Showing papers in "IEEE Transactions on Biomedical Engineering in 1996"
TL;DR: Evaluating the relative sensitivity of center-of-pressure (COP)-based measures to changes in postural steadiness related to age found mean velocity of the COP was the only measure that identified age-related changes in both eye conditions, and differences between eye conditions in both age groups.
Abstract: Measures of postural steadiness are used to characterize the dynamics of the postural control system associated with maintaining balance during quiet standing. The objective of this study was to evaluate the relative sensitivity of center-of-pressure (COP)-based measures to changes in postural steadiness related to age. A variety of time and frequency domain measures of postural steadiness were compared between a group of twenty healthy young adults (21-35 years) and a group of twenty healthy elderly adults (66-70 years) under both eyes-open and eyes-closed conditions. The measures that identified differences between the eyes-open and eyes-closed conditions in the young adult group were different than those that identified differences between the eye conditions in the elderly adult group. Mean velocity of the COP was the only measure that identified age-related changes in both eye conditions, and differences between eye conditions in both age groups. The results of this study will be useful to researchers and clinicians using COP-based measures to evaluate postural steadiness.
1,538 citations
TL;DR: A simple extension of a previously reported object recognition technique has been used to implement a six-degree-of-freedom position/orientation estimator for the measurement of knee replacement motion from two-dimensional fluoroscopic images, indicating it is uniquely well suited for performing dynamic kinematic measurements on individuals with knee replacements.
Abstract: A simple extension of a previously reported object recognition technique has been used to implement a six-degree-of-freedom position/orientation estimator for the measurement of knee replacement motion from two-dimensional (2-D) fluoroscopic images. Computer modeling studies and controlled mechanical tests were performed to assess the accuracy of the technique. The results indicate that knee rotations can be measured with an accuracy of approximately one degree and that sagittal plane translations can be measured with an accuracy of approximately 0.5 mm. The measurement technique is uniquely well suited for performing dynamic kinematic measurements on individuals with knee replacements, and for performing comparative studies among subjects with different designs of knee replacements.
608 citations
TL;DR: It is demonstrated that the coupling coefficient can be considerably enhanced, if the turns of the coils are not concentrated at the circumferences, but distributed across the diameters.
Abstract: This paper presents a geometric approach for the enhancement of the coupling coefficient between two magnetically coupled coils. It is demonstrated that the coupling coefficient can be considerably enhanced, if the turns of the coils are not concentrated at the circumferences, but distributed across the diameters. For analysis, each of the two coils is assumed to be composed of concentric circular loops. The experimental results are in very good agreement with the theoretical results.
401 citations
TL;DR: An image processing technique is presented for finding and localizing the centroids of cylindrical markers externally attached to the human head in computed tomography (CT) and magnetic resonance (MR) image volumes.
Abstract: An image processing technique is presented for finding and localizing the centroids of cylindrical markers externally attached to the human head in computed tomography (CT) and magnetic resonance (MR) image volumes. The centroids can be used as control points for image registration. The technique, which is fast, automatic, and knowledge-based, has two major steps. First, it searches the entire image volume to find one voxel inside each marker-like object. The authors call this voxel a "candidate" voxel, and they call the object a candidate marker. Second, it classifies the voxels in a region surrounding the candidate voxel as marker or nonmarker voxels using knowledge-based rules and calculates an intensity-weighted centroid for each true marker. The authors call this final centroid the "fiducial" point of the marker. The technique was developed on 42 scans of six patients-one CT and six MR scans per patient. There are four markers attached to each patient for a total of 168 marker images. For the CT images the false marker rate was zero. For MR the false marker rate was 1.4% (Two out of 144 markers). To evaluate the accuracy of the fiducial points, CT-MR registration was performed after correcting the MR images for geometrical distortion. The fiducial registration accuracy averaged 0.4 mm and was better than 0.6 mm for each of the eighteen image pairs.
228 citations
TL;DR: An ultrasound indentation system for biomechanical assessment of soft tissues in vivo was developed and the specific application is for stump tissue assessment in the design of prosthetics.
Abstract: An ultrasound indentation system for biomechanical assessment of soft tissues in vivo was developed. The pen-size, hand-held probe was composed of an ultrasound transducer and a load cell. The ultrasound transducer was at the tip of the probe serving also as the indentor. The thickness and deformation of the soft tissue layer were determined from the ultrasound echo. A compressive load cell was connected in series with the ultrasound transducer to record the force response. A validation experiment was performed on porcine tissues. Force and deformation acquired with the present system was in good comparison with those obtained from a Housfield material testing machine. Material constants were obtained via a curve-fitting procedure by predicting the force transient response from the deformation-time data using a quasilinear viscoelastic model. In addition, deformation in the fat and in the muscle could be differentiated. The potential applications of this type of indentation probes are many. The specific application of this current development is for stump tissue assessment in the design of prosthetics.
213 citations
TL;DR: This study determined that shorter PW's allowed more spatially selective stimulation of nerve fibers and generated larger torques before spillover and created a larger dynamic range of currents between threshold and spillover.
Abstract: Choice of stimulus parameters is an important consideration in the design of neural prosthetic systems. The objective of this study was to determine the effect of rectangular stimulus pulsewidth (PW) on the selectivity of peripheral nerve stimulation. Computer simulations using a cable model of a mammalian myelinated nerve fiber indicated that shorter PW's increased the difference between the threshold currents of fibers lying at different distances from an electrode. Experimental measurements of joint torque generated by peripheral nerve stimulation demonstrated that shorter PW's generated larger torques before spillover and created a larger dynamic range of currents between threshold and spillover. Thus, shorter PW's allowed more spatially selective stimulation of nerve fibers. Analysis of the response of a passive cable model to different duration stimuli indicated that PW dependent contributions of distributed sources to membrane polarization accounted for the observed differences in selectivity.
191 citations
TL;DR: A morphological model of colon tissue fluorescence is developed which connects the clinically observed spectra with their underlying microscopic origins and quantitated these spectral features in terms of the intrinsic fluorescence properties of tissue and its general histological characteristics.
Abstract: Fluorescence spectroscopy of tissue is a promising technique for early detection of precancerous changes in the human body. Investigation of the microscopic origin of the clinically observed tissue fluorescence can provide valuable information about the tissue's histology. The objective of this study was the development of a morphological model of colon tissue fluorescence which connects the clinically observed spectra with their underlying microscopic origins. Clinical colon tissue fluorescence spectra were modeled by measuring the intrinsic fluorescence properties of colon tissue on a microscopic level and by simulating light propagation in tissue using the Monte-Carlo method. The computed spectra were in good agreement with the clinical spectra acquired during colonoscopy, and exhibited the characteristic spectral features of the in vivo collected spectra. The authors' analysis quantitated these spectral features in terms of the intrinsic fluorescence properties of tissue and its general histological characteristics. The fluorescence intensity difference between normal and adenoma observed in vivo was found to be due to the increased hemoglobin absorption, the reduced mucosal fluorescence intensity, and the absence of submucosal fluorescence in adenomatous polyps. The increased red fluorescence in adenoma was found to be associated with the dysplastic crypt cell fluorescence.
190 citations
TL;DR: A method which incorporates nonlinear alignment of the EPs into the averaging operation is developed, which addresses the problems associated with averaging brain responses evoked through a repetitive application of an external stimulus.
Abstract: Addresses the problems associated with averaging brain responses evoked through a repetitive application of an external stimulus. In order to improve the estimate of the evoked potential (EP) through signal averaging, a method which incorporates nonlinear alignment of the EPs into the averaging operation is developed. The method makes no prior assumptions about the properties of the EP or which response in the set best characterizes the EP to be estimated. The nonlinear alignment procedure is designed to pairwise generate optimally aligned EPs by backtracking along the optimal alignment path. The nonlinear alignment and averaging operations are systematically combined to develop methods to estimate the EP. Results from a series of experiments conducted on simulated and real sets of responses show that, through nonlinear alignment and averaging, the events in the EPs are preserved and the estimates of the EP are quite robust.
183 citations
TL;DR: Numerical simulations indicate that alterations in wall viscoelastic properties and in arterial pressure pulse amplitude may significantly affect the accuracy of pressure estimates, leading to errors as great as 15-20% in the computation of diastolic and systolic arterials pressure.
Abstract: A mathematical lumped parameter model of the oscillometric technique for indirect blood pressure measurement is presented. The model includes cuff compliance, pressure transmission from the cuff to the brachial artery through the soft tissue of the arm, and the biomechanics of the brachial artery both at positive and negative transmural pressure values. The main aspects of oscillometry are simulated i.e., the increase in cuff pressure pulsatility during cuff deflation manoeuvres, the existence of a point of maximum pulsations (about 1.5 mmHg) at a cuff pressure close to mean arterial pressure, and the characteristic ratios for cuff pressure pulsatility at systole and diastole (0.52 and 0.70, respectively, with this model, using basal parameters and an individual set of data for the arterial pressure waveform). Subsequently, the model is used to examine how alterations in some biomechanical factors may prejudice the accuracy of pressure measurement. Numerical simulations indicate that alterations in wall viscoelastic properties and in arterial pressure pulse amplitude may significantly affect the accuracy of pressure estimates, leading to errors as great as 15-20% in the computation of diastolic and systolic arterial pressure. By contrast, changes in arterial pressure mean value and cuff compliance do not seem to have significant influence on the measurement. Evaluation of mean arterial pressure through a characteristic ratio is not robust and may lead to misleading results. Mean arterial pressure may be better evaluated as the lowest pressure at which cuff pulse amplitude reaches a plateau. The obtained results may help to explain the nature of errors which usually limit the reliability of arterial pressure measurement (for instance in the elderly).
182 citations
TL;DR: With a 360 Hz sample rate and an adaptation time of approximately 0.3 s for a 1 mV 60-Hz signal, the adaptive implementation is less complex and introduces less noise, particularly in the ST-segment, into a typical ECG signal.
Abstract: We have investigated the relative performance of an adaptive and nonadaptive 60-Hz notch filter applied to an ECG signal. We evaluated the performance of the two implementations with respect to adaptation rate (or transient response time), signal distortion, and implementation complexity. We also investigated the relative effect of adaptive and nonadaptive 60-Hz filtering on ECG data compression. With a 360 Hz sample rate and an adaptation time of approximately 0.3 s for a 1 mV 60-Hz signal, the adaptive implementation is less complex and introduces less noise, particularly in the ST-segment, into a typical ECG signal. When applied to ECG signals, prior to data compression by average beat subtraction and residual differencing, the residual signal resulting from the adaptively filtered signal had an average entropy 0.31 bits per sample (bps) lower than the unfiltered signal. The nonadaptive 60-Hz filter produced an average entropy decrease of 0.08 bps relative to the unfiltered ECG.
175 citations
TL;DR: This method provides the physician with nonsubjective numerical values for four criteria of malignancy based on the shape and the size analysis of the observed cells and more specifically on the use of geodesy.
Abstract: Presents a new method for automatic recognition of cancerous tissues from an image of a microscopic section. Based on the shape and the size analysis of the observed cells, this method provides the physician with nonsubjective numerical values for four criteria of malignancy. This automatic approach is based on mathematical morphology, and more specifically on the use of geodesy. This technique is used first to remove the background noise from the image and then to operate a segmentation of the nuclei of the cells and an analysis of their shape, their size and their texture. From the values of the extracted criteria, an automatic classification of the image (cancerous or not) is finally operated.
TL;DR: The test results show that the fuzzy ARTMAP neural network can classify cardiac arrhythmias with greater than 99% specificity and 97% sensitivity.
Abstract: The authors have investigated the QRS complex, extracted from electrocardiogram (EGG) data, using fuzzy adaptive resonance theory mapping (ARTMAP) to classify cardiac arrhythmias. Two different conditions have been analyzed: normal and abnormal premature ventricular contraction (PVC). Based on MIT/BIH database annotations, cardiac beats for normal and abnormal QRS complexes were extracted from this database, scaled, and Hamming windowed, after bandpass filtering, to yield a sequence of 100 samples for each QRS segment. From each of these sequences, two linear predictive coding (LPC) coefficients were generated using Burg's maximum entropy method. The two LPC coefficients, along with the mean-square value of the QRS complex segment, were utilized as features for each condition to train and test a fuzzy ARTMAP neural network for classification of normal and abnormal PVC conditions. The test results show that the fuzzy ARTMAP neural network can classify cardiac arrhythmias with greater than 99% specificity and 97% sensitivity.
TL;DR: A two-dimensional (2-D) prototype of a quasi real-time microwave tomographic system was constructed and was utilized to reconstruct images of physiologically active biological tissues such as an explanted canine perfused heart.
Abstract: Microwave tomographic imaging is one of the new technologies which has the potential for important applications in medicine. Microwave tomographically reconstructed images may potentially provide information about the physiological state of tissue as well as the anatomical structure of an organ. A two-dimensional (2-D) prototype of a quasi real-time microwave tomographic system was constructed. It was utilized to reconstruct images of physiologically active biological tissues such as an explanted canine perfused heart. The tomographic system consisted of 64 special antennae, divided into 32 emitters and 32 receivers which were electronically scanned. The cylindrical microwave chamber had an internal diameter of 360 mm and was filled with various solutions, including deionized water. The system operated on a frequency of 2.45 GHz. The polarization of the incident electromagnetic field was linear in the vertical direction. Total acquisition time was less than 500 ms. Both accurate and approximation methods of image reconstruction were used. Images of 2-D phantoms, canine hearts, and beating canine hearts have been achieved. In the worst-case situation when the 2-D diffraction model was used for an attempt to "slice" three-dimensional (3-D) object reconstruction, the authors still achieved spatial resolution of 1 to 2 cm and contrast resolution of 5%.
TL;DR: The Selective Discrete Fourier transform (DFT) Algorithm provides a reliable tool for the evaluation and quantification of the control exerted by the Central Nervous System, during clinical and experimental procedures resulting in nonstationary signals.
Abstract: The Selective Discrete Fourier transform (DFT) Algorithm [SDA] method for the calculation and display of time-frequency distribution has been developed and validated. For each time and frequency, the algorithm selects the shortest required trace length and calculates the corresponding spectral component by means of DFT. This approach can be extended to any cardiovascular related signal and provides time-dependent power spectra which are intuitively easy to consider, due to their close relation to the classical spectral analysis approach. The optimal parameters of the SDA for cardiovascular-like signals were chosen. The SDA perform standard spectral analysis on stationary simulated signals as well as reliably detect abrupt changes in the frequency content of nonstationary signals. The SDA applied during a stimulated respiration experiment, accurately; detected the changes in the frequency location and amplitude of the respiratory peak in the heart rate (HR) spectrum. It also detected and quantified the expected increase in vagal tone during vagal stimuli. Furthermore, the HR time-dependent power spectrum displayed the increase in sympathetic activity and the vagal withdrawal on standing. Such transient changes in HR control would have been smeared out by standard heart rate variability (HRV), which requires consideration of long trace lengths. The SDA provides a reliable tool for the evaluation and quantification of the control exerted by the Central Nervous System, during clinical and experimental procedures resulting in nonstationary signals.
TL;DR: Realistically shaped head models are developed for three subjects with electrical sources implanted at known locations in the brain and localization accuracy is found to be significantly better in the realistically shaped bead models than in spherical models if EEGs with good signal-to-noise ratio are used.
Abstract: A model of the head must be used in making estimates of the locations of electrical sources in the brain using electroencephalograms (EEGs) measured on the scalp. In part, the accuracy of these estimates is dependent on how accurately the model represents the actual head. In most work performed to date, spherical models of the head have been used. This paper presents results in which the estimates of source location are made in realistically shaped head models. Techniques for accurately and conveniently developing realistically shaped head models from CTs, MRIs, X-rays, and/or physical measurements are also presented. Realistically shaped head models are developed for three subjects with electrical sources implanted at known locations in the brain. Localization accuracy is found to be significantly better in the realistically shaped bead models than in spherical models if EEGs with good signal-to-noise ratio are used.
TL;DR: The quantitative evaluation of imaging performance has revealed potential advantages in a two-tiered receiver antenna configuration whose measured field values are more sensitive to target region changes than the typical tomographic type of approach which uses reception sites around the full target region perimeter.
Abstract: A prototype microwave imaging system is evaluated for its ability to recover two-dimensional (2-D) electrical property distributions under transverse magnetic (TM) illumination using multitarget tissue equivalent phantoms. Experiments conducted in a surrounding lossy saline tank, demonstrate that simultaneous recovery of both the real and imaginary components of the electrical property distribution is possible using absolute imaging procedures over a frequency range of 300-700 MHz. Further, image reconstructions of embedded tissue-equivalent targets are found to be quantitative not only with respect to geometrical factors such as object size and location but also electrical composition. Quantitative assessments based on full-width half-height criteria reveal that errors in diameter estimates of reconstructed targets are less than 10 mm in all cases, whereas, positioning errors are less than 1 mm in single object experiments but degrade to 4-10 mm when multiple targets are present. Recovery of actual electrical properties is found to be frequency dependent for the real and imaginary components with background values being typically within 10-20% of their correct size and embedded object having similar accuracies as a percentage of the electrical contrast, although errors as high as 50% can occur. The quantitative evaluation of imaging performance has revealed potential advantages in a two-tiered receiver antenna configuration whose measured field values are more sensitive to target region changes than the typical tomographic type of approach which uses reception sites around the full target region perimeter. This measurement strategy has important implications for both the image reconstruction algorithm where there is a premium on minimizing problem size without sacrificing image quality and the hardware system design which seeks to economize on the amount of measured data required for quantitative image reconstruction while maximizing its sensitivity to target perturbations.
TL;DR: It is concluded that chest expansion can contribute significantly to the conductivity images of lung ventilation and should be taken into account in the interpretation of these images.
Abstract: Electrical Impedance Tomography (EIT) uses surface electrical measurements to image changes in the conductivity distribution within a medium. When used to measure lung ventilation, however, measurements depend both on conductivity changes in the thorax and on rib cage movement. Given that currently available reconstruction techniques assume that only conductivity changes are present, certain errors are introduced. A finite element model (FEM) is used to calculate the effect of chest expansion on the reconstructed conductivity images. Results indicate that thorax expansion accounts for up to 20% of the reconstructed image amplitude and introduces an artifact in the center of the image tending to "move" the reconstructed lungs closer together. Although this contribution varies depending on anatomical factors, it is relatively independent of inspiration depth. For certain applications in which one is only interested in changes in the level of physiological activity, the effect of the expansion can be neglected because it varies linearly with impedance changes. It is concluded that chest expansion can contribute significantly to the conductivity images of lung ventilation and should be taken into account in the interpretation of these images.
TL;DR: A closed-loop functional neuromuscular stimulation system that uses afferent neural activity from muscle spindle fibers as feedback for controlling position of the ankle joint is described.
Abstract: Describes a closed-loop functional neuromuscular stimulation system that uses afferent neural activity from muscle spindle fibers as feedback for controlling position of the ankle joint. Ankle extension against a load was effected by neural stimulation through a dual channel intrafascicular electrode of a fascicle of the tibial nerve that innervated the gastrocnemius muscle. Ankle joint angle was estimated from recordings of tibialis anterior and lateral gastrocnemius spindle fiber activity made with dual channel intrafascicular electrodes. Experiments were conducted in neurally intact anesthetized cats and in unanesthetized decerebrate cats to demonstrate the feasibility of this system. The system was able to reach and maintain a fixed target ankle position in the presence of a varying external moment ranging in magnitude between 7.3 and 22 N-cm opposing the action of the ankle extensor, as well as track a sinusoidal target ankle position up to a frequency of 1 Hz in the presence of a constant magnitude 22- or 37-N-cm external moment.
TL;DR: Simulation with a lumped-parameter model of the cochlea confirms the focusing action of the quadrupole in the layers of the inner ear andField measurements in saline solution and in the scala tympani of guinea pigs show that focusing occurs with the Quadrupolar mode.
Abstract: Cochlear implants are electrically driven in monopolar, bipolar, or common ground mode. Ideally, a quadrupolar mode is created with three colinear electrodes, where the outer poles are half the inverse polarity value of the center electrode. The resulting field is highly focused. Models of point sources show that the quadrupolar paradigm offers a greater choice of parameters to shape the field. Simulation with a lumped-parameter model of the cochlea confirms the focusing action of the quadrupole in the layers of the inner ear. Field measurements in saline solution and in the scala tympani of guinea pigs show that focusing occurs with the quadrupolar mode. It is conceivable that quadrupolar stimulation will affect the pitch place coding, reduce channel interaction and limit facial or tactile stimulation induced by current spread.
TL;DR: The preliminary application of the selected methods to real RR interval time series obtained during some autonomic tests has shown that the TFR are capable to correctly represent the transient of the series in the joint time-frequency domain.
Abstract: Spectral analysis of cardiovascular series has been proposed as a noninvasive tool for investigating the autonomic control of the cardiovascular system. The analysis of such series during autonomic tests requires high resolution estimators that are capable to track the transients of the tests. A comparative evaluation has been made among classical (FFT based), autoregressive (both block and sequential mode) and time-frequency representation (TFR) based power spectral estimators. The evaluation has been performed on artificial data that have typical patterns of the nonstationary series. The results documented the superiority of the TFR approach when a sharp time resolution is required. Moreover, the test on a RR-like series has shown that the smoothing operation is effective for rejecting TFR crossterms when a simple, two-three components series is concerned. Finally, the preliminary application of the selected methods to real RR interval time series obtained during some autonomic tests has shown that the TFR are capable to correctly represent the transient of the series in the joint time-frequency domain.
TL;DR: A preliminary investigation of a wavelet packet based algorithm for the compression of single lead ECG is presented, which generates significantly lower data rates with less than one-third the computational effort.
Abstract: A preliminary investigation of a wavelet packet based algorithm for the compression of single lead ECG is presented. The algorithm combines the efficiency and flexibility of wavelet packet expansions with the methodology of the Karhunen-Loeve transform (KLT). For selected records from the MIT-BIH arrhythmia database, an average data rate of 184.7 bits per second, corresponding to a compression ratio of 21.4:1, is achieved. When compared with the KLT applied to the same data, the wavelet packet algorithm generates significantly lower data rates with less than one-third the computational effort.
TL;DR: Transverse current from an anode positioned opposite the stimulating cathode was found to improve spatial selectivity, and selectivity was enhanced when the ratio of transverse current to longitudinal current was increased.
Abstract: The degree of spatial selectivity which can be obtained with longitudinal dot tripoles in an insulating cuff was quantified in terms of the overlap between fiber populations activated by different tripoles. Previous studies have failed to take into account the relative influences of transverse current and longitudinal current on position-selective activation, and furthermore have not controlled for the differing sensitivities of large and small nerve fibers to electrical stimuli. In this study, these factors were taken into account. Transverse current from an anode positioned opposite the stimulating cathode was found to improve spatial selectivity, and selectivity was enhanced when the ratio of transverse current to longitudinal current was increased. Large fibers were excited before small fibers, irrespective of fiber position, indicating a combination of position and size selectivity.
TL;DR: The best neuron patterning was observed on substrates produced by the SSR process where cytophilic aminosilane is uniformly deposited and selectively removed from the background, and an average of 77% of background regions were free of neurites or cells connected to the pattern.
Abstract: Toward the goal of creating patterns of primary hippocampal neurons in low density culture, the authors investigated techniques to fabricate microminiature grids of organofunctional silanes on glassy surfaces. A new photoresist (PR) process, Selective Silane Removal (SSR), was developed and compared to two previously developed techniques which use PR and laser patterning. The grid patterns consisted of 27 combinations of path width, length, and intersection (node diameter). The background consisted of squares bounded for the paths. The best neuron patterning was observed on substrates produced by the SSR process where cytophilic aminosilane is uniformly deposited and selectively removed from the background. Controlling water during aminosilane deposition was critical to good neuronal growth and patterning. Oxygen plasma etching of background regions prior to cytophobic phenylsilane binding significantly reduced off-pattern cell growth. Up to 90% of somata grown on these substrates complied to the pattern, and an average of 77% of background regions were free of neurites or cells connected to the pattern. The highest laser energy density, 120 mJ/cm/sup 2/, produced the best compliance on lased substrates, with an average of 35% of background regions free of connected cells and neurites, but considerable variation across the surface. On substrates with excellent patterning, compliance to nodes was found to be dependent on pattern dimensions, with 20-/spl mu/m node diameters and 80-/spl mu/m internodal path lengths increasing compliance.
TL;DR: System nonlinearity model dynamics is found to be highly sensitive to time delay and changes of this parameter within a physiological range cause the model to exhibit different patterns of behavior.
Abstract: The influence of time delay in the baroreflex control of the heart activity is analyzed by using a simple mathematical model of the short-term pressure regulation. The mean arterial pressure in a Windkessel model is controlled by a nonlinear feedback driving a nonpulsatile model of the cardiac pump in accordance with the steady-state characteristics of the arterial baroreceptor reflex. A pure time delay is placed in the feedback branch to simulate the latent period of the baroreceptor regulation. Because of system nonlinearity model dynamics is found to be highly sensitive to time delay and changes of this parameter within a physiological range cause the model to exhibit different patterns of behavior. For low values of time delay (shorter than 0.5 s) the model remains in a steady state. When time delay is longer than 0.5 s, a Hopf bifurcation is crossed and spontaneous oscillations occur with frequencies in the high-frequency (HF) band. Further increases of time delay above 1.2 s cause the oscillations to become more complex, and following the typical Feigenbaum cascade, the system becomes chaotic. In this condition heart rate, pressure, and how show evident variability. The heart rate power spectrum exhibits a peak whose frequency moves from the HF to LF band depending on whether simulated time delay is as short as the vagal-mediated control or long as the sympathetic one.
TL;DR: An experimental study of induced-current electrical impedance tomography verifies that image quality is enhanced by employing six rather than three induction coils by increasing the number of independent measurements, and reconstructs images of two-dimensional conductivity perturbations.
Abstract: An experimental study of induced-current electrical impedance tomography verifies that image quality is enhanced by employing six rather than three induction coils by increasing the number of independent measurements. However, with an increasing number of coils, the inverse problem becomes more sensitive to measurement noise. Using 16 electrodes to measure surface voltages, it is possible to collect 6/spl times/15=90 independent measurements. For comparison purposes, images of two-dimensional conductivity perturbations are reconstructed by using the data for three and six coils with the truncated pseudoinverse algorithm. By searching for the optimal truncation index that minimizes the noise error plus the resolution error, the signal-to-noise ratio of the data acquisition system was established as 58 db. Images obtained with this six-coil system reveal the sizes and locations of the conductivity perturbations. This system also provides images within the central region of the object space, a capability not achieved in previous experimental studies using only three circular coils. Nevertheless, the three-coil system can identify the conductivity perturbations near the periphery. However, it displays shifts in the locations and spread in the sizes of perturbations near the center of the object.
TL;DR: A method to compute L/sub s/ on the surface of an inhomogeneous volume conductor of arbitrary shape resulting from assumed electrical sources is introduced, and in two applications the sensitivity of the body surface Laplacian is compared to that of body surface potentials.
Abstract: The use of the surface Laplacian of the potential (L/sub s/) in bioelectricity is discussed Different estimates of L/sub s/, in particular the field measured by coaxial electrodes, are compared to that of the true Laplacian A method to compute L/sub s/ on the surface of an inhomogeneous volume conductor of arbitrary shape resulting from assumed electrical sources is introduced In two applications the sensitivity of the body surface Laplacian is compared to that of body surface potentials This comparison is carried out for dipolar sources within the human brain as well as for distributed sources within the heart
TL;DR: A combined head-eye tracking system suitable for use with free head movement during natural activities and providing an integrated head and eye position measurement while allowing for a large range of head movement.
Abstract: The authors present a combined head-eye tracking system suitable for use with free head movement during natural activities. This system provides an integrated head and eye position measurement while allowing for a large range of head movement (approx 1.8 m of head translation is tolerated). Six degrees of freedom of head motion and two degrees of freedom of eye motion are measured by the system. The system was designed to be useful for the evaluation of the vestibulo-ocular reflex (VOR). The VOR generates compensatory eye movements in order to stabilize gaze during linear or rotational motion of the head. Current clinical and basic research evaluation of the VOR has used a restricted range of head motion, mainly low-frequency, yaw rotation. An integrated eye-head tracking system such as the one presented here allows the VOR response to linear and angular head motion to be studied in a more physiologically relevant manner. Two examples of the utility of the integrated head and eye tracking system in evaluating the vestibular response to linear and angular motion are presented.
TL;DR: The performance of three methods for evaluation of motion on synthesized 2-D echo image sequences with features similar to real ones are examined and a discussion about the suitability of the above-mentioned techniques for evaluationof motion on real echocardiographic images is presented.
Abstract: The performance of three methods for evaluation of motion on synthesized 2-D echo image sequences with features similar to real ones are examined. The selected techniques based on the computation of optical flow are of the differential type and assume that the image brightness pattern is constant over time. They differ in the choice of the smoothing term and in the local or global treatment of the domain. The images were synthesized by simulating the process of echo formation, considering the interaction between ultrasonic fields and human tissues. Moreover, two different approaches were followed to generate the sequences: (1) a known motion field was applied to the intensity distribution of the synthesized images; (2) a known motion field was applied directly to the point scatterer distribution of the tissue. Favorable results were obtained by applying Lucas-Kanade and Horn-Schunck techniques to the sequences of the first type, while all the techniques produced large errors when applied to the other type of sequences. A discussion about the suitability of the above-mentioned techniques for evaluation of motion on real echocardiographic images is also presented together with some results.
TL;DR: A matched-dipole type catheter antenna is described for intracavitary and/or transluminal microwave treatment of diseases that may be responsive to thermal ablation therapy.
Abstract: A matched-dipole type catheter antenna is described for intracavitary and/or transluminal microwave treatment of diseases that may be responsive to thermal ablation therapy. This cap-choke antenna consists of an annular cap and coaxial choke design that can yield specific absorption rate (SAR) distributions in, and can produce heating of, tissue surrounding the distal end of the catheter antenna. The cap-choke antenna is simple in construction, provides the desired SAR distribution, and is efficient, i.e., the measured power reflection coefficient is 2% in tissue equivalent phantom modeling materials.
TL;DR: A technique for assessing the error associated with the impulse responses calculated from estimated ARMA parameterizations is presented, and a method for obtaining a Zineurizution of this mapping is derived, which leads to a simple procedure to approximate the confidence bounds.
Abstract: This paper presents a new approach to AutoRegressive Moving Average (ARMA or ARX) modeling which automatically seeks the best model order to represent investigated linear, time invariant systems using their inputloutput data. The algorithm seeks the ARMA parameterization which accounts for variability in the output of the system due to input activity and contains the fewest number of parameters required to do so. The unique characteristics of the proposed system identification algorithm are its simplicity and efficiency in handling systems with delays and multiple inputs. We present results of applying the algorithm to simulated data and experimental biological data. In addition, a technique for assessing the error associated with the impulse responses calculated from estimated ARMA parameterizations is presented. The mapping from ARMA coefficients to impulse response estimates is nonlinear, which complicates any effort to construct confidence bounds for the obtained impulse responses. Here a method for obtaining a Zineurizution of this mapping is derived, which leads to a simple procedure to approximate the confidence bounds.