Biomedical subjects

R H Smallwood

Publications and source records attributed to R H Smallwood.

At least 19 recordsLinked to original sources

PubMed2001

Mk3.5: a modular, multi-frequency successor to the Mk3a EIS/EIT system.

This paper describes the Sheffield Mk3.5 EIT/EIS system which measures both the real and imaginary part of impedance at 30 frequencies between 2 kHz and 1.6 MHz. The system uses eight electrodes with an adjacent drive/receive electrode data acquisition protocol. The system is modular, containing eight identical data acquisition boards, which contain DSPs to generate the drive frequencies and to perform the FFT used for demodulation. The current drive is in three sequentially applied packets, where each packet contains ten summed sine waves. The data acquisition system is interfaced to a host PC through an optically isolated high speed serial link (RS485) running at 2 Mbaud (2 Mbits s(-1)). Measurements on a saline filled tank show that the average signal to noise performance of the system is 40 dB measured across all frequencies and that this figure is independent of frequency of measurement. These results suggest that the current system is 10 dB better in absolute terms than the previous Sheffield (Mk3a) system.

Electric Impedance↗

PubMed2000

Relation between tissue structure and imposed electrical current flow in cervical neoplasia.

BACKGROUND: When an electrical potential is applied to human tissue, the pattern of the resulting current flow is determined by the shapes, arrangements, and internal structure of the tissue cells. By measurement of the electrical current patterns over a range of frequencies, and use of an inverse modelling procedure, electrical variables describing the tissue structure can be calculated. We used this method to develop a screening technique for the detection of cervical precancers. METHODS: We used a pencil probe (diameter 5 mm) to measure electrical impedance spectra from eight points on the cervix in 124 women with abnormal cervical smears. Variables that should be sensitive to the expected tissue changes were calculated. These were compared with the colposcopic results. FINDINGS: The measured electrical impedance changes were those predicted on the basis of the expected tissue structures. Measurements made on normal squamous tissues were well separated from those made on precancerous tissues. We constructed receiver-operating-characteristic curves, comparing measurements made on normal tissue and that showing cervical intraepithelial neoplasia grade 2/3; the area under the curve was 0.951. These groups of women could be separated with a sensitivity of 0.92 and a specificity of 0.92. INTERPRETATION: Characteristics of the electrical impedance spectra of tissues can be explained by changes in cell arrangements (layering) and in the size of the nuclei. This relation opens the way to deriving tissue structure from electrical impedance spectral measurements. We show that this approach can be used to give good separation of normal and precancerous cervical tissues.

Biopsy↗

PubMed2000

Assessing the conditions for in vivo electrical virtual biopsies in Barrett's oesophagus.

It has previously been shown that it is possible to differentiate between squamous and columnar epithelia in rat and resected human tissues using an impedance probe to make in vitro measurements. This probe can be passed down an endoscope allowing measurements to be made in patients. However, the probe emerges parallel to the oesophageal wall, with little room to manoeuvre. The conditions of control required to give reliable readings have been investigated. The importance of pressure applied and the angle of approach to the oesophagus was assessed. Pressures in the range 26.6 Pa to 46.3 kPa and angles in the range 15-90 degrees were considered. In in vitro studies it was observed that it was possible to obtain consistent readings with pressures greater than 2.9 kPa and with angles greater than 15 degrees between the probe and the oesophagus. These conditions can be achieved in vivo, and readings obtained from twelve patients are shown (45 readings on normal squamous, 34 on Barrett's oesophagus and 22 on stomach). At low frequencies (9.6-153.2 kHz), a Mann-Whitney test shows a significant difference (p < 0.001) when comparing the means from squamous and columnar, and also when readings from Barrett's and normal gastric epithelia are compared (p < 0.001).

Animals↗

PubMed2000

Movement artefact rejection in impedance pneumography using six strategically placed electrodes.

In this paper, we have proposed a technique for reducing movement artefacts in impedance pneumography by placing six electrodes at appropriate locations and suitably combining the measurements obtained. The strategy for electrode placement was based on the observation that the electrodes appeared to slide over the rib cage along with the skin, during movement. A volume conductor model of the thoracic cavity was developed and movement artefacts were simulated by shifting the electrodes to a different location on the surface. The impedance changes due to movement in one of the measurements of a 'symmetrical pair' were 180 degrees out of phase with respect to those observed in the other measurement of that pair. However, the impedance changes due to breathing were in phase in both these measurements. Thus, it was possible to reduce movement artefacts by taking a mean of these measurements without affecting the breathing related changes. The six electrodes could be configured into two such symmetrical pairs. The same observation was made in experimental data recorded from human subjects. This indicated that movement artefacts were caused by sliding of electrodes along with the skin and could be reduced by using the six-electrode configuration.

Cardiography, Impedance↗

PubMed2000

Cole equation modelling to measurements made using an impulse driven transfer impedance system.

Electrical impedance measurements are used to obtain information about a subject, tissue sample or tissue model under test. There are several ways of obtaining these impedance data and thereafter analysing the data to obtain relevant parameters. This paper shows how a completely isolated drive and receive system using current pulses, as opposed to sine waves, achieves good fitted results with resistor-capacitor Cole phantoms.

Data Interpretation, Statistical↗

PubMed1999

Virtual biopsies in Barrett's esophagus using an impedance probe.

Preliminary results of electrical impedance measurements in squamous and columnar epithelia in rat and human tissues are presented. The aim of this work is to show the possibility of differentiating these two types of epithelia in terms of their electrical characteristics. For the measurements, we employed a 1.95-m-long, 3.2-mm-diameter, four-electrode probe designed to be used transendoscopically in the diagnosis of Barrett's esophagus (BE). BE is a condition in which the normal squamous epithelium of the esophagus is replaced by columnar epithelium of the intestinal type. This metaplasia is considered as a premalignant condition that puts patients at a 30-125-fold risk of developing adenocarcinoma of the esophagus. The diagnosis and surveillance of BE involve taking multiple biopsies, an expensive and time-consuming procedure. This study constitutes the first stage in the replacement of tissue biopsy by "virtual biopsies".

Animals↗

PubMed1999

A comparison of neonatal and adult lung impedances derived from EIT images.

An objective method of extracting respiratory data from lung images is presented, together with a technique for automatically generating regions of interest delineating the anterior and posterior regions of the lungs. The method is used to extract data on the change in lung impedance with frequency, and on calculated Cole parameters, from 19 normal neonates (gestational age 32 to 42 weeks) and 8 normal adults (age 21 to 82 years). A comparison of the impedance properties of neonatal and adult lungs was made. The variation of lung impedance with frequency in neonates, as derived from EIT images, is significantly different from that found for adults. The implications for a model of the electrical impedance of lung tissue are discussed.

Adult↗

PubMed1996

Three-dimensional electrical impedance tomography.

The electrical resistivity of mammalian tissues varies widely and is correlated with physiological function. Electrical impedance tomography (EIT) can be used to probe such variations in vivo, and offers a non-invasive means of imaging the internal conductivity distribution of the human body. But the computational complexity of EIT has severe practical limitations, and previous work has been restricted to considering image reconstruction as an essentially two-dimensional problem. This simplification can limit significantly the imaging capabilities of EIT, as the electric currents used to determine the conductivity variations will not in general be confined to a two-dimensional plane. A few studies have attempted three-dimensional EIT image reconstruction, but have not yet succeeded in generating images of a quality suitable for clinical applications. Here we report the development of a three-dimensional EIT system with greatly improved imaging capabilities, which combines our 64-electrode data-collection apparatus with customized matrix inversion techniques. Our results demonstrate the practical potential of EIT for clinical applications, such as lung or brain imaging and diagnostic screening.

Algorithms↗

PubMed1996

Parametric modelling for electrical impedance spectroscopy system.

Three parametric modelling approaches based on the Cole-Cole model are introduced. Comparison between modelling only the real part and modelling both the real and imaginary parts is carried out by simulations, in which random and systematic noise are considered, respectively. The results of modelling the in vitro data collected from sheep are given to reach the conclusions.

Animals↗

PubMed1996

A two-dimensional Kolmorogov-Smirnov test for binned data.

A new two-dimensional version of the Kolmorogov-Smirnov test has been developed for the testing of data which has been placed in a two-dimensional array of bins. Monte Carlo testing has been used to derive the test statistics for 8 x 8, 16 x 16 and 32 x 32 arrays. The predicted probabilities for non-square arrays and arrays with intermediate numbers of elements are compared to the probabilities derived from Monte Carlo testing. The new test is applied to two-dimensional histograms of movement vectors derived from electrical impedance images of the stomach.

Humans↗

PubMed1996

Determination of the relationship between the pH and conductivity of gastric juice.

Studies of gastric secretion were carried out on 14 subjects, some of whom had taken acid secretion inhibitors. In vitro studies were performed in an attempt to ascertain the effect of H+ and Na+ ions on conductivity. There is a strong correlation between intragastric pH and conductivity for pH < 2, but none of the gastric samples were isotonic. The measured conductivity of the samples was therefore considerably lower than predicted for isotonic gastric juice.

Adult↗

PubMed1996

Can electrical impedance tomography be used to detect gastro-oesophageal reflux?

A number of experiments have been carried out to investigate the feasibility of using electrical impedance tomography as an alternative method for identifying gastro-oesophageal reflux. Five subjects have been studied using simultaneous EIT and intraoesophageal pH measurements. A refluxogenic meal was given during the course of the recording to induce reflux in the subjects. Results show that there is some change in conductivity in the stomach region during some of the reflux episodes, but EIT is not able to detect all of the pH changes associated with the reflux. Other large conductivity changes, relating to gastric motility can be detected at times when no reflux is occurring.

Adult↗

PubMed1996

Reproducibility of electrical impedance tomographic spectroscopy (EITS) parametric images of neonatal lungs.

The reproducibility of electrical impedance tomographic spectroscopy (EITS) images of neonatal lungs have been investigated in 11 clinically stable babies. We have used the Sheffield Mark IIIa EITS system. An average inspiration frame was generated from the data frames associated with maximum inspiration. Frequency images were reconstructed from these frames. The frequency images were analysed to locate the pixel with the maximum change in the right lung field. The change was defined as the 614 kHz measurement relative to 9.6 kHz. A 3 x 3 pixel region of interest was centred at this point. The changes in impedance with frequency for this region of interest show good overall reproducibility between electrode applications for eight frequencies (95% limits of agreement +/- 28%). This reproducibility is improved (95% limits of agreement +/- 13%) by omitting the highest frequency (1.2 MHz) which is most subject to system noise. The parameters for the Cole model derived from data with the highest frequency omitted are less reproducible between electrode applications (95% limits of agreement, R/S +/- 0.83, fc +/- 81.6, RC +/- 0.52, SC +/- 0.39). We suspect that the parametric model used may have an effect on this. The signals recorded at the highest frequency (1.2 MHz) are a major source of variability. The reproducibility results are improved by omitting this frequency from the analysis.

Body Water↗

PubMed1995

Data processing techniques for serial EIT spectroscopy images: a review of some preliminary results.

Mutlifrequency EIT imaging should allow specific organs within the body to be identified by their impedance spectrum, and the use of parametric imaging should lead to a much greater freedom from movement artefacts. This will make EIT more attractive as a monitoring technique, but the data rate will require automated processing of the images. The application of dynamic regions of interest, generated on a frame by frame basis, is described, with examples from the imaging of neonatal lungs and adult stomach. The lung can be objectively identified on a single frame from the fRSC, SC and RC images, but the stomach could only be identified on the dynamic images.

Animals↗

PubMed1995

Detecting oesophageal-related changes using electrical impedance tomography.

Preliminary work has been carried out using the Sheffield mark II real time EIT system, looking for changes in conductivity which occur in the stomach and oesophagus following a swallow of a small volume of either a conducting or a non-conducting liquid. This has been done using three different configurations: a conventional transverse array placed around the thorax, a rosette array on the abdomen, and a rosette array placed on the back. Results show a significant difference between the two liquids, which can be detected equally well by the three electrode configurations.

Adult↗

PubMed1995

Multifrequency and parametric EIT images of neonatal lungs.

The aims of the study were to investigate the problems involved in making multifrequency EIT measurements on neonates and to compare the images obtained with the results from a group of normal adults. The Sheffield electrical impedance tomographic spectroscopy (EITS) system acquires multifrequency data using a set of eight drive and eight receive electrodes. EITS measurements were made on an inhomogeneous group of 10 neonates admitted to the special care baby unit for observation and feeding. R/S, characteristic frequency, RC and SC parameters were generated using the Cole equation. Comparisons of the parameters were made with data collected from normal adults in another study. We have shown that it is possible to obtain EITS parametric images of neonatal lungs and that there are some differences in Cole parameters between the adult and neonatal groups.

Adult↗

PubMed1995

Identifying oesophageal contents using electrical impedance tomography.

Investigations have been carried out using the Sheffield mark II real-time EIT system, to look at changes in conductivity associated with swallowing. A ring of 16 electrodes was placed around the neck of 10 subjects, who then performed swallows with four liquids of different conductivities, ranging from water (sigma = 0.03 mS cm-1) to salty soup (sigma = 35.8 mS cm-1). Results showed that the conductive and non-conductive liquids could be distinguished. Bolus transit times were calculated from region of interest curves, and the average transit time for the 10 subjects was found to be 320 +/- 100 ms.

Deglutition↗

PubMed1994

Multi-frequency imaging and modelling of respiratory related electrical impedance changes.

Two studies concerning multi-frequency impedance measurements are presented. The first uses tetrapolar measurements made on the thorax and the second electrical impedance tomography images, also made from the thorax. The way in which the impedance and the changes in impedance with ventilation depend upon frequency are investigated using Cole-Cole modelling and also a physiological model of lung tissue. There is an excellent fit to the Cole-Cole model, and the results show that it should be possible to identify tissue on the basis of the impedance spectrum and the spectrum of the changes in impedance during breathing.

Cardiography, Impedance↗