Showing papers in "IEEE Transactions on Biomedical Engineering in 1990"

An interactive graphics-based model of the lower extremity to study orthopaedic surgical procedures

Abstract: A model is developed of the human lower extremity to study how changes in musculoskeletal geometry and musculotendon parameters affect muscle force and its moment about the joints. The lines of action of 43 musculotendon actuators were defined based on their anatomical relationships to three-dimensional bone surface representations. A model for each actuator was formulated to compute its isometric force-length relation. The kinematics of the lower extremity were defined by modeling the hip, knee, ankle, subtalar, and metatarsophalangeal joints. Thus, the force and joint moment that each musculotendon actuator develops can be computed for any body position. The joint moments calculated with the model compare well with experimentally measured isometric joint moments. A graphical interface to the model has also been developed. It allows the user to visualize the musculoskeletal geometry and to manipulate the model parameters to study the biomechanical consequences of orthopaedic surgical procedures. For example, tendon transfer and lengthening procedures can be simulated by adjusting the model parameters according to various surgical techniques. Results of the simulated surgeries can be analyzed quickly in terms of postsurgery muscle forces and other biomechanical variables. >

A comparison of the noise sensitivity of nine QRS detection algorithms

Abstract: The noise sensitivities of nine different QRS detection algorithms were measured for a normal, single-channel, lead-II, synthesized ECG corrupted with five different types of synthesized noise: electromyographic interference, 60-Hz power line interference, baseline drift due to respiration, abrupt baseline shift, and a composite noise constructed from all of the other noise types. The percentage of QRS complexes detected, the number of false positives, and the detection delay were measured. None of the algorithms were able to detect all QRS complexes without any false positives for all of the noise types at the highest noise level. Algorithms based on amplitude and slope had the highest performance for EMG-corrupted ECG. An algorithm using a digital filter had the best performance for the composite-noise-corrupted data. >

Charge density and charge per phase as cofactors in neural injury induced by electrical stimulation

Abstract: Stimulating electrodes of various sizes were used to investigate the interactions of two stimulus parameters, charge density and charge per phase, in determining the threshold of neural injury induced by electrical stimulation. Platinum electrodes ranging in size from 0.002 to 0.5 cm/sup 2/ were implanted over the parietal cortex of adult cats. Ten days after implantation, the electrodes were pulsed continuously for 7 h using charge-balanced, current-regulated, symmetric pulse pairs 400 mu s per phase in duration and at a repetition rate of 50 Hz. The results show that charge density (as measured at the surface of the stimulating electrode) and charge per phase interact in a synergistic manner to determine the threshold of stimulation-induced neural injury. This interaction occurs over a wide range of both parameters: for charge density from at least 10 to 800 mu C/cm/sup 2/, and for charge per phase from at least 0.05 to 5.0 mu C per phase. The significance of these findings in elucidating the mechanisms underlying stimulation-induced injury is discussed. >

ECG data compression techniques-a unified approach

Abstract: Electrocardiogram (ECG) compression techniques are compared, and a unified view of these techniques is established. ECG data compression schemes are presented in two major groups: direct data compression and transformation methods. The direct data compression techniques are ECG differential pulse code modulation (DPCM) and entropy coding, AZTEC, Turning-point, CORTES, Fan and SAPA algorithms, peak-picking, and cycle-to-cycle compression methods. The transformation methods include Fourier, Walsh, and Karhunen-Loeve transforms. The theoretical bases behind the direct ECG data compression schemes are presented and classified into three categories: tolerance-comparison compression, DPCM, and entropy coding methods. A framework for evaluation and comparison of ECG compression schemes is presented. >

An approach to cardiac arrhythmia analysis using hidden Markov models

Abstract: A new approach to ECG arrhythmia analysis is described. It is based on hidden Markov modeling (HMM), a technique successfully used since the mid 1970s to model speech waveforms for automatic speech recognition. Many ventricular arrhythmias can be classified by detecting and analyzing QRS complexes and determining R-R intervals. Classification of supraventricular arrhythmias, however, often requires detection of the P wave in addition to the QRS complex. The HMM approach combines structural and statistical knowledge of the ECG signal in a single parametric model. Model parameters are estimated from training data using an iterative, maximum-likelihood reestimation algorithm. Initial results suggest that this approach can provide improved supraventricular arrhythmia analysis through accurate representation of the entire beat, including the P-wave. >

A new mode of communication between man and his surroundings

Abstract: The feasibility of establishing an alternative mode of communication between man and his surroundings was studied. The form of communication proposed uses only the subject's brain waves, with no overt physical action required. The subject's electroencephalograms (EEG) were recorded while various mental tasks designed to elicit hemispheric responses were performed. Features formed from the EEG recording were then used as inputs into a Bayes quadratic classifier to test classification accuracy between the various tasks. The results obtained indicate that it is possible to accurately distinguish between any two of the five tasks investigated. A comparison between three different methods for creating the feature sets is also presented. >

A model of the stimulation of a nerve fiber by electromagnetic induction

Abstract: A model is presented to explain the physics of nerve stimulations by electromagnetic induction. Maxwell's equations predict the induced electric field distribution that is produced when a capacitor is discharged through a stimulating coil. A nonlinear Hodgkin-Huxley cable model describes the response of the nerve fiber to this induced electric field. Once the coil's position, orientation, and shape are given and the resistance, capacitance, and initial voltage of the stimulating circuit are specified, this model predicts the resulting transmembrane potential of the fiber as a function of distance and time. It is shown that the nerve fiber is stimulated by the gradient of the component of the induced electric field that is parallel to the fiber, which hyperpolarizes or depolarizes the membrane and may stimulate an action potential. The model predicts complicated dynamics such as action potential annihilation and dispersion. >

Electrical stimulation with Pt electrodes. VIII. Electrochemically safe charge injection limits with 0.2 ms pulses (neuronal application)

Abstract: For pt.VII see J. Neurosci. Methods, vol.9, p.301-8 (1983). The charge injection limits of a Pt electrode using 0.2 ms charge balanced, biphasic current pulses range from 50 to 150 mu C/cm/sup 2/ geometric if the potential excursions of the electrode are kept below those at which H/sub 2/ or O/sub 2/ are produced. These charge densities are three to ten times smaller than the currently accepted value based on earlier experiments in which the reversible surface reactions were fully utilized and the pulse widths were longer. Several qualifying comments are made regarding the experimental results. >

High-efficiency coupling-insensitive transcutaneous power and data transmission via an inductive link

Abstract: A new approach is presented for transmitting RF power and signal via an inductive link. Such an approach optimizes the power efficiency of the overall transmission scheme comprising the power amplifier plus the inductive link. Power amplification is based on the single ended class E concept. The power amplification stage is self-oscillating, and thus the oscillation frequency is influenced by the coupling of the coils. The resulting operating frequency offset yields improved power transmission performance of the circuit, since the oscillation frequency tracks the absolute transmission efficiency maximum. A detailed analysis is given. Realization of the approach requires a minimal number of circuit components. Experimental and theoretical results are in good agreement. >

Restoring unassisted natural gait to paraplegics via functional neuromuscular stimulation: a computer simulation study

Abstract: A computer simulation of functional neuromuscular stimulation (FNS)-assisted bipedal gait shows that it is difficult, but possible, to attain undisturbed, level gait at normal speeds provided the electrically stimulated ankle plantarflexors exhibit near-normal strengths or are augmented by an orthosis, and at least seven muscle groups in each leg are stimulated. A combination of dynamic programming and an open-loop, trial-and-error adjustment process was used to find a suboptimal set of discretely varying muscle stimulation patterns needed for a 3-D, 8 degree-of-freedom dynamic model to sustain a step. An ankle-foot orthosis was found to be especially useful, as it helped to stabilize the stance leg and simplified the task of controlling the foot during swing. It is believed that the process of simulating natural gait with this model will serve to highlight difficulties to be expected during laboratory and clinical trials. >

Ventricular tachycardia and fibrillation detection by a sequential hypothesis testing algorithm

Abstract: An algorithm for detecting ventricular fibrillation (VF) and ventricular tachycardia (VT) by the method of sequential hypothesis testing is presented. The algorithm first generates a binary sequence by comparing the signal to a threshold. The probability distribution of the time intervals of the binary sequence is obtained, and the sequential hypothesis testing procedure of A.J. Wald and J. Wolfowitz (1948) is employed to discriminate the arrhythmias. Sequential hypothesis testing of 85 cases resulted in identification of (1) 97.64% VF and 97.65% VT episodes after 5 s and (2) 100% identification of both VF and VT after 7 s. The desired false positive and false negative error probabilities can be programmed into the algorithm. An important feature of the sequential method is that extra time for detection can be traded off for improved accuracy, and vice versa. >

A nerve cuff technique for selective excitation of peripheral nerve trunk regions

Abstract: The numerical modeling and experimental testing of a nerve cuff technique for selective stimulation of superficial peripheral nerve trunk regions are presented. Two basic electrode configurations ('snug' cuff monopolar and tripolar longitudinally aligned dots) have been considered. In addition, the feasibility of steering excitation into superficial nerve trunk regions using subthreshold levels of current flow from an electrode dot located on the opposite side of the nerve has been tested. The modeling objectives were to solve for the electric field that would be generated within a representative nerve trunk by each electrode configuration and to use a simple nerve cable model to predict the effectiveness of each configuration in producing localized excitation. In three acute experiments on cat sciatic nerve the objective was to characterize the effectiveness of each electrode configuration in selectively activating only the medial gastrocnemius muscle. Modeling and experimentation both suggest that longitudinally aligned tripolar dot electrodes on the surface of a nerve trunk, and bounded by a layer of insulation (such as a nerve cuff), will restrict excitation to superficial nerve trunk regions more successfully than will monopolar dot electrodes. Excitation steering will improve the spatial selectivity of monopolar and tripolar electrode configurations. >

Sampling frequency of the electrocardiogram for spectral analysis of the heart rate variability

Abstract: A model is developed to describe and quantitate the error in R-R interval measurement due to the finite sampling frequency. The measured R-R interval is modeled as the sum of the true R-R interval and the error measurement. The first- and second-order statistics of the error are computed in order to investigate its influence on the heart-rate-variability (HRV) power spectrum. The statistics are found to be functions only of the ECG sampling frequency, and it is shown that the power spectrum of the error contributes an additive high-pass-filter-like term (colored noise) to the power spectrum of the HRV. The accuracy of the model is tested by a simulation procedure. The model indicates that the relative balance between the HRV and error power spectra is important and should be checked before any variability analysis of heart rate. >

The application of neural networks to myoelectric signal analysis: a preliminary study

Abstract: Two neural network implementations are applied to myoelectric signal (MES) analysis tasks. The motivation behind this research is to explore more reliable methods of deriving control for multi-degree-of-freedom arm prostheses. A discrete Hopfield network is used to calculate the time series parameter for a moving average MES model. It is demonstrated that the Hopfield network is capable of generating the same time series parameters as those produced by the conventional sequential least-squares algorithm. Furthermore, it can be extended to applications utilizing larger amounts of data, and possibly to higher-order time series models, without significant degradation in computational efficiency. The second neural network implementation involves using a two-layer perceptron for classifying a single-site MES on the basis of two features, the first time series parameter and the signal power. >

Automatic stance-swing phase detection from accelerometer data for peroneal nerve stimulation

Abstract: The development of implantable peroneal nerve stimulators has increased interest in sensors which can detect the different phases of walking (stance and swing). Accelerometers with a potential for implantation are studied as detectors for the swing phase of walking to replace footswitches. Theoretically, one can show that accelerometers can be used to distinguish between stance and swing phase. Using accelerometers attached between the ankle and knee joints, the equivalent acceleration of the ankle joint was calculated. This resulted in a typical and reproducible signal in which the different walking phases were identified. Automatic detection algorithms based on cross-correlation calculations were developed and tested. Measurements from four healthy and four hemiplegic subjects resulted in a total of 317 and 272 steps, respectively. The testing of one of the hemiplegic subjects was considered to be a failure due to large disturbances in the acceleration signal during the swing phase of walking, which may be related to the use of crutches. >

Muscle sounds are emitted at the resonant frequencies of skeletal muscle

Abstract: The changes in mechanical resonant frequency of whole muscles during twitch and tetanic contractions are compared to changes in frequency components of the pressure wave produced by muscles during contraction. Resonant frequencies were determined by imposing sinusoidal length changes on a muscle and observing transverse standing waves when the frequency of length change matched the muscle's resonant frequency or a harmonic of the resonant frequency. Acoustic signal instantaneous frequency spectra were calculated using time-frequency transformations including the Wigner transform and the exponential distribution. During a tetanic muscle contraction, the peak instantaneous frequency initially increased and then became constant as the force plateau was reached. The resonant frequencies determined during the force plateau and during relaxation spanned the same range as the peak instantaneous frequency of the acoustic signal. These results suggest that the acoustic signal may be useful as a noninvasive monitor of muscle resonant frequency during contraction. >

Strength characterization of silicon microprobes in neurophysiological tissues

Abstract: Experimentally determined strength characteristics of thin-silicon probes in neural tissues are discussed. It is shown that by proper selection of the substrate length, width, and thickness, silicon substrates can be designed and used to penetrate a variety of biological tissues without breakage or excessive dimpling. Thin-silicon structures have a maximum fracture stress which is a factor of six larger than that of bulk silicon and are very flexible and capable of bending to angles larger than 90 degrees . Silicon substrates 15 mu m thick*30 mu m wide can easily penetrate guinea pig and rat pia arachnoid layers with minimum dimpling and no breakage, while substrates 30 mu m thick*80 mu m wide can penetrate guinea pig and rat dura mater repeatedly without breakage. Quantitative comparison on the relative toughness of neurophysiological tissues in rat and guinea pig have also been experimentally obtained. >

Stability in contractive nonlinear neural networks

Abstract: Models of the form mu x=-x+p+WF(x), where x=x(t) is a vector whose entries represent the electrical activities in the units of a neural network are considered W is a matrix of synaptic weights, F is a nonlinear function, and p is a vector (constant or slowly varying over time) of inputs to the units If the map WF(x) is a contraction, then the system has a unique equilibrium which is globally asymptotically stable; consequently, the network acts as a stable encoder in that its steady-state response to an input is independent of the initial state of the network Considered are some relatively mild restrictions on W and F(x), involving the eigenvalues of W and the derivative of F, that are sufficient to ensure that WF(x) is a contraction It is shown that, in the linear case with spatially homogeneous synaptic weights, the eigenvalues of W are simply related to the Fourier transform of the connection pattern This relation makes it possible, given cortical activity patterns as measured by autoradiographic labeling, to construct a pattern of synaptic weights which produces steady-state patterns showing similar frequency characteristics >

Medical imaging with a microwave tomographic scanner

Abstract: A microwave tomographic scanner for biomedical applications is presented. It consists of a 64-element circular array with a useful diameter of 20 cm. Electronically scanning the transmitting and receiving antennas allows multiview measurements with no mechanical movement. Imaging parameters-a spatial resolution of 7 mm and a contrast resolution of 1% for a measurement time of 3 s-are appropriate for medical use. Measurements on tissue-simulating phantoms and volunteers, together with numerical simulations, are presented to assess the system for absolute imaging of tissue distribution and for differential imaging of physiological, pathological, and induced changes in tissues. >

Scaling limitations of silicon multichannel recording probes

Abstract: The scaling limitations of multichannel recording probes fabricated for use in neurophysiology using silicon integrated circuit technologies are described. Scaled silicon probe substrates 8- mu m thick and 16- mu m wide can be fabricated using boron etch-stop techniques. Theoretical expressions for calculating the thickness and width of silicon substrates have been derived and agree closely with experimental results. The effects of scaling probe dimensions on its strength and stiffness are described. The probe shank dimensions can be designed to vary the strength and stiffness for different applications. The scaled silicon substrates have a fracture stress of about 2*10/sup 10/ dyn/cm/sup 2/, which is about six times that of bulk silicon, and are strong and very flexible. Scaling the feature sizes of recording electrode arrays down to 1 mu m is possible with less than 1% electrical crosstalk between channels. >

Effects of head shape on EEGs and MEGs

Abstract: Results are presented of computer modeling studies of the effects of head shape on electroencephalograms (EEGs) and magnetoencephalograms (MEGs) and on the localization of electrical sources in the brain using these measurements. The effects of general, nonspherical head shape are determined by comparisons of EEG and MEG maps from nonspherical head models with corresponding maps from a spherical head model. The effects on source localization accuracy are determined by calculating moving dipole inverse solutions in a spherical head model using EEGs and MEGs from the nonspherical models and comparing the solutions with those for known sources. It was found that nonspherical head shape can produce significant changes in the maps produced by some sources in the cortical region of the brain but produce localization errors of less than approximately 1 cm. No significant differences in the effects of such deviations on EEGs and MEGs were found. >

In vivo reflectance of blood and tissue as a function of light wavelength

Abstract: The reflectance of light from in vivo tissue is described for wavelengths in the range from 420 to 940 nm, based on photon diffusion theory and on experimental results from studies of 17 subjects. The results show a minimum reflectance and a peak sensitivity to the blood pulsations in the wavelength range from 510 to 590 nm. Skin pigmentation is seen to attenuate reflectance rather than altering the character of the modulation spectra. Based on the model presented, the dependence of modulation spectra on mean blood fractional volume as well as wavelength is described theoretically and corroborated by further experimental data at 570 and 630 nm. At these latter wavelengths, the signal-to-noise ratio is calculated for the blood volume pulsation signal in the presence of physiological noise. The median for calculated ratios of reflectance modulation by blood pulsation and ratios of signal to noise between the two wavelengths are 13.1 and 7.5, respectively, for 93 sites in nine subjects. These results are seen to be consistent with the theory. >

An improved method for localizing electric brain dipoles

Abstract: An improved method for noninvasive localization of equivalent dipoles in the brain is presented and evaluated. The method has been tested and evaluated on humans in vivo. The head is represented by a three-layer spherical model. The potential on any point on the scalp due to any source is found by a closed formula, which is not based on matrix rotations. The formulas will accept any surface electrode as the reference electrode. The least-squares procedure is based on optimal dipoles, reducing the number of unknowns in the iterations from six to three. The method was evaluated by localizing five implanted dipolar sources in the human sensorimotor cortex. The distances between the locations of the sources as calculated by the method and the actual locations were between 0.4 and 2.0 cm. The sensitivity of the method to uncertainties encountered whenever a real head has to be modeled by a three-layer model has also been assessed. >

Functional assessment of control systems for cybernetic elbow prostheses. II. Application of the technique

Abstract: For pt.I see ibid., vol.37, no.11, p.1025-36 (1990). An application of the technique described in pt.I for the functional assessment of two myoelectric elbow-prosthesis controllers is presented: the controller implemented in the Boston Elbow, a state-of-the-art elbow prosthesis, and a proposed control scheme that, to a crude degree, mimics control of the intact elbow. Assessment of the controllers was achieved by evaluating an amputee subject's functional capability as he performed a constrained motion task with a prosthesis that implemented each control scheme. Evaluation of the amputee's capability was accomplished offline using sampled values of myoelectric activity, limb kinematics, and the interface forces at the constraint. Superior performance was observed with the proposed scheme. Namely, the subject exhibited better synergy among elbow and shoulder muscles. Furthermore, when using the proposed controller, some natural patterns of muscle coordination were exhibited. >

Identifiability analysis and parameter identification of an in vivo ligand-receptor model from PET data

Abstract: The identification of the model parameters from data obtained with a single tracer injection leads to disappointing numerical results, since most of the parameters have to be considered as unidentifiable. A protocol including two injections, a first injection of the labeled ligand and a second injection of the cold ligand (displacement experiment), leads to two very different numerical solutions, which is surprising, since such multiplicity of solutions was not indicated by a preliminary theoretical identifiability study. It is shown that a three-injections protocol, including both a displacement and a coinjection experiment, makes it possible to determine which of these two solutions is biologically valid. >

Frequency analysis of the peripheral pulse wave detected in the finger with a photoplethysmograph

Abstract: A photoelectric plethysmograph that performs a frequency analysis of the peripheral volume pulse wave using a portable computer is described. It was used to determine how the pulse shape varied with age using 54 subjects in three age groups: 10-29, 30-59, and 60-89 years. The youngest group had a larger power in the second harmonic (normalized to the fundamental), with p >

An eye movement communication-control system for the disabled

Abstract: The discrete electrooculographic control system (DECS) has been developed as a communication tool for persons with severe handicaps. The system can be used as a means of adaptive control allowing persons with handicaps, especially those with only eye-motor coordination, to live more independent lives. Initial information about the design and capability of the DECS is provided. >

A nonlinear model of the arterial system incorporating a pressure-dependent compliance

Abstract: An examination is made of the consequences of incorporating a pressure dependent compliance in a modified arterial system model. This nonlinear model is evaluated under control and acute pressure-loading conditions. Results show that the nonlinear compliance model in general can more accurately predict the measured pressure waveforms during control and during acute pressure loading. The difference between the predicted waveforms is more pronounced when blood pressure is high and when the pulse pressure is large. >

The dynamic response model of nine different skeletal muscles

Abstract: The frequency response model of nine different skeletal muscles in the hindlimb of the cat was determined with the aid of electrical nerve stimulation, which allows orderly stimulation of motor units concurrently with firing rate increase. It was shown that the general model consists of a linear second-order system with double real poles and a pure time delay. The pole values were different for the different muscles, ranging from 1.55 to 2.8 Hz. Similarly, the pure time delay varied from muscle to muscle, ranging from 8 to 17 ms. Statistical analysis demonstrates that, under isometric contraction with force oscillations in the range of 10-90% of maximal, the model poles are determined, and could be predicted, from the muscle's functional and anatomical location in the limb and from its pennation pattern. >

Blood glucose measurement by multiple attenuated total reflection and infrared absorption spectroscopy

Abstract: The difficulty of measuring physiological concentrations of glucose in blood by conventional infrared absorption spectroscopy is due to the intrinsic high background absorption of water. This limitation can be largely overcome by the use of a CO/sub 2/ laser as an infrared source in combination with a multiple attenuated total reflection (ATR) technique. To demonstrate the applicability of this technique, in vitro measurements of glucose in blood obtained from an experimental infrared laser spectrometer were compared with independent measurements made by a standard YSI 23A laboratory glucose analyzer. The capability of continuous measurement of blood glucose concentration is of primary importance in the future development of a glucose sensor for diabetic patients. >