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Y Z Ider

Publications and source records attributed to Y Z Ider.

6 recordsLinked to original sources
PubMed1992

Electrical impedance tomography using induced and injected currents.

A two-dimensional forward problem formulation is introduced for electrical impedance tomography (EIT) using induced currents. The forward problem is linearised around a certain resistivity distribution and the inverse problem is formulated as a solution of a linear system of equations. Sensitivity of boundary measurements to resistivity variations are analysed for spatially uniform, linear and quadratic fields. The formulation, however, is suitable for studying the effects of a general magnetic field applied to induce the currents in the conductive object. A similar inverse problem formulation is also developed for EIT using injected currents. Simulation studies are performed by reconstructing images of a simulation distribution using both methods separately with generalised inversion. It is also shown that the derived formulations for the inverse problems of the two methods can be combined to solve a larger set of equations with a greater number of independent measurements.

Electric Conductivity↗
PubMed1992

Analysis of three-dimensional software EIT (electrical impedance tomography) phantoms by the finite element method.

In electrical impedance tomography, two-dimensional (2D) finite element solutions are used in the imaging algorithms. It is assumed that a major part of the current flowing through the object is restricted to the measurement plane (i.e. the plane determined by the electrodes which are used for measuring voltage differences) and the current flowing elsewhere is negligible. However, there is usually a three-dimensional (3D) variation of the conductivity distribution and if there are regions of high contrast close to the measurement plane, the measured voltage values may be considerably affected. In this work a 3D finite element analysis is utilised to demonstrate the previously mentioned effects. Examples are given to show the measured voltage differences for conductivity distributions which are identical on the measurement plane but different elsewhere.

Algorithms↗
PubMed1992

Electrical impedance tomography. Determination of the boundary of an object inserted into a water-filled cylinder.

In order to circumvent the electrode position determination problem in static electrical impedance tomography, it is possible to insert the object to be imaged into a water-filled cylinder on which the electrodes are at fixed and known positions. It has previously been shown that if the boundary of the internally placed object and the conductivity of the salty water in the cylinder are known, then a significant improvement in the conductivity image of the object is obtained. An algorithm for finding the boundary of an internally placed object is developed based on the finite element method (FEM). The boundary is assumed to obey a parametric model and the parameters are estimated by inverting a matrix representing the sensitivity of the boundary voltage measurements to parameter variations. The algorithm assumes that the object's internal conductivity is uniform and known. Simulation studies show that if the internal conductivity is not uniform to the extent found in the arm cross-sections, up to 9% error in the boundary, as measured from a centrally placed reference point, may result. It is also shown that if previous knowledge about the boundary shape is used to model the boundary with fewer numbers of parameters, then the boundary may be found with less error.

Algorithms↗
PubMed1990

A new technique for line interference monitoring and reduction in biopotential amplifiers.

It has been reported that power line frequency must be accurately known if line interference is to be accurately subtracted from the output of a bipotential amplifier. In this paper, hardware developed to record the common mode line frequency signal on the body simultaneously with the ECG lead signals of a 15-channel computerized cardiograph is described. This interference reference signal and its quadrature, obtained by software, are linearly combined to be subtracted from any one of the channels to reduce line interference to below the quantization level of the 12 b A/D converter. Coefficients of the linear combination are estimated using linear regression which is applied to the relatively isoelectric regions of the data, excluding the QRS complexes. Since the interference reference signal is available in real time, simultaneously with the ECG signals, another software approach is also adopted in which an adaptive interference reduction algorithm is used to cope with varying interference. A recursive least squares algorithm with forgetting factor is used to update the coefficients. This updating mechanism is gated by the output of a software QRS detector. Results regarding the performance of both the off-line and the adaptive algorithms are given, and the effects of nonisoelectric portions of the ECG lead signals on the estimation of the coefficients are quantified.

Algorithms↗
PubMed1979

Quantitative estimation of insulin sensitivity.

We have evaluated the feasibility of using a mathematical model of glucose disappearance to estimate insulin sensitivity. Glucose was injected into conscious dogs at 100, 200, or 300 mg/kg. The measured time course of insulin was regarded as the "input," and the falling glucose concentration as the "output" of the physiological system storing and using glucose. Seven mathematical models of glucose uptake were compared to identify the representation most capable of simulating glucose disappearance. One specific nonlinear model was superior in that it 1) predicted the time course of glucose after glucose injection, 2) had four parameters that could be precisely estimated, and 3) described individual experiments with similar parameter values. Insulin sensitivity index (SI), defined as the dependence of fractional glucose disappearance on plasma insulin, was the ratio of two parameters of the chosen model and could be estimated with good reproducibility from the 300 mg/kg injection experiments (SI = 7.00 X 10(-4) +/- 24% (coefficient of variation) min-1/(microU/ml) (n = 8)). Thus, from a single glucose injection it is possible to obtain a quantitative index of insulin sensitivity that may have clinical applicability.

Animals↗