Time series of EIT chest images using singular value decomposition and Fourier transform.

The aim of this study is to propose a useful method for exploring regional ventilation and perfusion in the chest. The paper describes two methods based on singular value decomposition (SVD) and Fourier transform (FT) respectively. This work shows that power spectral density (PSD) and phase images (derived from the Fourier transform) are easier to interpret and more useful tools for exploiting in vivo EIT data in healthy volunteers in order to explore the cardiovascular and respiratory systems.

Generalized optimal current patterns and electrical safety in EIT.

There are a number of constraints which limit the current and voltages which can be applied on a multiple drive electrical imaging system. One obvious constraint is to limit the maximum ohmic power dissipated in the body. Current patterns optimizing distinguishability with respect to this constraint are singular functions of the difference of transconductance matrices with respect to the power norm (the optimal currents of Isaacson). If one constrains the total current (L1 norm) the optimal patterns are pair drives. On the other hand if one constrains the maximum current on each drive electrode (an L(infinity) norm), the optimal patterns have each drive channel set to the maximum source or sink current value. In this paper we consider appropriate safety constraints and discuss how to find the optimal current patterns with those constraints.

Dielectric relaxation time spectroscopy.

A new mathematical method is developed to recover the permittivity relaxation spectrum of living tissue from measurements of the real and imaginary parts of the impedance. Aiming to derive information about electrical properties of living tissue without the prior selection of any impedance model, the procedure calculates the relaxation time distribution. It provides new characteristic independent parameters: time constants, their distribution, and the amplitudes of the associated dispersion. As the beta-dispersion is the most important in the area of electrical impedance spectroscopy of tissue, the paper gives an estimate of the essential frequency range to cover the whole relaxation spectrum in that area. Results are presented from both simulation and known lumped--constant element circuit.

Development of a real-time adaptive current tomograph.

Following the successful development of a multiple-drive electrical impedance tomography system OXPACT-II featuring a voltage-driven current method for in vitro studies, research work currently being undertaken at the EIT research group in Oxford is aimed at developing a real-time multiple-drive adaptive system, called the Oxford Brookes Adaptive Current Tomograph Mark-III (OXBACT-III) which will operate at several frequencies in between 10-160 kHz. The objective of this system development is to enable EIT clinical studies to be undertaken based on the adaptive current method. One of the most important issues addressed in the new system design is to achieve high data acquisition speed while maintaining sufficient system accuracy. This paper will describe the overall data acquisition system structure and relevant system performance specifications.

A high output impedance current source.

A high output impedance current source was required for electrical impedance tomography (EIT) applications capable of operating up to 200 kHz. The architecture is based on operational-amplifier power-supply current sensing and produces a predominantly capacitive output impedance, which for the design presented is approximately 1.2 pF.

An adaptive current tomography using voltage sources.

This paper describes the development of an adaptive electric current tomography system which contains a novel front-end analog architecture. Programmable voltage sources have been used to deliver currents into the study object and to avoid the difficulties of obtaining high quality current sources. Through inverting an admittance matrix, the system is capable of achieving a desired current drive pattern by applying a computed voltage pattern. The tomograph, operating at 9.6 kHz, comprises 32 driving electrodes and 32 voltage measurement electrodes. The study of system noise performance shows high SNR in the data acquisition which is enhanced by a digital demodulation scheme. In vitro reconstruction images have been obtained with the data collected by the tomograph.

Electrical impedance tomography. Electrode current determination from programmable voltage sources.

The accuracy requirements for adaptive current electrical impedance tomography (EIT) measurements exceed the capability of available current sources. A new architecture for an EIT system is described in which the electrode current is set indirectly from a voltage-drive structure. A numerically inverted admittance matrix, obtained from current measurements and driving voltages, has been used to achieve the desired current pattern from programmable voltage sources.

Monitoring of end-tidal carbon dioxide partial pressure during high frequency jet ventilation.

A new method has been developed to measure end-tidal carbon dioxide partial pressure (PECO2) during high frequency jet ventilation (HFJV). A digital flow controller incorporated in a computerized high frequency jet ventilator was used to deliver either a single deep breath or a series of three deep breaths. On user request, HFJV was interrupted and the deep breaths delivered, after which HFJV was resumed. Using a mathematical model, we were able to predict accurately the pressures to which the lungs would be inflated during deep breaths. The effect of varying the deep breath pressure (Pdb) on the ratio of end-tidal PCO2 to arterial (PCO2 (PECO2:PaCO2) was studied in three dogs. In all the dogs, within an optimum Pdb range of 5-10 cm H2O, PECO2 during the first deep breath was found to be similar (+/- 0.2 kPa) to the PaCO2 immediately before the onset of deep breaths. Deep breaths delivered above or below the optimum Pdb range resulted in a decrease in the ratio PECO2:PaCO2. The frequency of jet ventilation (12-200 b.p.m.) before the onset of the deep breaths did not affect PECO2:PaCO2.

The automatic sampling shield: a device for sampling suckled breast milk.

The automatic sampling shield uses the negative pressure exerted by the breast-feeding baby during sucking to withdraw a series of small samples from the milk stream. Shield performance was studied at four feeds for each of 13 babies. Acceptance of the shield was good (96% of feeds) and samples were obtained in 88% of feeds. Sample weight was large enough for micro-biochemical analysis of composition (median weight 1.3 g), without depriving the baby of important amounts of milk (median percentage of weight of milk intake was 4.3% and sample weight was less than 5 g in all cases). The shield should sample a constant proportion throughout the feed, in order to obtain a sample that is representative of the infant's nutrient intake. Fractional test-weighing of feeds showed a tendency for a larger proportion of the feed to be collected at slower flow rates. Modelling on the basis of the observed flow profiles in conjunction with a theoretical fat concentration profile showed that the observed 'disproportionality' would have over-estimated fat intake by a mean of 2.6% per feed (S.D.9.1 range -18.9% to + 16.9%). Using the shield in random sequence at two out of the first four feeds after 0800 h, showed that an infant's mean milk intake with the shield was on average 32% less than on control feeds. Allowing for possible compensation by the infant when control feeds followed low shield intakes, mean milk intakes with the shield were 17% smaller than without the shield.

Evaluating breast-milk intake by test weighing: a portable electronic balance suitable for community and field studies.

A K-Tron Digital Scale (Model DS-1) was used to weigh a group of 52 new born babies. The regression of the weights on true weights obtained with a Sartorius electronic balance 3806MP was 0.9911, with an estimated residual variance of 5.25. With the same babies weighed on a Marsden Weighmaster, the estimated residual variance was 68.93. Data from published studies on other balances are presented for comparison. The K-Tron is a reliable, portable, battery operated balance suitable for measuring milk intake by test weighing in homes or in the field.

Studies on the control of respiration and behaviour during development in ewe-reared lambs.

Radiotelemetric recordings have been made in a series of newborn lambs with implanted electrodes and transducers and an external back-pack transmitter. Cardiac, respiratory and behavioural data have been gathered. These data have been analysed firstly with respect to age. The cardiac and respiratory data have then been related to both sleep state and age. An unexpected rise in both heart and respiratory rates was found after birth and marked differences were obvious in respiratory control in REM and non-REM sleep states.