Biomedical subjects

B H Brown

Publications and source records attributed to B H Brown.

At least 19 recordsLinked to original sources

PubMed1992

Back-projection distortions in applied potential tomography images due to non-uniform reference conductivity distributions.

An investigation was conducted to determine and quantify the distortions in applied potential tomography (APT) images reconstructed from data originating from bodies of non-uniform reference conductivity distributions. The results show that the distortions in the images are dependent on the reference conductivity distribution and on whether the images are formed by back projection along the assumed equipotentials of a uniform reference conductivity distribution or along the equipotentials of the true conductivity distribution. We believe that this last finding is significant since our previously held expectation, similar to that of Yorkey and Webster (1987), that back projection along the true equipotentials of the reference conductivity distribution should result in an accurate reconstruction, is shown to be incorrect.

Electric Conductivity↗

PubMed1992

Noise and spatial resolution of a real-time electrical impedance tomograph.

The Sheffield real-time electrical impedance tomograph produces 25 images per second, using 16 electrodes with adjacent-pair current drive and non-iterative image reconstruction. We describe the data acquisition timing of this instrument and present quantitative measurements of its signal-to-noise ratio and spatial resolution.

Electric Conductivity↗

PubMed1992

Blood flow imaging using electrical impedance tomography.

It is shown that a real-time electrical impedance tomography (EIT) system can be used to image the flow of saline through the human vascular system. A 10 ml bolus of 0.9% saline injected intravenously distal to an EIT imaging plane allows venous flow to be observed. Measurements on a cylindrical tank with flow along axial conductive tubes have been used to establish that the area under a concentration against time curve can be obtained from the EIT images and used to determine the flow rate down the tube. In vivo results show that flow images of the venous system in a limb can be obtained and that there is adequate sensitivity to follow the passage of a saline bolus though the cardiac chambers.

Blood Flow Velocity↗

PubMed1992

Monitoring body fluid distribution in microgravity using impedance tomography (APT (applied potential tomography)).

For an astronaut, the excitement of going into orbit is accompanied by a shift of 1 to 1.5 l of fluid from the legs into the upper body. Information on the way the redistributed fluid is handled by the body is very useful to space physiologists studying the process of adaptation to zero-gravity. Applied potential tomography (APT) can be used to image changes in fluid distribution. To ensure that the technique was capable of measuring fluid shifts induced by changing gravitational forces on the body, a standard Sheffield APT system was used to study several subjects during the eight ESA parabolic flight campaign. The results clearly demonstrated the feasibility of using APT for monitoring fluid redistribution during space flight. A battery-powered, body-worn APT system has now been developed for use in space. The equipment was tested on the eleventh parabolic flight campaign. The data collected with the miniaturised system was comparable to that obtained in the earlier experiment. Ergonomic tests indicated that the equipment is no more difficult to operate and maintain under weightless conditions than on earth. The system is undergoing space qualification tests in Munich. If no problems arise it will be used by German astronauts on missions to MIR and Skylab.

Body Fluids↗

PubMed1991

Local blood volume changes in women with pelvic congestion measured by applied potential tomography.

1. Applied potential tomography is a new, non-invasive technology for observing changes in blood volume. We have used it to study 12 women with lower abdominal pain caused by pelvic congestion, and 15 control subjects. 2. A significant increase in blood volume of 1.8% was observed in the pelvis of women with congestion when changing from the supine to the erect position, and of 2.7% in the control subjects (P less than 0.0002). The difference between the groups was not significant. 3. The distribution of the area over which blood volume changes took place was significantly different between the two groups (P less than 0.002). More of the posterolateral part of the pelvis was involved in women with pelvic congestion than in the control subjects. 4. Applied potential tomography distinguishes between normal women and those having pelvic congestion with a specificity of 87% and a sensitivity of 75%.

Blood Volume↗

PubMed1991

Trans-thoracic fluid shifts and endocrine responses to 6 degrees head-down tilt.

A tomographic method of measuring electrical impedance known as Applied Potential Tomography (APT) has been used to image the impedance changes within the thoraxes of 8 healthy volunteers (4 male, 4 female) during 4-h periods of 6 degrees head-down tilt (HDT). A large decrease in impedance, reflecting an increase in thoracic fluid, was apparent within 1 min of tilting, peaked after 45 min, and was maintained throughout, although during the 4 h there was an 8% return towards baseline resistivity. Resistivity changes were most obvious in the region of the lungs. Simultaneous measurements of the key fluid regulating hormones revealed a significant increase in atrial natriuretic peptide (ANP) and a significant decrease in angiotensin II (AII) and aldosterone. There was no significant difference in plasma antidiuretic hormone level. These results illustrate the dynamic nature of fluid shifts during HDT, the spatial distribution of the fluid within the thorax and the associated endocrine responses.

Adult↗

PubMed1990

Electrical impedance assessment of muscle changes following exercise.

Electrical impedance measurements have been assessed as a method of detecting changes in striated muscle following vigorous exercise. Transverse and longitudinal resistivities of the calf and thigh have been measured before and after four subjects ran a half marathon (21 km). No changes were observed in longitudinal resistivity but transverse resistivity rose by an average of 7% following the race. These results are consistent with changes in the muscle fibre membranes or interstitial fluid content.

Adult↗

PubMed1988

Determination of upper arm muscle and fat areas using electrical impedance measurements.

An electrical impedance technique is described which enables the cross-sectional areas of fat and muscle in the upper arm to be recorded. By making comparisons with measurements obtained using the x-ray technique of computerised tomography (CT) scanning it is shown that fat can be determined to a mean accuracy of 2.3 cm2 and muscle to a mean accuracy of 1.5 mm2. These results are more accurate than a parallel set of measurements made using the traditional anthropometric technique.

Adipose Tissue↗

PubMed1988

Comparison of applied potential tomography and impedance epigastrography as methods of measuring gastric emptying.

Two new non-invasive methods of measuring gastric emptying, impedance epigastrography (IE) and applied potential tomography (APT) have been compared. Measurements in vitro showed that there is a good correlation between the square of the radius of a glass rod placed in the centre of a tank and values obtained by IE or APT. However, if the rod is moved anteriorly in the tank IE values increase markedly, whereas APT values are unchanged. Both APT and IE can be used to follow gastric emptying of liquid meals; however, the results obtained using APT are more reproducible and have a better correlation with those obtained simultaneously by scintigraphy. Neither method was able accurately to follow gastric emptying unless gastric acid secretion was inhibited by cimetidine.

Adult↗

PubMed1988

Methods of cardiac gating applied potential tomography.

Electrical impedance imaging of the heart, pulmonary perfusion and the great blood vessels can only be achieved by synchronising the data collection with cardiac activity. Due to low signal-to-noise ratio, temporal averaging is needed to improve the image quality. In this study several methods of ECG gating are attempted to synchronise the applied potential tomography (APT) serial data collection with the cardiac cycle. They allow us to collect sequential images time-locked with the R-wave of the patient, and hence image the pulsatile movement of blood. Different methods are examined for their sampling speeds, noise levels and ability to image before systole. A method of image data rearrangement in order to provide an apparent increase in speed is also discussed.

Coronary Vessels↗

PubMed1988

The effect of the skull of low-birthweight neonates on applied potential tomography imaging of centralised resistivity changes.

An investigation is presented into the likely effects of the neonatal skull on impedance images produced by applied potential tomography (APT) by imaging impedance changes inside the skull of a human infant of occipito-frontal circumference 30 cm. Measurements have been made with the skull immersed in a tank of saline and electrodes fixed to the perimeter of the tank. Sensitivity measurements have been assessed for imaging a small target close to the centre of the skull as compared with images produced without the skull. The results obtained compare favourably with measurements on a more realistic model of the neonatal head constructed by filling the skull with agar jelly to leave only a thin exterior coating of jelly to simulate the scalp. These experiments suggest that in the central region of the head of a neonate, measured changes by the APT technique are about 44% of that expected from a homogeneous phantom, but that this might vary from 32% to 55% at different points in the image in a very complex manner.

Brain↗

PubMed1988

Problems of cardiac output determination from electrical impedance tomography scans.

Impedance variations within the thorax related to cardiac activity have been localised using cardiac gated electrical impedance images. Since quantitative measurements of local variations can be made from those images, electrical impedance tomography gives more valuable information than impedance cardiography (ICG). However, because of the three-dimensional (3D) and non-uniform nature of the sensitivity function, localised measurements from electrical impedance tomography (EIT) scans are related to the position and geometry of the regions in which a resistivity change occurs. For accurate determination of volume changes from conductivity variations, the 3D sensitivity distribution needs to be known.

Cardiac Output↗

PubMed1988

Applied potential tomography: a new technique for monitoring pulmonary function.

An electrical impedance tomographic imaging system has been developed which can monitor changes in the resistivity of the thorax at a rate of 5 frames per second. There is a high correlation (r greater than 0.95) between changes in resistivity of the lungs and the volume of air inspired. Calibration of the system allows continuous monitoring of the level of ventilation on exercise up to a minute volume of 45 l min-1. The volumetric accuracy of the system is generally within +/- 10% of spirometric measurements. Studies of the effect of changes in posture on the calibration of the system show changes of between +9.5% and -3.8% in normal male subjects. The performance of the system compares favourably with existing techniques for the noninvasive monitoring of ventilation.

Humans↗

PubMed1988

An assessment of dynamic images by applied potential tomography for monitoring pulmonary perfusion.

Applied potential tomography (APT) images can be collected at a rate of 24 per second and data collection can be synchronised with the ECG. Images thus obtained from a thoracic plane allow the spatial separation of impedance changes originating in the heart, aorta and lungs and have raised the possibility of detecting pulmonary perfusion abnormalities from the cardiac-related impedance changes in the lungs. We have recently started a study to compare isotope perfusion scans with APT images and present here a few initial examples which suggest that further investigation of this field may prove rewarding.

Electrodes↗

PubMed1988

Errors in reconstruction of resistivity images using a linear reconstruction technique.

Reconstruction of electrical impedance images using the filtered back projection method of Barber and Brown makes several important assumptions about the object being imaged. These are principally that the object has a circular boundary, is two-dimensional and of impedance close to uniform, and has electrodes equally spaced on its boundary. In practice few of these assumptions are met, yet the method appears to give sensible and useful images. This paper looks at errors of reconstruction produced by non-ideal placement of the electrodes and shows that the reconstruction method is insensitive to such placement errors.

Algorithms↗

PubMed1988

Possibilities and problems of real-time imaging of tissue resistivity.

We show that the surface potential profiles used for electrical impedance tomography are largely determined by body shape and to a much lesser extent by internal resistivity distribution. The results of 2D finite element modelling show that changes of profile amplitude greater than a factor of four can be expected for the thorax. This offers the possibility of determining body shape from surface potential profiles and hence the determination of static images. Some of the other problems and possibilities of impedance imaging are discussed and in particular the design of a system for real time cardiac gated imaging.

Animals↗

PubMed1987

Theoretical limits to sensitivity and resolution in impedance imaging.

In any practical impedance imaging system it is important to be able to predict the image quality which can be expected from particular measurements. It is of interest both to establish the smallest object that can be detected for a certain noise level and to determine the maximum resolution for a certain number of electrodes. In impedance imaging this is not straightforward. The reason is that the resolution and the accuracy of an image which represents a conductive region are related to the number of electrodes and to the noise on the measurements. They also vary with position in the image and depend on the particular distribution of conductivity itself. It is therefore not possible, in general, to make quantitative statements about the resolution and accuracy. It is of course possible to make qualitative statements, but they are not of much use in any particular situation. Formulations are presented here which do allow quantitative assessment of the resolution and accuracy in a certain class of conductive regions. The regions to which they apply are two-dimensional and have a circular boundary shape. The details of the approach are included, both mathematically and descriptively. The quantitative improvement in image quality which can be obtained by reducing the noise, is shown both in terms of accuracy and resolution. The limit to the improvement in quality which can be obtained by taking unlimited independent measurements (i.e. using an unlimited number of electrodes) is calculated. It is shown how to predict the smallest sized object that can just be detected by measurements with a known level of noise.

Electric Conductivity↗

PubMed1987

Limitations in hardware design in impedance imaging.

The collection of data suitable for impedance imaging is a well defined task. Once the number of electrodes is chosen, it is possible to specify the number of independent measurements which must be made. Having done so, a data collection system can be designed; preferably with the view to both maximising the speed of data collection and minimising the noise on the measurements. The former is desirable to eliminate aliasing when taking measurements on regions in which the conductivity varies with time, the latter to ensure maximum image quality. When designing such a system many practical problems become apparent. Some are a result of the electrical components used. In principle these can be overcome, although in practice they will always be important. Other problems arise from the nature of the measurements and the way in which they must be taken. These problems do not depend on how the hardware is implemented. They impose fundamental constraints on the quality of the measurements. The problems in the design of a data collection system are considered here. The design is analysed at the functional rather than electronic level, so the results are of general use. Factors considered include the number of measurements, speed of data collection, noise, bandwidth, isolation, common mode feedback, dynamic range, and quantisation.

Electric Conductivity↗