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Biomedical subjects

B H Blott

Publications and source records attributed to B H Blott.

17 recordsLinked to original sources
PubMed2001

Adaptive mesh refinement techniques for electrical impedance tomography.

Adaptive mesh refinement techniques can be applied to increase the efficiency of electrical impedance tomography reconstruction algorithms by reducing computational and storage cost as well as providing problem-dependent solution structures. A self-adaptive refinement algorithm based on an a posteriori error estimate has been developed and its results are shown in comparison with uniform mesh refinement for a simple head model.

Algorithms↗
PubMed2000

High fidelity imaging and high performance computing in nonlinear EIT.

We show that nonlinear EIT provides images with well defined characteristics when smoothness of the image is used as a constraint in the reconstruction process. We use the gradient of the logarithm of resistivity as an effective measure of image smoothness, which has the advantage that resistivity and conductivity are treated with equal weight. We suggest that a measure of the fidelity of the image to the object requires the explicit definition and application of such a constraint. The algorithm is applied to the simulation of intra-ventricular haemorrhaging (IVH) in a simple head model. The results indicate that a 5% increase in the blood content of the ventricles would be easily detectable with the noise performance of contemporary instrumentation. The possible implementation of the algorithm in real time via high performance computing is discussed.

Algorithms↗
PubMed1998

Nonlinear reconstruction constrained by image properties in electrical impedance tomography.

It is proposed that image quality, for example the degree of roughness, in electrical impedance tomography is the essential measure required to regularize nonlinear reconstruction. Most previously published work has addressed efficiency, stabilization and speed of reconstruction and has overlooked the targeted image qualities. The measure of quality adopted is the mean square gradient of the logarithm of resistivity which, in combination with the chi2 statistic as a measure of the fit to the data, is minimized by iteration until convergence to a stable image is achieved. This penalty function is invariant to the scale of the resistivity and to the interchange of resistivity and conductivity. The algorithm is tested on computer simulated data and on measurements from a cylindrical tank of electrolyte. The results demonstrate the increased image definition that it would be possible to achieve as data acquisition systems are improved. The images show how a reduction in resolution can be traded for reduced noise artefacts, by selecting an appropriate target chi2.

Algorithms↗
PubMed1998

Electrical impedance tomography with compensation for electrode positioning variations.

Ideally electrical impedance tomography (EIT) should not be oversensitive to electrode positions, but this conflicts with efforts to produce high-resolution images. Two procedures are presented that balance reducing the sensitivity to electrode position errors with generating practicable EIT images. The first provides a criterion based on electrode sensitivity for regularizing the reconstruction through spectral expansion. The main consequences of this are that smoother images are produced and the number of artefacts and their magnitude are generally reduced. The second modification uses the recorded data to compensate for electrode movements that have occurred after the reference data were measured. Image smoothness is used as the criterion for the readjustment. Computer simulation tests have shown that this modification produces improved image fidelity.

Biophysical Phenomena↗
PubMed1997

An optical trigger source for hydrophone-based ultrasound measurement systems: a feasibility study.

Ultrasound is routinely used in many medical applications. Concern for the safety of subjects undergoing investigations has led to the development of systems to quantify the acoustic output of such devices. One system, the NPL ultrasound beam calibrator, uses a multi-element hydrophone to determine rapidly the temporal and spatial characteristics of the acoustic output. When investigating pulsed systems it is necessary to synchronise data acquisition with the launch of the acoustic pulse, a procedure that currently can be both time consuming and difficult. This article examines the feasibility of using an alternative, optical approach, generating a trigger signal in response to the changes in refractive index associated with propagation of the acoustic pulse.

Acoustics↗
PubMed1996

EIT data noise evaluation in the clinical environment.

In the clinical environment the reliable interpretation of EIT images depends on the quality of the data. In the electrically noisy hospital environment the system performance needs to be assessed for each clinical investigation. From the model of noise presented, a figure of merit for comparisons of system performance with a known standard, or with previous studies, can be generated. The method depends on calculating the variances of the differences in reciprocity measurements as a function of the distance between the current drive electrodes and the receive voltage electrodes. These measurements fit the noise model, with minimal interference from physiological variability, and permit a figure of merit to be calculated which is a representation of the noise at the point to the system. Typical figures of merit are 7.36 +/- 0.03 microV for a test card and 10.50 +/- 16 microV for subject data.

Analysis of Variance↗
PubMed1996

Optimal filtering of EIT data in spectral expansion analysis.

The signal-to-noise rations for some EIT measurements are very low, and for in vivo EIT measurements these are dependent on the electrode positioning and the distance from the current drive. The effect of removing noisy measurements to produce higher-fidelity images was investigated for the case of gastric emptying data. A consequence of this filtering was the reduction in the size of the sensitivity matrix and its subsequent singular-value decomposition. Several different filters were tested and for each of these spectral expansion regularization filter was optimized using a chi 2 test. Filtering out the measurement made by the spinal electrode, where the spinal bone barrier lies directly in the current path to the stomach, produced improved images by reducing the artefact content in the spinal sector of the conductivity map. For stomach imaging little useful information is produced by the spinal electrode, and the benefits of filtering dominate. However artefact images may be generated. In contrast consistent small improvements were produced by filtering out some of the weakest signals.

Data Interpretation, Statistical↗
PubMed1995

The dependence of EIT images on the assumed initial conductivity distribution: a study of pelvic imaging.

EIT measurements on humans are often made in regions of the body where the conductivity distribution is far from uniform. This paper addresses the problem of deriving accurate quantitative data in one such region: the conductivity changes associated with the accumulation of blood in the pelvic bowl. A computer map of the bone in the pelvic region was constructed, from which an appropriate reconstruction matrix was generated. Both computer simulations and tank tests were performed to assess whether this bone reconstruction matrix produced impedance images with closer fidelity to the measured object than images produced using a reconstruction based on a uniform conductivity distribution. As expected, images produced by the computer simulation indicated that the bone reconstruction matrix produced images of better fidelity than did the uniform reconstruction matrix. However, in the case of the tank data only a moderate improvement was achieved. The reconstruction matrix based on a uniform conductivity distribution was found to produce satisfactory images for both bone and near-uniform objects, but for regions further into the pelvic bowl, where the signal was lower, the uniform reconstruction matrix was less satisfactory.

Computer Simulation↗
PubMed1992

Review of neural network applications in medical imaging and signal processing.

The current applications of neural networks to in vivo medical imaging and signal processing are reviewed. As is evident from the literature neural networks have already been used for a wide variety of tasks within medicine. As this trend is expected to continue this review contains a description of recent studies to provide an appreciation of the problems associated with implementing neural networks for medical imaging and signal processing.

Diagnostic Imaging↗
PubMed1992

Light scattering study of irradiated lipid bilayer.

Vesicular phospholipid bilayer membranes in the form of giant unilamellar vesicles (GUVs) of dipalmitoylphosphatidylcholine (DPPC) were irradiated with fast neutron fluences ranging from 10(4) to 10(7) n cm-2. The phase behaviour of both non-irradiated and irradiated GUVs was investigated using an angular light scattering technique. A model independent size distribution of the samples and their optical anisotropy (delta) were determined using a maximum entropy technique and the theory of light scattering from spherical shells composed of anisotropic cylindrical molecules arranged radially in the shells. The structural changes in the lipid bilayer exposed to fission neutrons are discussed on the basis of the damaging mechanisms of fast neutrons to both the hydrophobic and hydrophilic regions of the lipid bilayer.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1992

Neural networks for electrical impedance tomography image characterisation.

The Southampton electrical impedance tomography (EIT) system used a Sheffield data acquisition unit and a PC based 'Harlequin' transputer card to reconstruct and display images of the distribution of internal conductivity within the thorax. The system produces real-time images relating to both cardiac and pulmonary function. As a first step towards diagnosis using these images neural nets have been applied to the identification of regions of interest in the EIT images for which some activity with time, such as ventricular ejection, is sought. This paper addresses the use of a back-projection network to identify characteristic regions within the images. The network facilitates the production of automated real-time activity plots by defining their effective extent in the images of specific organs. The application is novel within the medical imaging field as the aim is to use neural networks for real-time image analysis.

Electric Conductivity↗
PubMed1992

Pulmonary perfusion and ventricular ejection imaging by frequency domain filtering of EIT (electrical impedance tomography) images.

While EIT images can produce clinically useful qualitative information, the extraction of quantitative data is essential in clinical monitoring. In the case of imaging of the thorax the parameters available relate to cardiac activity and pulmonary perfusion. Imaging the relatively small changes in the resistivity of the lungs due to pulmonary perfusion in the presence of noise and the larger ventilation component is difficult. Suggested solutions involve multiple time averaging of cardiac gated data or reconstructed images. The required number of data frames for this type of processing is large (at least 100 cardiac cycles). Because the ventilation and perfusion components of the resistivity signals are well separated in the frequency domain, they can be differentiated by filtering. We report the results of this analysis which requires a data collection period of typically 15 s.

Blood Flow Velocity↗
PubMed1991

The spectral expansion of a head model in electrical impedance tomography.

This paper examines whether electrical impedance tomography (EIT) can provide information of use to magneto-encephalographic modelling (MEG). The EIT image domain is expanded in a complete set of orthogonal basis images, the number of which is given by the number of independent measurements (104 for 16 electrodes). They are ordered according to their sensitivity to data noise, with more centrally located features and higher spatial frequency components appearing towards the higher terms in the series, in the case of uniform resistivity distribution. This indicates that the resolution can be improved at the expense of degrading signal-to-noise in the reconstructed image. Applied to an approximate model of the head, the technique generates a set of basis images that emphasises central features of the head.

Brain↗
PubMed1990

A transputer implemented algorithm for electrical impedance tomography.

Electrical impedance tomography (EIT) offers a non-invasive technique of great scope for producing clinically important information in a number of medical applications. Initial work by Brown used an isopotential approach to solving the complex backprojection problem. In this study a less restrictive algorithm for EIT has been developed based on the work of Yorkey and of Kaczmarz. Since considerably more processing is required this has been implemented on a transputer in Occam 2 with the aim of achieving real-time imaging. Data were collected, using a Sheffield prototype EIT system, from a test object and a human thorax. Image processing used both the 'Sheffield' and the Yorkey/Kaczmarz algorithms. Our initial results indicate that the images generated using the latter approach were more representative of the source impedance distributions.

Adult↗
PubMed1984

The superdiamagnetic effect of magnetic fields on one and two component multilamellar liposomes.

For multilamella vesicles of DMPC, DPPC, DSPC, binary mixtures of DMPC-DPPC, DMPC-DSPC, DMPC-DPPE, DOPC and egg lecithin, the optical turbidity decreases significantly on the application of a magnetic field in excess of about 0.2 T, provided that the temperature is above the pretransition value. The turbidity reaches a limiting value for magnetic fields of about 2 T. The effect is attributed to augmentation of the diamagnetic anisotropy of the lipid molecules by clustering within the bilayer, with consequent orientation of either the individual 'superdiamagnetic' clusters or the whole liposome. It is suggested that, since most animal cell membranes are largely in the liquid crystalline phase, it is possible that homogeneous magnetic fields as low as 0.2 T may cause biologically significant changes within the membrane.

Liposomes↗
PubMed1983

Dye permeability at phase transitions in single and binary component phospholipid bilayers.

By encapsulating a pH-sensitive dye, phenol red, in multilamellar liposomes of DMPC, DPPC and DMPC/DPPC mixtures, the permeability of these phospholipid bilayers to dye as a function of temperature has been studied. For both DMPC and DPPC liposomes, dye release begins well below the main gel-to-liquid-crystalline phase transition (24 degrees C and 42 degrees C, respectively) at temperatures corresponding to the onset of the pretransition (about 14 degrees C and 36 degrees C, respectively) with DPPC liposomes exhibiting a permeability anomaly at the main phase transition (42 degrees C). The perturbation occurring in the bilayer structure that allows the release of encapsulated phenol red (approx. 5 A diameter) is not sufficient to permit the release of encapsulated haemoglobin (approx. 20 A diameter, negatively charged). In liposomes composed of a range of DMPC/DPPC mixtures, dye release commences at the onset of the pretransition range (determined by optical absorbance measurements) and increases with increasing temperature until the first appearance of liquid crystalline phase after which no further dye release occurs. Interestingly, the dye retaining properties of DMPC and DPPC liposomes well below their respective pretransition temperature regions are very different: DMPC liposomes release much encapsulated dye at incubation temperatures of 5 degrees C whilst DPPC liposomes do not.

Animals↗