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Parallel, multi frequency EIT measurement, suitable for recording impedance changes during epilepsy

Abstract

Electrical Impedance Tomography (EIT) has been proposed as a method for imaging and localising epileptic activity in the brain. No existing EIT system meets all of the requirements for effective imaging of epilepsy. A parallel EIT system, employing frequency division multiplexing, is described, which is optimised for measuring impedance changes during epilepsy. The system is capable of imaging short duration, spontaneous events in a saline filled tank, using as little as 1ms of recorded data. In-vivo impedance measurements recorded during epilepsy in a rat model are presented.

Open access
Compression-dependency of soft tissue bioimpedance for in-vivo and in-vitro tissue testing

Abstract

The present study determines the effect of compression over bioimpedance of healthy soft tissue (in-vitro and in-vivo). Electrical impedance spectroscopy (EIS) is a promising tissue characterization and tumor detection technique that uses tissue impedance or admittance to characterize tissue and identify tissue properties as well as cell structure. Variation in EIS measurements while applying pressure suggests that compression tends to affect soft tissue bioimpedance. Moreover, the displacements in tissue caused by applied compression may provide useful information about the structure and state of the tissue. Thus combining the changes to the electrical properties of tissue resulted by applied compression, with the changes in tissue displacements caused by applied compression, and consequently measuring the effect that electrical and mechanical properties have on each other, can be useful to identify tissue structure. In this study, multifrequency bioimpedance measurements were performed on in-vitro and in-vivo soft tissue at different pressure levels. Increasing compression on the in-vitro tissue results in an increase in both extracellular resistance and membrane capacitance while it causes a reduction in the intracellular resistance. However, as the compression over the in-vivo samples increases, the intracellular and extracellular resistance increase and the membrane capacitance decreases. The in-vivo measurements on human body are also tested on contra-lateral tissue sites and similar tissue impedance variation trends are observed in the contra-lateral sites of human body. The evidence from these tests suggests the possibility of using this EIS-Pressure combined measurement method to improve tumor detection in soft tissue. Based upon the observations, the authors envision developing an advanced model based upon the Cole model, which is dependent on tissue displacements.

Open access
A fast method to estimate body capacitance to ground at mid frequencies

Abstract

Impedance measurements that involve the human body are affected by the capacitance between the body and earth ground. This paper describes a fast method to estimate that capacitance at 10 kHz, which is valid for impedance analyzers intended to measure ungrounded impedances. The method does not require any external component other than two common capacitors and two conductive electrodes in contact with the body.

Open access
Impedance surprises
Open access
Optically isolated current source

Abstract

There is a need for isolated current sources for use in selected bioimpedance measurement circuits. The requirement for good isolation is particularly important in medical settings because of safety concerns. A new circuit for producing voltage-controlled current is presented. Measurements have been made on a prototype and simulations have been done on a SPICE model. The presented circuit is an H-bridge where the output devices are the output photodiodes of high-linearity optocouplers. Five operational amplifiers, four high linearity optocouplers, and passive components are used. Output current capability is ±35 μA with an output impedance that is more than 1 M Ω. It is possible to achieve bandwidths above 1 MHz for small load impedances. This circuit is well suited for medical applications thanks to the isolation in the optocouplers.

Open access
Investigating the quasi-oscillatory behavior of electrical parameters with the concentration of D-glucose in aqueous solution

Abstract

The impedance, capacitance and conductance of deionized water-glucose polar solution is measured by employing impedance spectroscopy and a quasi-oscillatory nature of variation with glucose content in the solution is observed. Such quasi-oscillatory nature is attributed to the randomly distributed water-water, water-glucose and glucose-glucose dipole interactions at the molecular level in the solution. A relevant analytical model is developed on the basis of such random distribution of the molecular dipoles and the experimental data agree well with those obtained from the theoretical model. The electrical parameters are measured in the frequency range of 100Hz to 4MHz for the volume fractions of glucose with respect to water in the range of 0.1 to 0.5. The impedance, capacitance and conductance are obtained to be in the range of 1.03 kΩ – 112 kΩ, 34.9 pF – 1.66 nF, and 8.95 μS – 52.9 μS respectively for the glucose volume fraction range considered.

Open access
Development of a real-time, semi-capacitive impedance phlebography device

Abstract

Chronic venous insufficiency of the lower limbs is a disease which is caused by an increased blood pressure inside the veins of the leg and the resulting increase of the contained blood volume. This work focuses on developing a device which uses impedance plethysmography, also known as impedance phlebography, to obtain information about the blood volume in the lower leg and provides the possibility to measure the impedance semi contact-less, e.g. through compression stockings. Furthermore a real-time beat-to-beat interval detection algorithm was implemented. Finally, the function of the developed impedance measuring system and the whole system was verified by comparing it with a gold standard. In comparison to the conductive coupling, the system performed similarly. The analysis showed that the developed system is suitable for semi-capacitive IPG. The algorithm was implemented conservatively since it provided a good false-positive rate of 0 %, but only a moderate sensitivity of about 68 %. Reliable and continuous measurement of the pulse signal was only possible in periods of immobility.

Open access
Simplified estimation of membrane potentials induced by high-frequency electric signals

Abstract

In this paper we show why the poorly conducting cytoplasmic membranes have little effect on the overall impedance of the tissue above a certain frequency, and derive an estimate of this upper frequency. It is further shown that the induced transmembrane potentials at different sites over the membrane can be found through a simple formula for frequencies above the threshold, without the need to analytically or theoretically model the membranes directly. The findings are validated for an irregular cell shape through rigorous numerical modeling.

Open access
The feasibility of using compression bioimpedance measurements to quantify peripheral edema

Abstract

The accurate assessment of body fluid volume is important in many clinical situations, especially in the determination of “dry weight” in a dialysis setting. Currently, no clinically applicable diagnostic system exists to determine the mechanical properties that accurately characterize peripheral edema in an objective and quantitative manner. We have developed a method for quantifying the impact of compression on the electrical properties of tissue by measuring stress-induced changes in bioimpedance (BIS). Using this method, we simultaneously measured the impedance and mechanical response of a tissue mimicking material (tofu) under both quasi-static and dynamic loading conditions. Our results demonstrate a temporal quantification of viscoelastic properties using a viscoelastic phantom tissue model.

Open access
Modelling the Ability of Rheoencephalography to Measure Cerebral Blood Flow

Abstract

Despite the long history of rheoencephalography (REG), some important aspects of the method are still debatable. Bioimpedance measurements offer great potential benefit for study of the human brain, but the traditional four or six electrode method suffers from potential misinterpretations and lack of accuracy. The objective of this paper is to study the possible mechanism of REG formation by means of numerical modelling using a realistic finite element model of the human head. It is shown that the cardiac related variations in electrical resistivity of the scalp contributes more than 60% to the REG amplitude, whereas the brain and cerebrospinal fluid are mutually compensated by each over.

Open access