Biomedical subjects

J A Crowe

Publications and source records attributed to J A Crowe.

At least 19 recordsLinked to original sources

PubMed2006

Comparison of methods for reducing the effects of scattering in spectrophotometry.

Light scattering provides a problem in optical spectroscopy as the relationship between attenuation and absorption in the presence of scattering is nonlinear. Three simple methods of reducing the effects of scattering and hence returning to an approximately linear relationship are considered in this paper, namely, extracting light that has maintained its original polarization state through subtraction of orthogonal polarization states, use of an added absorber, and spatial filtering. These can all be applied relatively easily to conventional spectrophotometers. However, there is an inevitable trade-off between the accuracy of the measurement and the signal-to-noise ratio as scattered light is rejected from the detector. It is demonstrated that polarization subtraction is the most efficient technique at selecting weakly scattered photons from a scattered light background as it enables the relationship between attenuation and absorption coefficient to become more linear while maintaining a higher number of detected photons. In practical use, the drawback of polarization subtraction over added absorber and spatial filtering methods is that a large dc background light level is maintained, which contributes to a higher shot noise. This means that when the scattering coefficient is high (micros > or = 7 mm(-1)) the added absorber method offers better performance for shot noise limited detection.

Journal Article↗

PubMed2004

Detrended fluctuation analysis: a suitable method for studying fetal heart rate variability?

We evaluate the suitability of an enhanced detrended fluctuation analysis for studying fetal heart rate series involving imperfect quality of information. Our results indicate that to explore persistent long-range correlations, or fractality, the collection requirements of the data can be relaxed by allowing the possibility of using averaged fetal heart rate series. In addition, it also appears feasible to employ, without producing major alterations in the long-range scaling behaviour, fragmented fetal heart rate series involving up to 50% of random missing values, or up to 50 min of consecutive missing samples in recordings of approximately equal to 8 h length. These are crucial advantages to overcome the often variable quality of fetal data. Consequently, these findings may open the possibility of obtaining information concerning the development of neural processes from fetal heart rate series, despite their non-stationary and fragmented nature.

Algorithms↗

PubMed2003

Interpretation of heart rate variability via detrended fluctuation analysis and alphabeta filter.

Detrended fluctuation analysis (DFA), suitable for the analysis of nonstationary time series, has confirmed the existence of persistent long-range correlations in healthy heart rate variability data. In this paper, we present the incorporation of the alphabeta filter to DFA to determine patterns in the power-law behavior that can be found in these correlations. Well-known simulated scenarios and real data involving normal and pathological circumstances were used to evaluate this process. The results presented here suggest the existence of evolving patterns, not always following a uniform power-law behavior, that cannot be described by scaling exponents estimated using a linear procedure over two predefined ranges. Instead, the power law is observed to have a continuous variation with segment length. We also show that the study of these patterns, avoiding initial assumptions about the nature of the data, may confer advantages to DFA by revealing more clearly abnormal physiological conditions detected in congestive heart failure patients related to the existence of dominant characteristic scales.

Adult↗

PubMed2001

Compact long-term recorder for the transabdominal foetal and maternal electrocardiogram.

Foetal heart rate (FHR) monitoring is a proven means of assessing foetal health during the antenatal period. Currently, the only widely available instrumentation for producing these data is based on Doppler ultrasound, a technology that is unsuitable for long-term use. For nearly a century, it has been known that the foetal electrocardiogram (FECG) can be detected using electrodes placed on the maternal abdomen. Although these signals suggest an alternative means of FHR derivation, their use has been limited owing to problems of poor signal-to-noise ratio. However, the eminent suitability of the transabdominal FECG for long-term FHR monitoring has suggested that perseverance with the technique would be worthwhile. The paper describes the design, construction and use of a compact, long-term recorder of three channels of 24 h antenatal transabdominal data. Preliminary use of the recorder in around 400 short recording sessions demonstrates that FHR records of equivalent quality to those from Doppler ultrasound-based instruments can be extracted from such data. The success of FHR derivation is, on average, around 65% of the recording period from around 20 weeks gestation (although this figure is reduced from around 28-32 weeks, and the success rates exhibit a wide range when individual subjects are considered). These results demonstrate that the technique offers, not only a means of acquiring long-term FHR data that are problematic to obtain by other means, but also a more patient-friendly alternative to the Doppler ultrasound technique.

Electrocardiography, Ambulatory↗

PubMed2001

Application of empirical mode decomposition to heart rate variability analysis.

The analysis of heart rate variability, involving changes in the autonomic modulation conditions, demands specific capabilities not provided by either parametric or non-parametric spectral estimation methods. Moreover, these methods produce time-averaged power estimates over the entire length of the record. Recently, empirical mode decomposition and the associated Hilbert spectra have been proposed for non-linear and non-stationary time series. The application of these techniques to real and simulated short-term heart rate variability data under stationary and non-stationary conditions is presented. The results demonstrate the ability of empirical mode decomposition to isolate the two main components of one chirp series and three signals simulated by the integral pulse frequency modulation model, and consistently to isolate at least four main components localised in the autonomic bands of 14 real signals under controlled breathing manoeuvres. In addition, within the short time-frequency range that is recognised for heart rate variability phenomena, the Hilbert amplitude component ratio and the instantaneous frequency representation are assessed for their suitability and accuracy in time-tracking changes in amplitude and frequency in the presence of non-stationary and non-linear conditions. The frequency tracking error is found to be less than 0.22% for two simulated signals and one chirp series.

Adult↗

PubMed2001

The information content of Doppler ultrasound signals from the fetal heart.

Knowledge of the content of Doppler ultrasound signals from the fetal heart is essential if the performance of fetal heart rate (FHR) monitors based upon this technology is to be improved. For this reason instrumentation was constructed to enable the simultaneous collection of Doppler audio signals and the transabdominal fetal ECG (for signal registration), with a total of 22 recordings being made with an average length of around 20 minutes. These data demonstrate the transient nature of the Doppler audio data with wide variations in the signal content observable on a beat-to-beat basis. Short-time Fourier analysis enabled the content of the Doppler signals to be linked to six cardiac events, four valve and two wall motions, with higher frequency components being associated with the latter. This differing frequency content together with information regarding the direction of movement that can be discerned from Doppler signals provided a potential means of discriminating between these six events (which are unlikely to all contribute to the Doppler signal within the same cardiac cycle). Analysis of 100 records showed that wall contractions generate the most prominent signals, with atrial contraction recognisable in all records and ventricular wall contraction in 95% (although its amplitude is only around 30% of that of the atrial signal). Valve motion, with amplitudes between 15 and 25% that of the atrial wall signal, were visible in 75% of records. These results suggest means by which the six events that contribute to the Doppler signal may be distinguished, providing information that should enable an improvement in the current performance of Doppler ultrasound-based FHR monitors.

Electrocardiography↗

PubMed1999

A method for foetal heart rate monitoring during magnetic resonance imaging using Doppler ultrasound.

A means of monitoring foetal heart rate (FHR) during magnetic resonance imaging (MRI) is presented. Foetal heart rate was measured using a modified standard Doppler ultrasound based monitor. The transducer and lead from the monitor required alteration to reduce interference and distortion in the MR images to acceptable levels. These changes enabled high quality images to be produced with insignificant additional noise and distortion when the foetal heart rate was recorded simultaneously.

Equipment Safety↗

PubMed1998

Towards multi-patient leadless and wireless cardiotocography via RF telemetry.

During labour the condition of the fetus is monitored by a cardiotocograph which displays fetal heart rate and a measure of uterine contractions. Ultrasound and tocodynamometer transducers are placed on the mother's abdomen and connected to a bedside monitor and display unit via a cable. This paper describes a prototype wireless system aimed at demonstrating the potential elimination of the cable which is undesirable in a number of respects. The radio link utilised operates at a frequency of 418 MHz with data compression and time division multiplexing (TDM) employed to allow up to 10 units to operate simultaneously on the same frequency in the same area. Data compression is achieved by extracting the Doppler ultrasound signal envelope and representing the frequency content of the signal using a zero crossing counting technique. Two Medium Access Control (MAC) protocols were considered, with a synchronised time division multiplexing system shown to offer greater throughput and resistance to interference than Carrier Sense Multiple Access (CSMA). This wireless RF telemetry system could be readily adapted for other multi-patient monitoring applications.

Cardiotocography↗

PubMed1997

Detection of fetal electrocardiogram signals using matched filters with adaptive normalisation.

The authors discuss the application of matched litters to the detection of R-waves in fetal electrocardiogram (FECG) data, recorded during labour using a scalp electrode. When using the basic matched filter, one correlates a template representing the clean signal with the noisy signal. This method is optimal when the underlying noise is white in nature. However, it is known that false detection of R-waves can occur in the presence of extraneous peaks which have a similar shape to the fetal R-wave. It is proposed to switch between two different normalisations of the impulse response of the matched filter to alleviate this problem. When the signal-to-noise ratio is lower than a predetermined threshold, then normalisation to the geometric mean of the template and noisy data energies is carried out, otherwise only normalisation to the template energy is made. In the former case, the background noise and spikes that are larger than the underlying FECG are attenuated, hence increasing the probability of detection of the R-waves. In the latter case, noise, which has a lower amplitude than the underlying R-wave, is reduced. The effectiveness of this method is demonstrated by application to scalp electrode data.

Electrocardiography↗

PubMed1997

Data compression of fetal Doppler ultrasound audio signals using zero-crossings analysis.

This paper outlines a method of reducing the data rate for transmitting fetal Doppler ultrasound audio signals. A specific application is cited where compression of the fetal Doppler signal is required to transmit information to cardiotocographs (CTGs) using radio telemetry. The method involves splitting the signal into amplitude and frequency components. The amplitude is represented by samples of the signal envelope whilst the frequency information is represented by the number of zero-crossings within fixed intervals (windows). With a careful choice of window size, it is shown that this method can be used to reproduce the signal with no audible difference when compared with the original waveform. A reduction in data rate of 15:1 is achieved.

Algorithms↗

PubMed1996

Antenatal assessment using the FECG obtained via abdominal electrodes.

During the past decade a variety of intrapartum fetal monitors have been constructed that process the entire fetal electrocardiogram (FECG), obtained via a scalp electrode. They therefore differ from conventional monitors in aiming to extract relevant timing and magnitude information from the morphology of the FECG rather than simply the RR interval and hence heart rate. An intrapartum monitor such as this has been successfully developed by ourselves. This paper describes the early results obtained whilst attempting to extend this form of monitoring forward into the antenatal period. In order to achieve this the FECG must be acquired via surface electrodes placed on the maternal abdomen, which yields a signal containing the FECG amidst a number of noise sources. Our investigations into the feasibility of "antenatal abdominal FECG analysis" have been on two fronts. The first has been to produce a bedside monitor similar in function to our intrapartum device, whilst the second has been to address the possibility of performing such monitoring in ambulant subjects. At present the antenatal bedside monitor has successfully extracted and processed the FECG in approximately 75% of the cases studied, with subjects ranging from 20 weeks through to term having been monitored. We also have demonstrated the feasibility of the long term monitoring of maternal and fetal heart rate using a portable instrument.

Abdomen↗

PubMed1995

Iterative method to derive an approximate matched filter template for fetal electrocardiogram signals.

Fetal electrocardiogram (FECG) signals that are extracted from the maternal abdomen have a signal-to-noise ratio that is so low that the determination of the times of location of the R-waves can be difficult. A matched filter could, in principle, be used, but in theory this requires prior knowledge of the shape of the QRS complex of the FECG. In the work that is described in this paper, a digital low-pass filter, with an impulse response that is triangular in shape, is applied to the first M complexes of a simulated FECG signal. An average based on the detected R-wave locations is determined and this is used as an approximation to the matched filter template for the next block of M complexes. It is shown that this method can be iterated to obtain an effectiveness in detecting R-wave locations that is competitive with the corresponding performance that is obtained with the pure matched filter. The resilience of this technique to increasing noise levels is investigated.

Algorithms↗

PubMed1995

Sequential recording of the abdominal fetal electrocardiogram and magnetocardiogram.

The aim of this study was to demonstrate the acquisition of both the fetal magnetocardiogram (FMCG) and fetal electrocardiogram (FECG) from the abdomen of the same subject. This contrasts with previous reported studies which have generally recorded one or other of these signals but not the two together. Both signals were successfully recorded and averaged to produce a typical complex, thus allowing their direct comparison.

Electrocardiography↗

PubMed1995

The feasibility of long-term fetal heart rate monitoring in the home environment using maternal abdominal electrodes.

It is well established that fetal and maternal electrocardiograms (ECGs) can be obtained from the maternal abdomen using standard surface electrodes, although this cannot be guaranteed. The unobtrusive and non-invasive nature of such monitoring lends itself naturally to the long-term ambulatory collection of this data on cardiac activity. By employing suitable algorithms it would then be possible to extract records of both fetal and maternal heart rate. This article presents results of the collection of raw electrophysiological signals, containing both fetal and maternal ECGs from a single volunteer from the 20th week of gestation until term. The significance of the data is that they were recorded by the mother herself in her own environment. Previously written software was then used to extract fetal and maternal heart rate data. These results demonstrate the feasibility of using this method for the long-term recording of fetal and maternal heart rate in the mother's normal surroundings.

Abdomen↗

PubMed1994

A windows application for real-time fetal ECG analysis.

Analysis of the fetal electrocardiogram (FECG) from abdominal recordings may be used as a noninvasive technique to assess fetal well being. Consequently, interest has arisen in the development of a real-time fetal ECG monitoring system. In this paper, we present a Windows user interface developed for our antenatal FECG analysis system. The program (FECGV1), written in C++, consists of three principal modules: System Setup, Real-Time FECG Analysis, and Result Review. By adopting the Windows' graphical user interface and object-oriented programming methodology, the system software accomplished the goals of visualization, easy use, extensibility, and user adaptability. Preliminary clinical application has shown that this approach provides a strong basis for a solution to real-time antenatal FECG analysis.

Computer Graphics↗

PubMed1993

The application of finite impulse response filters to the detection of fetal electrocardiogram signals.

An investigation is made into the potential application of linear phase digital filters to the detection of fetal electrocardiogram signals buried in noise. Such an assessment is made by applying both matched and linear phase filters to six computer simulated fetal signals and also to experimental data. The number of times that the R-wave locations are correctly located (N), the RMS error in R-wave location (RMS) and the correlation coefficient between the averaged and clean signals are computed. It is found that the averaged fetal complexes computed using these two types of filter are almost identical. However, for three of the signals, the values for N and RMS obtained using the linear phase filter are inferior to the corresponding results obtained with the matched filter. It is suggested that the averaged complex obtained using the linear phase filter could be used as an approximation to the matched filter template; it is found that this procedure results in an effectiveness of detecting R-waves that is, for the most part, comparable with the performance of a matched filter based on the QRS complex.

Abdomen↗

PubMed1992

Wavelet transform as a potential tool for ECG analysis and compression.

The recently introduced wavelet transform is a member of the class of time-frequency representations which include the Gabor short-time Fourier transform and Wigner-Ville distribution. Such techniques are of significance because of their ability to display the spectral content of a signal as time elapses. The value of the wavelet transform as a signal analysis tool has been demonstrated by its successful application to the study of turbulence and processing of speech and music. Since, in common with these subjects, both the time and frequency content of physiological signals are often of interest (the ECG being an obvious example), the wavelet transform represents a particularly relevant means of analysis. Following a brief introduction to the wavelet transform and its implementation, this paper describes a preliminary investigation into its application to the study of both ECG and heart rate variability data. In addition, the wavelet transform can be used to perform multiresolution signal decomposition. Since this process can be considered as a sub-band coding technique, it offers the opportunity for data compression, which can be implemented using efficient pyramidal algorithms. Results of the compression and reconstruction of ECG data are given which suggest that the wavelet transform is well suited to this task.

Electrocardiography↗

PubMed1990

Signal processing of the fetal electrocardiogram.

There has been considerable interest in recent years in the monitoring of the well-being of the fetus by the analysis of changes in morphology of the fetal electrocardiogram (FECG) signal. In this article, a brief review is first made of the use of the scalp electrode method to obtain an enhanced FECG complex. Subsequently, the problems in extracting the fetal signal from measurements taken from the abdomen of the mother are discussed. A comparison is made between two existing algorithms to extract the fetal signal: (1) the method of Akselrod; and (2) the adaptive filtering algorithm of Widrow. These algorithms are evaluated by considering the errors that could occur in locating the fetal R wave in the presence of noise.

Algorithms↗