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Biomedical subjects

R Kuc

Publications and source records attributed to R Kuc.

14 recordsLinked to original sources
PubMed2001

Transforming echoes into pseudo-action potentials for classifying plants.

Animals perceive their environment by converting sensory stimuli into action potentials, or temporal point processes, that are interpreted by the brain. This paper investigates the information content of point processes extracted from echoes from in situ plants in an effort to understand how bats recognize landmarks in the field. A mobile sonar converts echoes into biologically similar temporal point processes. termed pseudo-action potentials (PAPs), whose inter-PAP interval relates to echo amplitude. The sonar forms a sector scan of an object to produce a spatial-temporal PAP field. Classifier neurons apply delays and coincidence detection to the PAP field to identify three distinct echo types, glints, blobs, and fuzz, which characterize plant features. Glints are large amplitude echoes exhibiting coherence over successive echoes in the sector scan, typically produced by favorably oriented isolated specular reflectors. Blobs are large echoes lacking coherence, typically bordering glints or formed by collections of interfering reflectors. Fuzz represents weak echoes, typically produced by collection of weak scatterers or by reflectors on the beam periphery. A small mirror reflector models a flat leaf surface and motivates the glint criteria. Classifiers are applied to experimental data from two types of tree trunks, a glint-producing sycamore (Platanus occidenatalis) and a glint-absent Norway maple (Acer platanoides) and two plants, a glint-producing rhododendron (Rhododendron maximus) and a glint-absent yew (Taxus media). We speculate that our narrow-band sonar models the activity of a single frequency bin in the frequency-modulated (FM) sweep emitted by bats, and that one function of the frequency bins in the FM sweep is to form a sector scan of the environment.

Animals↗
PubMed2000

Foliage echoes: a probe into the ecological acoustics of bat echolocation.

The research reported here aims at understanding the biosonar system of bats based on the properties of its natural inputs (ecological acoustics). Echoes from foliages are studied as examples of ubiquitous, natural targets. The echo properties and their qualitative relationship to plant architecture are described. The echoes were found to be profoundly stochastic and in general neither Gaussian nor stationary. Consequently, features useful for discrimination of such target classes will be confined to estimated random process parameters. Several such statistical signal features which are sufficiently invariant to allow a classification of the used example plants were identified: the characteristic exponent and the dispersion of an alpha-stable model for the amplitude distribution, a crest factor defined as the ratio of maximum squared amplitude and signal energy, the dispersion of the first threshold passage distribution, the structure of the correlation matrix, and a nonstationarity in sound channel gain. Discrimination error probability could be reduced by combining features pairwise. The best combination was the crest factor and the correlation coefficient of a log-linear model of the time-variant sound channel gain; it yielded an estimated Bayes risk of 6.9% for data pooled from different views.

Acoustics↗
PubMed1996

Magnetometer spacing criterion for biomagnetic source current imaging.

A criterion for determining the maximum spacing between magnetometers for measuring the magnetic field is derived. A two-dimensional (2-D) filter model is employed to determine the maximum spatial frequency component present in the magnetic field that is above the spectral noise level. This maximum frequency component is then sampled at a rate greater than twice per period as indicated by the Nyquist criterion, yielding the required magnetometer spacing. It is shown that the rule-of-thumb employed in current clinical biomagnetic array systems, that the spacing between the coils should be approximately equal to the depth of the source, is adequate when the signal-to-noise power ratio is less than 28.4 (14.5 dB). The analysis also quantitatively demonstrates that reducing the separation between the measurement and source planes has a greater effect on the resolution than decreasing the noise level by the same factor. This result is important for employing high Tc superconductor magnetometers that allow thinner thermal insulating layers at the cost of higher thermal noise.

Electromagnetic Fields↗
PubMed1994

Sensorimotor model of bat echolocation and prey capture.

A model of the bat sensorimotor system is developed using acoustics, signal processing, and control theory to illustrate the fundamental issues in accomplishing prey capture with echolocation. This model indicates that successful nonpredictive tracking of an ideal prey can be accomplished with a very simple system. Circular apertures approximate the mouth and ears for deriving acoustic beam patterns, using the big brown bat Eptesicus fuscus as a model. Fundamental and overtone frequency components in the emissions allow two simultaneous acoustic beams to be defined. A pair of nonlinear, time-variable, sampled-data controllers alter the bat's heading by applying yaw and pitch heading corrections. The yaw correction attempts to position the prey in the midsagittal plane by nulling the interaural intensity difference of the fundamental component. The pitch correction compares the intensities of the overtone and fundamental components and acts to null their difference. By initiating pitch correction when the overtone intensity first exceeds that of the fundamental, the ambiguity problem is solved and the prey is directed to the capture region. Simulations of passive prey capture indicate that the capture probability decreases as the prey speed increases. Both quick and sluggish prey are considered, with sluggish prey found to be caught with slightly better efficiency. The magnitude of the prey's lateral motion just prior to capture is observed to be an important factor determining capture. The presence of a blind stage is considered, during which the interference of the emission with the echo is assumed to disrupt any sonar information. The presence of such a blind stage is found to have negligible effect on capture efficiency.

Acoustics↗
PubMed1986

A computer model for simulating reflected ultrasound signals.

This letter describes a model for simulating an ultrasound signal reflected from a medium composed of randomly distributed scatterers, as typified by soft biological tissue, such as liver. The model is specified in terms of the effective transducer beam radius B and the mean scatterer spacing S. The novel feature of the model is that the transducer field is partitioned by packing cylinders, called microbeams, into concentric annular regions that lie parallel to the transducer axis. The radii of the microbeams and the annuli are related to S. An independent reflector sequence is generated for each microbeam, the microbeam sequences in each annulus are summed and convolved with the impulse response of a point reflector. The reflected waveform sequence is then generated by summing the annular contributions and convolving with a band-limited pulse waveform. Simulated signals were generated for different values of B and S and compared with actual signals reflected from two in vivo livers and a tissue-equivalent phantom. Estimates of the kurtosis for the simulated signals indicate the range of signals that can be generated by varying the values for S and B.

Computer Simulation↗
PubMed1986

Quantitative US attenuation in normal liver and in patients with diffuse liver disease: importance of fat.

Two methods are used to estimate ultrasound attenuation in liver. These were based on amplitude change and frequency change as a result of depth dependent attenuation. Evaluation of the two methods against a family of calibrated phantoms yielded correlation coefficients of 0.98 and 0.99, respectively. Liver attenuation in 26 control subjects was 0.50 and 0.52 dB/MHz/cm, respectively. Liver attenuation was estimated in 50 patients who later underwent liver biopsy. Comparison with quantitative histologic results showed that the presence of fat alone accounted for the increased attenuation associated with cirrhosis. Similar high attenuation values were found in patients with fatty infiltration. Fibrosis alone did not result in elevated liver attenuation. Cirrhotics without fatty infiltration had attenuation similar to that of the controls. Mechanisms of action are discussed.

Adolescent↗
PubMed1986

Presence of cepstral peak in random reflected ultrasound signals.

A peak in the power cepstrum, or the squared magnitude of the Fourier transform of the data log-power spectrum, is commonly observed when processing reflections from plate-like structures, such as membranes. In this case, the cepstral peak at the smallest nonzero time lag, or quefrency, value can be used to determine the thickness of the plate. For reflections from a medium composed of randomly distributed scatterers, such as liver tissue, a cepstral peak is also commonly observed, but cannot be so intuitively explained as in the deterministic case above. In this paper, it is demonstrated that the presence of a cepstral peak depends on the form of the probability density function (pdf) of the separation between reflectors. In the case where the pdf is uniform from O to SM, the cepstral peak is found to occur at the quefrency corresponding to SM. For simple unimodal pdfs, a cepstral peak will occur at the location of the maximum probability. These observations are shown analytically and verified through simulations. The diagnostic value of these results lies in the interpretation of the relation of the cepstral peak location to the spacing of the scattering elements in the tissue.

Fourier Analysis↗
PubMed1985

Reduced-order autoregressive modeling for center-frequency estimation.

The center frequency of a narrowband, discrete-time random process, such as a reflected ultrasound signal, is estimated from the parameter values of a reduced, second-order autoregressive (AR) model. This approach is proposed as a fast estimator that performs better than the zero-crossing count estimate for determining the center-frequency location. The parameter values are obtained through a linear prediction analysis on the correlated random process, which in this case is identical to the maximum entropy method for spectral estimation. The frequency of the maximum of the second-order model spectrum is determined from these parameters and is used as the center-frequency estimate. This estimate can be computed very efficiently, requiring only the estimates of the first three terms of the process autocorrelation function. The bias and variance properties of this estimator are determined for a random process having a Gaussian-shaped spectrum and compared to those of the ideal FM frequency discriminator, zero-crossing count estimator and a correlation estimator. It is found that the variance values for the reduced-order AR model center-frequency estimator lie between those for the ideal FM frequency discriminator and the zero-crossing count estimator.

Computers↗
PubMed1984

Modeling acoustic attenuation of soft tissue with a minimum-phase filter.

Soft biological tissue has been observed to exhibit an acoustic attenuation log-magnitude characteristic which increases as an approximately linear function of frequency. This paper describes the implementation of a finite-impulse-response (FIR) digital filter model for simulating this behavior on a digital computer. To insure that the filter is causal, the minimum-phase constraint is imposed. For minimum-phase filters, the log-magnitude and phase characteristics form a Hilbert Transform pair. The discrete-time Hilbert Transform of the linear log-magnitude characteristic was evaluated to determine the phase of the filter. The inverse Fourier Transform of the resulting real and imaginary components of the frequency transform produces the finite-duration unit-sample response of the digital filter model. Experimental results using plexiglas material, which has a linear-with-frequency loss characteristic, indicate that the minimum-phase model is more accurate than the linear-phase model, resulting in a rms error between predicted and observed time waveforms that is 3 times smaller. The effects of varying the sampling period and the size of the FIR filter are discussed. A FORTRAN program to calculate the minimum-phase unit-sample response from the slope of the log-magnitude characteristic is included in the Appendix.

Computers↗
PubMed1982

Variation of acoustic attenuation coefficient slope estimates for in vivo liver.

Characterization of diffuse alcoholic disease is being attempted by estimating the slope of the acoustic attenuation coefficient with frequency, a parameter denoted by beta, from reflected ultrasound signals. The ultrasound signals reflected from within the body are distorted by the non-ideal properties of tissues. Two distorting mechanisms are considered: the propagation through nonhomogeneous media and the reflection from irregularly shaped interfaces. The mechanisms are modeled as random filters and their effect on estimating the value of beta are considered. The variances of three beta estimators are derived from the model. Clinical results on normals and patients with alcoholic liver disease are presented to verify the model.

Humans↗