electric field and excitation zones due to magnetic stimulation of peripheral nerves

    Page(s): 158 - 161
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    The main unknown factor in understanding magnetic stimulation of peripheral nerves is the distribution of the induced electric field. The authors have applied the so-called reciprocity theorem and developed an analytical model to compute the electric field and its spatial derivatives inside pseudocylindrical structures. The results can be used to predict the site of excitation in magnetic stimulation of peripheral nerves. View full abstract»

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  • The effect of morphological interdigitation on field coupling between smooth muscle cells

    Page(s): 162 - 171
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    The electrical control activity (ECA) in the distal stomach, small intestine, and colon has been modeled by populations of coupled nonlinear oscillators. Coupling has traditionally been explained through gap junctions, but gap junctions alone are inadequate, as they are not always present or cannot account for the observed behavior. Coupling through extracellular electric fields has been proposed as another coupling path which may work instead of, or in conjunction with, gap junctions. A morphological structure, the interdigitation, is studied for its effect on fields produced by a spherical cell. Using boundary element methods, the potential produced by a cell and the transmembrane potential induced in an adjacent cell are considered. Computer simulation results indicate that the presence of an interdigitation between two neighboring cells produces a 60% increase in extracellular potential and a 50% increase in induced transmembrane voltage. The interdigitation length is the most important factor, with radius playing a very small part in determining peak values of potential and voltage. These interdigitation fields may be of appreciable magnitude with regard to coupling. Also, the upstroke phase of the ECA can play a major role in intercellular communication. View full abstract»

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  • Computational studies of transthoracic and transvenous defibrillation in a detailed 3-D human thorax model

    Page(s): 172 - 184
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    A method for constructing and solving detailed patient-specific 3D finite element models of the human thorax is presented for use in defibrillation studies. The method utilizes the patient's own X-ray CT scan and a simplified meshing scheme to quickly and efficiently generate a model typically composed of approximately 400,000 elements. A parameter sensitivity study on one human thorax model to examine the effects of variation in assigned tissue resistivity values, level of anatomical detail included in the model, and number of CT slices used to produce the model is presented. Of the seven tissue types examined, the average left ventricular (LV) myocardial voltage gradient was most sensitive to the values of myocardial and blood resistivity. Incorrectly simplifying the model, for example modeling the heart as a homogeneous structure by ignoring the blood in the chambers, caused the average LV myocardial voltage gradient to increase by 12%. The sensitivity of the model to variations in electrode size and position was also examined. Small changes (<2.0 cm) in electrode position caused average LV myocardial voltage gradient values to increase by up to 12%. It is concluded that patient-specific 3D finite element modeling of human thoracic electric fields is feasible and may reduce the empiric approach to insertion of implantable defibrillators and improve transthoracic defibrillation techniques. View full abstract»

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  • Optimization of cardiac defibrillation by three-dimensional finite element modeling of the human thorax

    Page(s): 185 - 192
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    The goal of this study was to determine the optimal electrode placement and size to minimize myocardial damage during defibrillation while rendering refractory a critical mass of cardiac tissue of 100%. For this purpose, the authors developed a 3D finite element model with 55,388 nodes, 50,913 hexahedral elements, and simulated 16 different organs and tissues, as well as the properties of the electrolyte. The model used a nonuniform mesh with an average spatial resolution of 0.8 cm in all three dimensions, To validate this model, the authors measured the voltage across 3-cm 2 Ag-AgCl electrodes when currents of 5 mA at 50 kHz were injected into a human subject's thorax through the same electrodes. For the same electrode placements and sizes and the same injected current, the finite element analysis produced results in good agreement with the experimental data. For the optimization of defibrillation, the authors tested 12 different electrode placements and seven different electrode sizes. The finite element analyses showed that the anterior-posterior electrode placement and an electrode size of about 90 cm 2 offered the least chance of potential myocardial damage and required a shock energy of less than 350 J for 5-ms defibrillation pulses to achieve 100% critical mass. For comparison. The average cross-sectional area of the heart is ≈48 cm 2, about half of the optimal area. A second best electrode placement was with the defibrillation electrodes on the midaxillary lines under the armpits. Although this placement had higher chances of producing cardiac damage, it required less shock energy to achieve 100% critical mass. View full abstract»

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  • Estimation of shear modulus distribution in soft tissue from strain distribution

    Page(s): 193 - 202
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    In order to obtain noninvasively quantitative static mechanical properties of living tissue, the authors propose a new type of inverse problem by which the spatial distribution of the relative elastic modulus of the tissue can be estimated only from the deformation or strain measurement. The living tissue is modeled as a linear isotropic incompressible elastic medium which has the spatial distribution of the shear modulus, and the deformation or strain is supposedly measured ultrasonically. Assuming that there is no mechanical source in the region of interest, the authors derive a set of linear equations in which unknowns are the spatial derivatives of the relative shear modulus, and the coefficients are the strain and its spatial derivatives. By solving these equations, the spatial derivatives of the relative shear modulus are determined throughout the region, from which the spatial distribution of the relative shear modulus is obtained by spatial integration. The feasibility of this method was demonstrated using the simulated deformation data of the simple inclusion problem. The proposed method seems promising for the quantitative differential diagnosis on the lesion in the tissue in vivo. View full abstract»

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  • Multiple site electromyograph amplitude estimation

    Page(s): 203 - 211
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    Temporal whitening of individual surface electromyograph (EMG) waveforms and spatial combination of multiple recording sites have separately been demonstrated to improve the performance of EMG amplitude estimation. This investigation combined these two techniques by first whitening, then combining the data from multiple EMG recording sites to form an EMG amplitude estimate. A phenomenological mathematical model of multiple sites of the surface EMG waveform, with analytic solution for an optimal amplitude estimate, is presented. Experimental surface EMG waveforms were then sampled from multiple sites during nonfatiguing, constant-force, isometric contractions of the biceps or triceps muscles, over the range of 10-75% maximum voluntary contraction. A signal-to-noise ratio (SNR) was computed from each amplitude estimate (deviations about the mean value of the estimate were considered as noise). Results showed that SNR performance: 1) increased with the number of EMG sites, 2) was a function of the sampling frequency, 3) was predominantly invariant to various methods of determining spatial uncorrelation filters, 4) was not sensitive to the intersite correlations of the electrode configuration investigated, and 5) was best at lower levels of contraction. A moving average root mean square estimator (245-ms window) provided an average ± standard deviation (A±SD) SNR of 10.7±3.3 for single site unwhitened recordings. Temporal whitening and four combined sites improved the A±SD SNR to 24.6±10.4. On one subject, eight whitened combined sites were achieved, providing an A±SD SNR of 35.0±13.4. View full abstract»

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  • Muscle-joint models incorporating activation dynamics, moment-angle, and moment-velocity properties

    Page(s): 212 - 223
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    Muscle input/output models incorporating activation dynamics, moment-angle, and moment-velocity factors are commonly used to predict the moment produced by muscle during nonisometric contractions: the three factors are generally assumed to be independent. The authors examined the ability of models with independent factors, as well as models with coupled factors, to fit input/output data measured during simultaneous modulation of the fraction of muscle stimulated (recruitment) and joint angle inputs. The models were evaluated in stimulated cat soleus muscles producing ankle extension moment, with regard to their potential applications in neuroprostheses with either fixed parameters or parameter adaptation. Both uncoupled and coupled models predicted the output moment well for random angle perturbation sizes ranging from 10° to 30°. For the uncoupled model, the best parameter values depended on the range of perturbations and the mean angle. Introducing coupling between activation and velocity in the model reduced this parameter sensitivity; one set of model parameter values fit the data for all perturbation sizes and also fit the data under isometric or constant stimulation conditions. Thus, the coupled model would be the most appropriate for applications requiring fixed parameter values. In contrast, with continuous parameter adaptation, errors due to changing test conditions decreased more quickly for the uncoupled model, suggesting that it would perform well in adaptive control of neuroprostheses. View full abstract»

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  • An advanced signal processing technique for impedance cardiography

    Page(s): 224 - 230
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    A new design using the latest technique in signal processing, the time-frequency analysis method, was developed to process impedance cardiography signals. This technique, when used to determine the relevant calculation parameters, was found to be more accurate than conventional methods. It was shown to be advantageous in reducing ventilation artifacts and motion noise, resulting in greater accuracy. Its cardiac output values had a much better correlation coefficient when compared in the clinical setting to the standard thermodilution technique than did the values from conventional impedance cardiography devices. View full abstract»

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  • An extensive Markov system for ECG exact coding

    Page(s): 230 - 232
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    An extensive Markov process, which considers both the coding redundancy and the intersample redundancy, is presented to measure the entropy value of an ECG signal more accurately. It utilizes the intersample correlations by predicting the incoming n samples based on the previous m samples which constitute an extensive Markov process state. Theories of the extensive Markov process and conventional n repeated applications of m-th order Markov process are studied first. After that, they are realized for ECG exact coding. Results show that a better performance can be achieved by the authors' system. The average code length for the extensive Markov system on the second difference signals was 2.512 b/sample, while the average Huffman code length for the second difference signals was 3.326 b/sample. 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