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Biomedical subjects

R E Challis

Publications and source records attributed to R E Challis.

13 recordsLinked to original sources
PubMed2001

Convergence criteria for scattering models of ultrasonic wave propagation in suspensions of particles.

Scattering models used to simulate the attenuation and phase velocity of an ultrasonic wave propagating through a suspension of particles involve the summation of an infinite series of partial waves. The accuracy of computation is influenced by the number of terms included in the harmonic series, and the number of terms required depends upon the scatterer size compared with wavelength. It is shown that the errors in modelled attenuation and phase velocity resulting from premature truncation can be significant when modelling higher values of particle diameter-frequency product. A useful and simple heuristic is presented, in which the number of terms in the summation of the infinite series needed for satisfactory convergence to a final value is a function of the particle diameter-frequency product and of the compressive wave velocity in the continuous phase.

Journal Article↗
PubMed1996

Acoustic absorption due to proton transfer in solutions of proteins, peptides and amino acids at neutral pH.

Measurements of ultrasonic absorption in the frequency range 2 to 60 MHz have been made on solutions of the amino acid histidine, dipeptides glycyl tyrosine and histidyl tyrosine, and proteins lysozyme and bovine serum albumin. Measurements were carried out at 37 degrees C and covered the pH range 6.8 to 7.7. Mechanisms for the observed excess acoustic absorption involving bimolecular and intramolecular proton transfer are considered.

Acoustics↗
PubMed1991

Biomedical signal processing (in four parts). Part 2. The frequency transforms and their inter-relationships.

This is the second in a series of four tutorial papers on biomedical signal processing, and it concerns the relationships between commonly used frequency transforms. It begins with the Fourier series and Fourier transform for continuous time signals and extends these concepts for aperiodic discrete time data and then periodic discrete time data. The Laplace transform is discussed as an extension of the Fourier transform. The z-transform is introduced and the ideas behind the chirp-z transform are described. The equivalence between the time and frequency domains is described in terms of Parseval's theorem and the theory of convolution. The use of the FFT for fast convolution and fast correlation is described for both short recordings and long recordings that must be processed in sections.

Biomedical Engineering↗
PubMed1991

Biomedical signal processing (in four parts). Part 3. The power spectrum and coherence function.

This is the third in a series of four tutorial papers on biomedical signal processing and concerns the estimation of the power spectrum (PS) and coherence function (CF) od biomedical data. The PS is introduced and its estimation by means of the discrete Fourier transform is considered in terms of the problem of resolution in the frequency domain. The periodogram is introduced and its variance, bias and the effects of windowing and smoothing are considered. The use of the autocovariance function as a stage in power spectral estimation is described and the effects of windows in the autocorrelation domain are compared with the related effects of windows in the original time domain. The concept of coherence is introduced and the many ways in which coherence functions might be estimated are considered.

Biomedical Engineering↗
PubMed1990

Biomedical signal processing (in four parts). Part 1. Time-domain methods.

This is the first of a series of four tutorial papers on biomedical signal processing. It provides an introduction to terminology and basic ideas for testing for randomness and trend, and for the determination of basic signal properties in the time domain, given the uncertainties associated with the estimation process. Techniques outlined in the paper are: the coherent average, cross-correlation and covariance, autocorrelation and phase-shift averaging.

Biomedical Engineering↗
PubMed1989

Signal preprocessing system for the small intestinal electromyogram.

The intestinal EMG obtained from chronically implanted electrodes in canine preparations provides for the evaluation of intestinal motor activity and its control. The basic electrical rhythm (BER) and spike components on the EMG signal provide evidence of control activity and a measure of contraction intensity, respectively. A hardware system is presented in which these two components are separated by filters and the contraction spikes counted in fixed epochs to yield a contraction spike per unit time record against time. The signal is also available in parallel binary form at the end of each epoch, together with a data-ready signal for direct acquisition by computer. Tests of system performance and operating protocols are given. The preprocessor is used as a fast front end to a digital signal processing system specifically built for intestinal EMG analyses.

Animals↗
PubMed1984

A controllable artificial afterload for isolated heart studies.

This paper presents an artificial afterload, based on the Westerhof model, for use in isolated heart preparations. The system has adjustable elements representing peripheral resistance and total arterial compliance, together with a fixed element representing aortic impedance. It is controlled by a computer in a manner which incorporates feed forward and feedback techniques. The system maintains constant 'arterial' pressure in the face of large excursions of the flow through it. It can be programmed to change this pressure in a dynamic fashion. The time taken to recover from large changes in flow and to reset the pressure is at most 3 S. Absolute values of peripheral resistance are easily obtained without any knowledge of the actual flow being required. Tests of the system demonstrate its stability and rapid transient characteristics.

Biomedical Engineering↗
PubMed1983

The design of digital filters for biomedical signal processing. Part 3: The design of Butterworth and Chebychev filters.

The first two papers in this series reviewed the basic concepts which apply to digital filter theory and presented design techniques based on the z plane pole-zero plot. In this paper these methods are used to develop digital versions of Butterworth and Chebychev filters. The basic theory of both filter types is reviewed and the bilinear transformation is used to derive the z-transforms of the filters from their s-plane continuous time descriptions. Recurrence relationships which may be used to implement filters of various orders are developed. The impulse and frequency responses of the elements are illustrated and examples are given of their application to ECG data.

Analog-Digital Conversion↗
PubMed1983

The design of digital filters for biomedical signal processing. Part 2: Design techniques using the z-plane.

Design methods for digital filters using z-plane techniques are developed out of the basic concepts described in part 1. The relationship between impulse response shape and side lobe generation in the frequency response is investigated by means of rectangular, triangular and cosine window filters. The limited choice of cut-off frequencies available for simple linear phase digital filters is explained. Design direct from the frequency domain is summarized. z-plane techniques for the design of high- and band-pass digital filters and the digital equivalents of analogue filters are described. s-plane to z-plane mapping by means of the Bilinear transformation is compared to the direct z-transform method of filter design.

Biomedical Engineering↗
PubMed1982

A simply constructed system for mean blood flow measurement.

Blood flow measuring equipment and cardiac bypass pumps must be calibrated against a direct measurement of flow using timed collection. Single vessel timed collection systems have problems associated with their use. Repeated determinations of flow cannot be made rapidly, and frothing and fluid surface instability in the collecting vessel render the timing of a measured volume flow difficult and inaccurate. Collecting vessels which fill from the bottom may exert a changing back pressure on the source of flow. A semi-automated timed collection system is presented which overcomes these disadvantages. It has been used successfully to calibrate electromagnetic flowmeters and heart bypass pumps with 1% reproducibility or better. The system has also been used to measure coronary sinus flow directly in canine right heart bypass preparations.

Animals↗