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E T McAdams

Publications and source records attributed to E T McAdams.

15 recordsLinked to original sources
PubMed2002

Sauer's non-linear voltage division.

The non-linearity of the electrode-tissue interface impedance gives rise to harmonics and thus degrades the accuracy of impedance measurements. Also, electrodes are often driven into the non-linear range of their polarisation impedance. This is particularly true in clinical applications. Techniques to correct for electrode effects are usually based on linear electrode impedance data. However, these data can be very different from the non-linear values needed. Non-linear electrode data suggested a model based on simple assumptions. It is useful in predicting the frequency dependence of non-linear effects from linear properties. Sauer's treatment is a first attempt to provide a more general and rigorous basis for modelling the non-linear state. The paper reports Sauer's treatment of the non-linear case and points out its limitations. The paper considers Sauer's treatment of a series arrangement of two impedances. The tissue impedance is represented by a linear voltage-current characteristic. The interface impedance is represented by a Volterra expansion. The response of this network to periodic signals is calculated up to the second-order term of the series expansion. The resultant, time-dependent current is found to contain a DC term (rectification), as well as frequency-dependent terms. Sauer's treatment assumes a voltage clamp across the impedances and neglects higher-order terms in the series expansion. As a consequence, it fails adequately to represent some experimentally observed phenomena. It is therefore suggested that Sauer's expressions for the voltage divider should be combined with the non-linear treatments previously published by the co-authors. Although Sauer's work on the non-linear voltage divider was originally applied to the study of the non-linear behaviour of the electrode-electrolyte interface and biological tissues, it is stressed, however, that the work is applicable to a wide range of research areas.

Electric Impedance↗
PubMed2002

Harmonic analysis of low-frequency bioelectrode behavior.

This paper concerns the modeling and interpretation of harmonics observed as a result of the nonlinear electrical properties of biomedical electrode/electrolyte interfaces. The higher order harmonics have been calculated assuming that the nonlinearity of the interfacial impedance is dominated at low frequencies by the nonlinear faradaic charge transfer resistance. The harmonic distribution in the output signal is compared between 1) the author's theoretical model based on the Butler-Volmer equation and 2) Schwan's empirical model and results. The influence of different parameters such as the number of electrons involved in the faradaic reaction and the transfer coefficient was investigated in order to physically interpret the experimental results. A good agreement was found between the authors' model and some of the experimental data previously reported in the literature. Further, potentially productive areas of research have been identified.

Computer Simulation↗
PubMed2000

Nonlinear transient response of electrode-electrolyte interfaces.

The voltage transient response, V(t), of the electrode-electrolyte interface is known to be nonlinear. It has been shown by Onaral and Schwan in 1983 that the DC limit current of linearity, IAL, is proportional to t-beta were beta is the fractional power dependence of the linear, short pulse duration impedance. We now seek to explain the physical phenomena underlying this observation. We present an equivalent circuit model of the interface and highlight the major source of the observed nonlinear behaviour. Using the equivalent circuit model and associated formulae, an expression for the limit current of linearity is derived and compared with that found experimentally by Onaral and Schwan. According to this theoretical model, a current of as little as 22 nA can be sufficient to drive an electrode system into nonlinear behaviour at longer pulse durations.

Electric Impedance↗
PubMed1999

In vivo ac impedance spectroscopy of human skin. Theory and problems in monitoring of passive percutaneous drug delivery.

The use of impedance spectroscopy to evaluate transdermal drug delivery is discussed and new techniques and protocols are suggested to avoid or minimize potential problems. A novel multichannel impedance analyzer, exploiting the advantages of the "three-electrode" configuration, was employed to measure the effects of differing topically applied concentrations of the percutaneous local anesthetic amethocaine on the electrical properties of the treated skin sites. Each measured impedance spectrum was modeled by an equivalent circuit consisting of a resistor in series with the parallel combination of a pseudocapacitance and a resistor. Due to differences in skin sites and to the finite times taken to apply each electrode, it was difficult to satisfactorily compare and contrast the results obtained from adjacent skin sites. Normalization of data highlighted differences in relative impedance changes and aided the meaningful comparison of treated skin sites.

Administration, Cutaneous↗
PubMed1996

Factors affecting electrode-gel-skin interface impedance in electrical impedance tomography.

The magnitude, mismatch and temporal variations of the electrode-gel-skin interface impedance can cause problems in electrical impedance tomography (EIT) measurement. It is shown that at the high frequencies generally encountered in EIT the capacitive properties of the electrode interface, and especially those of the skin, are of primary importance. A wide range of techniques are reviewed that could possibly be used to minimise these problems. These techniques include the use of skin preparation, penetration enhancers, temperature and electrical impulses. Although several of these techniques appear very attractive, they are not without serious potential drawbacks. A combination of some of these techniques may well hold the key to success.

Electric Impedance↗
PubMed1995

Tissue impedance: a historical overview.

Over the past 150 years the study of the electrical properties of various biological tissues has been undertaken by researchers from a wide variety of scientific backgrounds. This has, unfortunately, led to the existing range of confusing and misunderstood terminology/concepts. Some of the most important are presented and explained.

Animals↗
PubMed1994

Physical interpretation of Schwan's limit voltage of linearity.

The electrode/electrolyte interface impedance can be represented by the parallel combination of a non-faradaic pseudocapacitance and a faradaic, charge transfer resistance. The non-linearity of the overall electrode/electrolyte interface impedance is largely due to that of the faradaic resistance which is derived from the Butler-Volmer equation. As the charge transfer resistance dominates the interface impedance at low frequencies, it is in this region that non-linearities are first observed. The voltage limit of linearity has been investigated and found to increase gradually for higher frequencies. Although relatively linear compared with the charge transfer resistance, the non-faradaic impedance becomes non-linear at large applied voltage amplitudes and dominates the high-frequency non-linear behaviour of the overall interface impedance. Mid-frequencies are affected by a combination of the faradaic and non-faradaic non-linearities.

Electric Impedance↗
PubMed1994

Portable cardiac mapping assessment of acute ischaemic injury.

A portable cardiac mapping system is used to improve the accuracy of diagnosis of acute ischaemic injury outside hospital. Patients presenting chest pain suggestive of myocardial infarction (MI) were mapped by attendant medical personnel operating from a mobile coronary unit. These first MI maps were compared against average normal maps using QRS and ST-T isointegral values. Discriminant function analysis performed on the parameters achieved a sensitivity of 90% and a specificity of 96%.

Body Surface Area↗
PubMed1992

A physical interpretation of Schwan's limit current of linearity.

In this the second of a series of papers on the nonlinearity of the electrode-electrolyte interface impedance, the wealth of experimental observations which exists in the literature on AC impedance nonlinearity is physically interpreted. The interface impedance is well represented by the parallel combination of a constant phase angle impedance and a charge transfer resistance. The charge transfer resistance is the major source of the observed nonlinearities. As a result, the current limit of linearity, iL, increases with frequency such that iL is proportional to omega beta. The series resistance, Rs, of the interface impedance initially increases with applied signal amplitude, reaches a maximum and then decreases. The series reactance, Xs, decreases monotonically with signal amplitude.

Electric Conductivity↗
PubMed1992

Optimal electrolytic chloriding of silver ink electrodes for use in electrical impedance tomography.

The electrode-electrolyte interface impedance may be simplistically modelled by an equivalent circuit comprising a resistance, RTOTAL, in series with an empirical, constant phase angle impedance, ZCPA. This pseudo-capacitance can be thought of as representing empirically the non-faradaic, double layer capacitance in the presence of specific adsorption and surface roughness effects. RTOTAL is the sum of the lead and electrolyte resistances. Depositing a thin layer of silver chloride on silver electrodes can yield improved electrical performance characteristics (potential and impedance) when used in conjunction with a chloride gel. An electrolytically deposited AgCl layer tends to have a rough surface profile thus leading to an increase in the effective interface area. This gives rise to a decrease in RTOTAL and ZCPA, both of which are desirable. Unfortunately AgCl is a relatively poor conductor. Increasing layer thickness causes RTOTAL to increase, thus adversely affecting the inter-electrode impedance at high frequencies. Electrode systems for use in electrical impedance tomography therefore require only relatively thin layers of AgCl.

Electric Conductivity↗
PubMedDate not supplied

A high frequency electrical impedance tomograph using distributed parallel input channels.

Electrical Impedance Tomography (EIT) is an imaging technique based on multiple impedance measurements using electronically multiplexed surface electrodes. The present study describes an EIT prototype tomograph which uses two frequencies of applied signal: 31.25 kHz (within the frequency range originally used in early EIT studies) and 250 kHz. The use of the latter frequency was made possible as a result of several technical innovations including the use of separate groups of electrodes for current injection and for differential voltage measurement and the use of parallel input channels distributed around the object. In vitro images were successfully obtained using the system at the above two frequencies. It is concluded from this study that Electrical Impedance Tomography can be used at sufficiently high frequencies to enable bio-electrical tissue characterisation.

Electric Conductivity↗