Showing papers in "IEEE Transactions on Biomedical Engineering in 1991"

Applications of adaptive filtering to ECG analysis: noise cancellation and arrhythmia detection

Abstract: Several adaptive filter structures are proposed for noise cancellation and arrhythmia detection. The adaptive filter essentially minimizes the mean-squared error between a primary input, which is the noisy electrocardiogram (ECG), and a reference input, which is either noise that is correlated in some way with the noise in the primary input or a signal that is correlated only with ECG in the primary input. Different filter structures are presented to eliminate the diverse forms of noise: baseline wander, 60 Hz power line interference, muscle noise, and motion artifact. An adaptive recurrent filter structure is proposed for acquiring the impulse response of the normal QRS complex. The primary input of the filter is the ECG signal to be analyzed, while the reference input is an impulse train coincident with the QRS complexes. This method is applied to several arrhythmia detection problems: detection of P-waves, premature ventricular complexes, and recognition of conduction block, atrial fibrillation, and paced rhythm. >

A silicon-based, three-dimensional neural interface: manufacturing processes for an intracortical electrode array

Abstract: A method is described for the manufacture of a three-dimensional electrode array geometry for chronic intracortical stimulation. This silicon based array consists of a 4.2*4.2*0.12 mm thick monocrystalline substrate, from which project 100 conductive, silicon needles sharpened to facilitate cortical penetration. Each needle is electrically isolated from the other needles, and is about 0.09 mm thick at its base and 1.5 mm long. The sharpened end of each needle is coated with platinum to facilitate charge transfer into neural tissue. The following manufacturing processes were used to create this array: thermomigration of 100 aluminum pads through an n-type silicon block, creating trails of highly conductive p/sup +/ silicon isolated from each other by opposing pn junctions; a combination of mechanical and chemical micromachining which creates individual penetrating needles of the p/sup +/ silicon trails; metal deposition to create active electrode areas and electrical contact pads; and array encapsulation with polyimide. >

Electrotactile and vibrotactile displays for sensory substitution systems

Abstract: Sensory substitution systems provide their users with environmental information through a human sensory channel (eye, ear, or skin) different from that normally used or with the information processed in some useful way. The authors review the methods used to present visual, auditory, and modified tactile information to the skin and discuss present and potential future applications of sensory substitution, including tactile vision substitution (TVS), tactile auditory substitution, and remote tactile sensing or feedback (teletouch). The relevant sensory physiology of the skin, including the mechanisms of normal touch and the mechanisms and sensations associated with electrical stimulation of the skin using surface electrodes (electrotactile, or electrocutaneous, stimulation), is reviewed. The information-processing ability of the tactile sense and its relevance to sensory substitution is briefly summarized. The limitations of current tactile display technologies are discussed, and areas requiring further research for sensory substitution systems to become more practical are suggested. >

A novel method for angle independent ultrasonic imaging of blood flow and tissue motion

Abstract: A simple algorithm for angle independent motion imaging is described. This method requires only one absolute difference operation per pixel, compared to eight operations for normalized cross correlation. Quantitative studies using speckle-generating targets translated by fixed amounts both axially and laterally indicate that the technique tracks moving speckle as accurately as correlation. Color flow images generated from clinical blood and liver data highlight the success of the technique for tracking both large and small motions in two dimensions. The algorithm's suitability for implementation in digital hardware makes possible the development of clinical instruments for angle independent ultrasonic imaging of blood flow and tissue motion in real time. >

A semiclosed-loop algorithm for the control of blood glucose levels in diabetics

Abstract: A theoretical analysis of the control of plasma glucose levels in diabetic individuals is undertaken using a simple mathematical model of the dynamics of glucose and insulin interaction in the blood system. Mathematical optimization techniques are applied to the mathematical model to derive insulin infusion programs for the control of blood levels in diabetic individuals. Based on the results of the mathematical optimization, a semiclosed-loop algorithm is proposed for continuous insulin delivery to diabetic patients. The algorithm is based on three hourly plasma glucose samples. A theoretical evaluation of the effectiveness of this algorithm shows that it is superior to two existing algorithms in controlling hyperglycemia. A glucose infusion term representing the effect of glucose intake resulting from a meal is introduced into the model equations. Various insulin programs for the control of plasma glucose levels following a meal are then assessed. The theoretical results suggest that the most effective short-term control is achieved by an insulin infusion program which incorporates an injection to coincide with the meal. >

Selective activation of small motor axons by quasitrapezoidal current pulses

Abstract: A method to activate electrically smaller nerve fibers without activating larger fibers in the same nerve trunk is proposed. The method takes advantage of the fact that action potentials are blocked with less membrane hyperpolarization in larger fibers than in smaller fibers. In this nerve stimulation system, quasitrapezoidal current pulses are delivered through a tripolar cuff electrode to effect differential blocking membrane hyperpolarization. The quasitrapezoidal pulses with a square leading edge, a 350- mu s plateau, and an exponential trailing phase ensure the blockage of propagating action potentials and prevent the occurrence of anodal break excitation. The tripolar cuff electrode design restricts current flow inside the cuff and thus eliminates the undesired nerve stimulation due to a virtual cathode. Experiments were performed on 13 cats. The cuff electrode was placed on the medial gastrocnemius nerve. Both compound and single fiber action potentials were recorded from L7 ventral root filaments. The results demonstrate that larger alpha motor axons could be blocked at lower current levels than smaller alpha motor axons, and that all alpha fibers can be blocked at lower current levels than gamma fibers. >

Methods to assess physical activity with special reference to motion sensors and accelerometers

Abstract: The evolution of motion sensors is reviewed from the mechanical pedometer to the electronic accelerometer. It is concluded from this review that for accurate assessment of physical activity under free living conditions the recently introduced accelerometer looks most promising. The reliability of an accelerometer with a three-directional sensor was examined. Intrainstrument variation in a bench test was less than 8% during four measurements over a week. Interinstrument variation during treadmill experiments, while subjects wore two accelerometers at the same time, was on average 22% and did not improve after adjustment for differences found in the bench test. Reproducibility in the treadmill experiment was approximately 76, 85. and 95% at 3, 5, and 7 km/h, respectively. Bench testing revealed that the sensitivity of a piezoelectric element was prone to shifts, probably due to mechanical. electromagnetic, and/or temperature shock which may be encountered during outdoor application. However, the results of the bench test did not correspond with the findings in subjects. >

A spherical-section ultrasound phased array applicator for deep localized hyperthermia

Abstract: Computer data are presented for a nonplanar phased-array proposed as an applicator for deep, localized hyperthermia. The array provides precise control over the heating pattern in three dimensions. The array elements form a rectangular lattice on a section of a sphere so that the array has a natural focus at its geometric center when all its elements are driven in phase. When compared to a planar array with similar dimensions, the spherical-section array provides higher focal intensity gain. The relative grating-lobe level (with respect to the focus) is lower for scanned foci synthesized with this array (compared to a planar array with equal center-to-center spacing and number of elements). The spherical-section array has been simulated for use as a spot-scanning applicator as well as an applicator producing directly synthesized heating patterns. >

CAD/CAM for dental restorations-some of the curious challenges

Abstract: The current state of the art for production of dental restorations is reviewed, and DentiCAD, a system being developed by Maryland University and Minnesota University which is capable of producing dental crowns, is described. DentiCAD can produce dental restorations that fit at least as well as those that are cast. Some of the challenges and difficulties that have arisen during development are addressed. Some of the questions that, because of the new technology, can now be addressed and that are presenting new challenges are discussed. >

An artificial oral environment for testing dental materials

Abstract: An artificial oral environment which closely matches the conditions of human mastication for dental materials testing is described. Two servo-hydraulic actuators were combined to produce the force movement cycle of human mastication. A closed-loop method of control which functions in a manner similar to the human neuromuscular system was used. The horizontal closed loop used a linearly variable differential amplifier (LVDT) to monitor the position. The vertical closed loop used an LVDT to monitor position and a load cell to monitor force, with a 'mode switch' being made between the two mechanisms of control. A function generator was used to produce a sinewave that was modified to produce the movement and force programs for the two servo-hydraulic actuators. An acrylic chamber which contains the test samples and maintains an environment similar to that of the mouth was constructed. Natural or artificial saliva maintained at 37 degrees C is continuously circulated through the chamber. The time to complete a single chewing cycle was decreased by eliminating that part of the cycle where no force is produced. This enables the chewing rate to be increased to 4 cycle/s. The wear produced in the artificial mouth was found to have a high correlation with clinical wear if 250000 cycles in the artificial mouth are assumed to be equivalent to one year in the human mouth. >

Simple photon diffusion analysis of the effects of multiple scattering on pulse oximetry

Abstract: Photon diffusion theory is used to derive analytical expressions that relate the AC-DC intensity ratios measured by transmission-mode and reflectance-mode pulse oximeters to arterial oxygen saturation (S/sub a/O/sub 2/). The effects of multiple scattering are examined by comparing the results of the photon diffusion analysis with those obtained using an analysis is based on the Beer-Lambert law which neglects scattering. It is shown that the difference between the average lengths of the paths traveled by red and infrared photons makes the calibration curve of oximeters sensitive to the total attenuation coefficients of the tissue in the two wavelength bands, as well as to absorption by the pulsating arterial blood. Therefore, the shape of the calibration curve is affected by tissue blood volume, source-detector placement, and other variables that change the wavelength dependence of the attenuation coefficient of the tissue. After evaluating the relationship between S/sub a/O/sub 2/ and the red/IR AC-DC ratio under a variety of physiological conditions, it is concluded that for oximeters utilizing fixed calibration curves based on measurements obtained from normal subjects, errors introduced by interfering variables should be less than a few percent when S/sub a/O/sub 2/ exceeds 70%. >

Alignment methods for averaging of high-resolution cardiac signals: a comparative study of performance

Abstract: A comparative study of the performance of three alignment methods (the double-level method, a new time-delay estimation method based on normalized integrals, and matched filtering) is presented. A real signal and additive random noise for several signal-to-noise ratios (SNRs) are selected to make an ensemble of computer-simulated beats. The relation between the standard deviation of temporal misalignment versus SNR is discussed. A second study with real ECG signals is also presented. Several morphologies of QRS and P waves are tested. The results are in agreement with the computer simulation study. Nevertheless, the power spectrum of the noise process can affect the results. Matched filter estimation has been tested in the presence of power line interferences (50 Hz), with poor results. An application of the three alignment methods as a function of the SNR is proposed. The new time-delay estimation method has been observed to be robust, even in the presence of nonwhite noise. >

A comparison of four new time-domain techniques for discriminating monomorphic ventricular tachycardia from sinus rhythm using ventricular waveform morphology

Abstract: The authors have designed four new computationally efficient time-domain algorithms for distinguishing ventricular electrograms during monomorphic ventricular tachycardia (VT) from those during sinus rhythm using direct analysis of the ventricular electrogram morphology. All four techniques are independent of amplitude fluctuations and three of the four are independent of baseline changes. These new techniques were compared to correlation waveform analysis, a previously proposed method for distinction of VT from sinus rhythm. Evaluation of these four new algorithms was performed on data from 19 consecutive patients with 31 distinct monomorphic ventricular tachycardia morphologies. Three of the algorithms performed as well as or better than correlation waveform analysis but with one-tenth to one-half the computational demands. >

A blind mobility aid modeled after echolocation of bats

Abstract: A new model of a mobility and for the blind was designed using a microprocessor and ultrasonic devices. This mobility and was evaluated based on psychophysical experiments. In this model, a downswept frequency-modulated (FM) ultrasound signal is emitted from a transmitting array with broad directional characteristics in order to detect obstacles. The ultrasound reflections from the obstacles are picked up by a two-channel receiver. The frequency of the emitted ultrasound is swept from 70 to 40 kHz within 1 ms, so it has almost the same characteristics as the ultrasound a bat produces for echolocation. The frequency of the reflected ultrasound wave is downconverted by about 50:1 by using a microcomputer with analog-to-digital and digital-to-analog converters. These audible waves are then presented binaurally through earphones. In this method obstacles may be perceived as localized sound images corresponding to the direction and the size of the obstacles. From the results of psychophysical experiments, it was found that downswept FM ultrasound was superior to other ultrasonic schemes for the recognition of small obstacles. With it a blind person can recognize a 1-mm-diameter wire. It was also proved that the blind could discriminate between several obstacles at the same time without any virtual images. This mobility aid is very effective at detecting small obstacles placed in front of the head. >

A method to effect physiological recruitment order in electrically activated muscle

Abstract: A stimulation is used to achieve physiological recruitment order of small-to-large motor units in electrically activated muscles. The use of quasitrapezoidal pulses and a tripolar cuff electrode make selective activation of small motor axons possible, thus recruiting slow-twitch, fatigue-resistant muscle units before fast-twitch, fatigable units in a heterogeneous muscle. Isometric contraction force from the medial gastrocnemius muscle was measured in five cats. The physiological recruitment order was evidenced by larger twitch widths at lower force levels and small twitch widths at higher force levels. The force modulation process was more gradual and fused contractions were obtained at lower stimulation frequencies when the proposed stimulation method was used. Muscles activated by the method were more fatigue-resistant under repetitive activation at low force levels. This stimulation method is simpler to implement and has fewer adverse effects on the neuromuscular system than previous blocking methods. It may therefore have applications in future functional neuromuscular stimulation systems. >

Feedback regulation of hand grasp opening and contact force during stimulation of paralyzed muscle

Abstract: A fixed-parameter, discrete-time, first-order, feedback control system is described for regulating grasp during electrical stimulation of paralyzed muscles of the hand. The stiffness of the grasp (the relationship between grasp force and grasp opening) is kept constant by linearly combining force and position feedback signals. Thus, a single continuous command signal can control the size of the grasp opening prior to object acquisition and both grasp and opening after contact. The controller achieves this change in controlled variables by scaling and summing the force and position feedback signals, rather than by a discrete switch in control strategy. Experimental tests of the control system in quadriplegic subjects show that control can be obtained over conditions ranging from unloaded position regulation is isometric force regulation as well as in the transition between these conditions. The robustness of the control system was evaluated during force regulation with isometric loads. Step response rise time and overshoot were much more dependent on system gain than on the location of the controller zero. Responses with a rise time of less than and 2 s an overshoot of less than 30% were obtained over a gain range up to 10, indicating good robustness to muscle gain reductions such as might be caused by fatigue. >

Development of medical pressure and temperature sensors employing optical spectrum modulation

Abstract: Fiber optic Fabry-Perot sensors have been developed whose optical reflectance varies with optical cavity depth (pressure) or with change in a material's refractive index (temperature). These sensors employ a unique combination of features: they are interrogated by an LED; they are designed to operate within a single reflectance cycle; and their returned light is analyzed by a dichroic ratio technique. The sensors use a step index glass fiber and are relatively insensitive to absolute light levels and fiber bending. They have an expanded linear operating range and can be built for low cost disposable applications. Sensor performance meets or exceeds established medical requirements. >

Sensitivity and selectivity of intraneural stimulation using a silicon electrode array

Abstract: A multielectrode array in silicon technology, as well as experimental paradigms and model calculations for sensitivity and selectivity measures, have been developed. The array consists of twelve platinum electrode sites (10*50 mu m and 50 mu m interdistance) on a 45- mu m thick tip-shaped silicon substrate and a Si/sub 3/N/sub 4/ insulating glass cover layer. The tip is inserted in the peroneal nerve of a rat during acute experiments to stimulate alpha motor fibers or the extensor digitorum longus muscle. Sensitivity calculations and experiments show a cubic dependence of the number of stimulated motor units on current amplitude of the stimulatory pulse (recruitment curves), starting at single motor level. Selectivity was tested by a method based on the refractory properties of neurons. At the lowest stimulus levels (for one motor unit) selectivity is maximal when two electrodes are separated by 200-250 mu m, which was also estimated on theoretical grounds. The study provides clues for future designs of two- and three-dimensional devices. >

Muscle recruitment with intrafascicular electrodes

Abstract: The authors have studied muscle recruitment with Teflon-insulated, 24 mu m diameter, Pt-Ir intrafascicular electrodes implanted in nerves innervating the gastrocnemius and soleus muscles of cats. The purpose was to measure the performance of these bipolar electrodes, which were designed to optimize their ability to record unit activity from peripheral nerves, as stimulating electrodes. Recruitment curves identified the optimal stimulus configuration as a biphasic rectangular pulse, with an interphase separation of about 500 mu s and a duration of about 50 mu s. The current required for a half-maximal twitch contraction was on the order of 50 mu A. Current and charge densities needed for stimulation were well below levels believed to be safe for the tissue and electrode materials, involved. The spinal reflex contribution was not the same in all the implants. >

Combined microwave heating and surface cooling of the cornea

Abstract: The authors investigated a nonsurgical means of reshaping the cornea to correct hyperopia, keratoconus, or myopia. The object was to heat the central stroma of the cornea to the shrinkage temperature of collagen, 55-58 degrees C. The heating device was an open-ended, coaxial, near-field applicator driven at 2450 MHz; it incorporates cooling of the cornea surface by slow of saline. The system was investigated theoretically by computing the 2-D, axisymmetric temperature distribution with the finite-element method. The system was investigated experimentally by heating excised steer corneas. Histology showed that the system could shrink the stroma to a depth of 0.6 mm while sparing the epithelium in 75% of the cases; the diameter of shrinkage was 1.3 mm. Theory predicted a significantly deeper and narrower region of shrinkage than was observed. >

Feedback control of coronal plane hip angle in paraplegic subjects using functional neuromuscular stimulation

Abstract: An investigation of feedback control of coronal plane posture in paraplegic subjects who stand using functional neuromuscular stimulation (FNS) is described. A feedback control system directed at regulating coronal plane hip angle in neutral position was designed, implemented, and evaluated in two paraplegic subjects. The feedback controller consisted of two-stages in cascade: a modified discrete-time proportional-integral-derivative (PID) stage and a nonlinear single-input, multiple-output stage to determine the stimulation to be sent to several muscles. The authors evaluate the performance of the feedback controller by comparing the response of the feedback-controlled system to that of an open-loop stimulation system. In an evaluation based on temporal response characteristics the controlled system exhibited at 41% reduction in root-mean-squared (RMS) error (where error is defined as the deviation from the desired angle), a 52% reduction in steady-state error, and a 22% reduction in hip compliance. The feedback-controlled system also exhibited significant reductions in the variability of these measures on several days. >

Compression of the ambulatory ECG by average beat subtraction and residual differencing

Abstract: A strategy is evaluated for compression of ambulatory electrocardiograms (ECGs) that uses average beat subtraction and Huffman coding of the differenced residual signal. A sample rate of 100 sps and a quantization level of 35 mu V are selected to minimize the mean-square-error distortion while maintaining a data rate that allows 24 h of two-channel ECG data to be stored in less than 4 MB of memory. With this method, sample rate, and quantization level, the ambulatory ECG is compressed and stored in real time with an average data rate of 174 b/s per channel. It is demonstrated that, for the low sample rate and coarse quantization required for ambulatory recording, without sufficient temporal resolution in beat location, beat subtraction does not significantly improve compression, and may even worsen compression performance. It is also shown that with average beat subtraction, compression is improved if multiple beat averages maintained. Improvement is most significant for ECG signals that exhibit frequent ectopic beats. >

An airjet actuator system for identification of the human arm joint mechanical properties

Abstract: A system for determining the mechanical properties of the human arm during unconstrained posture and movement is described. An airjet perturbation device is attached to the wrist with a special cuff, providing high-frequency stochastic perturbations in three orthogonal directions. The airjet operations as a fluidic flip-flop utilizing the Coanda effect. The design greatly reduces the mass of mechanical moving parts and enhances the frequency bandwidth dramatically. This airjet is intrinsically a bistable device that can generate arbitrary binary force sequences, such as pseudorandom binary sequences, colored white noise, and Walsh functions. The force transmissibility is identified. Cuff design and data acquisition are discussed. >

Multisite microprobes for neural recordings

Abstract: Multisite, passive microprobes have been developed to allow simultaneous recording of action potential activity from multiple neurons at different locations in the brain. The microprobes were fabricated using standard integrated-circuit techniques. The probe is a planar structure that consists of gold electrodes sandwiched between two polyimide dielectric layers and bonded to a molybdenum structural support. Windows in the top dielectric layer expose the electrode sites and bonding pads. In two distinct versions of the probe, four or six recording sites of approximately 25 mu m/sup 2/ are arranged on a dagger-shaped structure which can penetrate the pia. The bonding pads and interconnect wires at the probe head are entirely encapsulated in a tubular fixture that is packed with silicone RTV and sealed with epoxy to protect the interconnections from contact with body fluids. The site impedances at 1 kHz are typically between 2 and 4 M Omega . Probe lifetimes for continuous immersion in physiological saline solution, as measured by impedance, have exceeded 750 h. The failure mechanism is believed to be due to moisture and ion absorption in the top dielectric layer. >

A quantitative analysis of pendular motion of the lower leg in spastic human subjects

Abstract: Gravity-induced oscillations of the lower leg in normal and spastic subjects were examined with a view towards evaluating a clinical test of spasticity called the pendulum test. For passive limb motion (in which no reflex excitation occurred), a second-order linear model did not provide an adequate description of the motion for either spastic or normal legs. System equations including nonlinear mechanical properties simulating asymmetries in the swing and amplitude dependent variations in stiffness and damping provided a more accurate description. For spastic limb motion (in which reflex excitation did occur) accurate simulation required components accounting for abnormal reflex activation, coinciding with the time course of EMG activation. These included increased stiffness and damping with their gains related to reflex EMG magnitude, and changes in the rest length of the stiffness. Comparison of numerical with experimental data showed that the nonlinear model simulated the motion accurately, with the variance accounted for usually exceeding 90%. >

Magnetocardiographic functional localization using a current dipole in a realistic torso

Abstract: A fast and numerically effective biomagnetic inverse solution using a moving current dipole in a realistic homogeneous torso is described. The localization model and high-resolution magnetocardiographic (HR-MCG) mapping were applied to localize noninvasively the ventricular preexcitation site in ten patients suffering from Wolff-Parkinson-White syndrome. In all cases, the computed localization results were compared to the results obtained by the invasive catheter technique. Using a standard-size torso model in all cases, the average 3-D distance between the computed noninvasive locations and the invasively obtained results was 2.8+or-1.4 cm. When the torso was rescaled to better match the true shape of the subject, the 3-D average was improved to 2.2+or-1.0 cm. This accuracy is very satisfactory, suggesting that the method would be clinically useful. >

Lower extremity angle measurement with accelerometers-error and sensitivity analysis

Abstract: The use of accelerometers for angle assessment of the lower extremities is investigated. This method is evaluated by an error-and-sensitivity analysis using healthy subject data. Of three potential error sources (the reference system, the accelerometers, and the model assumptions) the last is found to be the most important. Model calculations based on data obtained by the Elite video motion analysis system showed the rigid-body assumption error to be dominant for high frequencies (>10 Hz), with vibrations in the order of 1 mm resulting in errors of one radial or more. For low frequencies ( >

Recursive parameter identification of constrained systems: an application to electrically stimulated muscle

Abstract: A method of incorporating constraints on model parameters is developed. This method is applicable to most recursive parameter-identification algorithms. It enforces linear equality constraints on identified parameters. The use of this method for the real-time identification of autoregressive-moving-average time series models, subject to parameter constraints, is described. These constraints may be time varying. The use of this algorithm is demonstrated in the identification of electrically stimulated quadriceps muscles in paraplegic human subjects, using percutaneous intramuscular electrodes. The nonlinear steady-state force versus pulsewidth recruitment characteristic of the electrode-muscle system is identified simultaneously with the input-output muscle response dynamics, using a Hammerstein-type model. Knowledge of the recruitment curve's shape is translated into constraints on the identified parameters. This information improves the experimental predictive quality of the identified model. >

A nystagmus strategy to linearize the vestibulo-ocular reflex

Abstract: Two important aspects of the vestibulo-ocular reflex (VOR) are addressed. First, the linear range of ocular responses is much more extensive than expected from the characteristics of central pathways (CNS), and this is shown to result directly from early convergence of fast and slow premotor signals in the central processes, associated with significant and intermittent changes in functional connectivity (effective structural modulation). Second, the presence of such structural modulation implies that responses must be analyzed using transient analysis techniques, rather than previous steady state approaches, in order to properly evaluate reflex dynamics. Simulation results with a recent model of the VOR are used to illustrate the arguments. Relying on known interconnections between saccadic burst circuits in the brainstem, and the ocular premotor areas of the vestibular nuclei, a viable strategy for the timing of nystagmus events is proposed. The strategy easily reproduces the characteristic changes in vestibular nystagmus with the amplitude of head velocities, and with the frequency of passive head oscillation. >

Modern spectral analysis techniques for blood flow velocity and spectral measurements with pulsed Doppler ultrasound

Abstract: Four spectral analysis techniques were applied to pulsed Doppler ultrasonic quadrature signals to compare the relative merits of each technique for estimation of flow velocity and Doppler spectra. The four techniques were (1) the fast Fourier transform method, (2) the maximum likelihood method, (3) the Burg autoregressive algorithm, and (4) the modified covariance approach to autoregressive modeling. Both simulated signals and signals obtained from an in vitro flow system were studied. Optimal parameter values (e.g. model orders) were determined for each method, and the effects of signal-to-noise ratio and signal bandwidth were investigated. The modern spectral analysis techniques were shown to be superior to Fourier techniques in most circumstances, provided the model order was chosen appropriately. Robustness considerations tended to recommend the maximum likelihood method for both velocity and spectral estimation. Despite the restrictions of steady laminar flow, the results provide important basic information concerning the applicability of modern spectral analysis techniques to Doppler ultrasonic evaluation of arterial disease. >