Imaging of free-space interference patterns used to manufacture fiber bragg gratings.

A technologically important use of the free-space interference patterns formed by phase gratings is in the creation of the refractive-index variation along optical fiber Bragg gratings. The patterns can be imaged directly by use of a tapered optical fiber tip, which acts as a local probe of the optical field. We present measurements of these patterns under varying conditions and compare them with theoretical predictions.

Comparison of algorithms for non-linear inverse 3D electrical tomography reconstruction.

Non-linear electrical impedance tomography reconstruction algorithms usually employ the Newton-Raphson iteration scheme to image the conductivity distribution inside the body. For complex 3D problems, the application of this method is not feasible any more due to the large matrices involved and their high storage requirements. In this paper we demonstrate the suitability of an alternative conjugate gradient reconstruction algorithm for 3D tomographic imaging incorporating adaptive mesh refinement and requiring less storage space than the Newton-Raphson scheme.

Optimal imaging with adaptive mesh refinement in electrical impedance tomography.

In non-linear electrical impedance tomography the goodness of fit of the trial images is assessed by the well-established statistical chi2 criterion applied to the measured and predicted datasets. Further selection from the range of images that fit the data is effected by imposing an explicit constraint on the form of the image, such as the minimization of the image gradients. In particular, the logarithm of the image gradients is chosen so that conductive and resistive deviations are treated in the same way.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.