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Biomedical subjects

W Gough

Publications and source records attributed to W Gough.

13 recordsLinked to original sources
PubMed2004

A primary field compensation scheme for planar array magnetic induction tomography.

In biomedical magnetic induction tomography (MIT), measurement precision may be improved by incorporating some form of primary field compensation/cancellation scheme. Schemes which have been described previously include gradiometric approaches and the use of 'back-off' coils. In each of these methods, however, the primary field cancellation was achieved only for a single transmitter/receiver combination. For the purpose of imaging, it would be desirable for a fully electronically scanned MIT system to provide a complete set of measurements, all with the primary field cancelled. A single channel suitable for incorporation into an MIT system with planar-array geometry is described. The transmitter is a 6-turn coil of wire 5 cm in diameter. The receiver is a surface mount inductor, of inductance 10 microH, mounted such that, in principle, no net primary field flux threads it. The results of measurements carried out with the single channel system suggest that the signal due to the primary excitation field can be reduced on average by a factor of 298 by the sensor geometry over the operating frequency range 1-10 MHz. The standard deviation and drift of the signal with the system adjusted for maximum primary field cancellation, expressed as a percentage of the signal when the receiver coil was rotated until its axis of sensitivity lay along the primary field, were 0.0009% and 0.009%, respectively. The filter time constant used was 30 ms.

Artifacts↗
PubMed2003

Circuit for the measurement of small phase delays in MIT.

A single-channel MIT measuring system for obtaining phase delays is given. The circuit, which is described in detail, uses a high-frequency analogue multiplier to measure the phase difference between the signal and a reference signal. The noise in the phase measurement is approximately 1.5 millidegree when the time constant of measuring is 0.1 s, and the drift over about 1 day is approximately 10 millidegree.

Electrodes↗
PubMed2003

Magnetic induction tomography: phase versus vector-voltmeter measurement techniques.

In magnetic induction tomography (MIT) the in-quadrature component, and hence the phase, of the received signal contains information about the conductivity of the tissue. The quality of imaging will depend on the precision with which phase can be measured. Preliminary studies suggest that a precision of 10 m degrees may be required for a practical biomedical MIT system operating at 10 MHz. This paper describes the results of measurements carried out with a 16-channel, downconverting, 10 MHz, MIT system utilizing two types of data extraction techniques: direct-phase measurement and measurement of the in-phase and in-quadrature components of the signal with a vector voltmeter. The basic precision provided by each technique was 50 m degrees, with thermal drift representing the major limiting factor. Preliminary measurements of average conductivity and permittivity for a human thigh in vivo are given.

Artifacts↗
PubMed2002

Frequency downconversion and phase noise in MIT.

High-frequency (3-30 MHz) operation of MIT systems offers advantages in terms of the larger induced signal amplitudes compared to systems operating in the low- or medium-frequency ranges. Signal distribution at HF, however, presents difficulties, in particular with isolation and phase stability. It is therefore valuable to translate received signals to a lower frequency range through heterodyne downconversion, a process in which relative signal amplitude and phase information is in theory retained. Measurement of signal amplitude and phase is also simplified at lower frequencies. The paper presents details of measurements on a direct phase measurement system utilizing heterodyne downconversion and compares the relative performance of three circuit configurations. The 100-sample average precision of a circuit suitable for use as a receiver within an MIT system was 0.008 degrees for input amplitude -21 dBV. As the input amplitude was reduced from -21 to -72 dBV variation in the measured phase offset was observed, with the offset varying by 1.8 degrees. The precision of the circuit deteriorated with decreasing input amplitude, but was found to provide a 100-sample average precision of <0.022 degrees down to an input amplitude of -60 dBV. The characteristics of phase noise within the system are discussed.

Amplifiers, Electronic↗
PubMed2001

A numerical model for magnetic induction tomographic measurements in biological tissues.

A finite-difference model has been developed for simulating measurements in magnetic induction tomography (MIT) for biological tissues. The model has three stages: (1) computation of the distribution of current induced in a volume of dielectric due to the magnetic field from an excitation coil; both the electrical conductivity and permittivity of the dielectric are taken into account: (2) computation of the e.m.f. induced in the sensing coil directly from the excitation coil; (3) computation of the e.m.f. induced in a sensing coil due to the current distribution in the dielectric. From the results of stages (2) and (3), the change in signal in the sensing coil due to the dielectric can be obtained, in magnitude and phase, as a fraction of the signal received in the absence of the dielectric. The peak values in the modelled curves agreed to within 14% of practical measurements at 10 MHz on volumes of saline solution with conductivities in the range 0.7 to 6 S m(-1).

Computer Simulation↗
PubMed1999

Magnetic induction tomography. A measuring system for biological tissues.

A single-channel magnetic induction system operating at 10 MHz has been constructed. The system consists of an excitation coil and a sensing coil, between which different objects can be scanned. The eddy currents induced in the object cause perturbations in the sensed magnetic field, which are measured with a phase-sensitive detector with backing off of the signal to improve sensitivity. Scans were obtained for saline solutions with conductivities ranging from 0.001 to 6 Sm-1, encompassing the range for biological tissues. The imaginary part of the perturbation in the sensed magnetic field was found to be proportional to saline conductivity, consistent with theoretical prediction, and had a constant of proportionality of -1.2% per Sm-1. A filtered back-projection algorithm was used to generate tomographic images from the scans.

Algorithms↗
PubMed1989

Weak reflection of ultrasound by elements arranged in the steps of a one-dimensional random walk, with reference to backscatter by blood.

Blood backscatters ultrasound primarily because the erythrocytes (red blood corpuscles) have a specific acoustic impedance slightly different from that of the surrounding plasma. In an attempt to explain some of the main results concerning such scatter, a one-dimensional model is set up and analysed. The results computed from it are in qualitative agreement with, and contribute a partial explanation of, the variation of reflection coefficient with erythrocyte concentration and the noise in the backscattered signal.

Erythrocytes↗
PubMed1988

Weak reflection of a wave by a one-dimensional array of randomly spaced elements, with reference to the scattering of ultrasound by blood.

The mean reflection coefficient is deduced for a line of elements each of the same thickness, placed at random between two fixed ends, where the reflection coefficient of each element is very small. This theory is applied to the problem of the scattering of ultrasound by blood, and some qualitative agreement with experimental data is found. A simplified theory is also presented, from which the variance of the reflection coefficient is given.

Blood↗
PubMed1979

Pathologic findings and long-term sequelae in Legionnaires' disease.

A recently recognized sporadic case of Legionnaires' disease occurring in North Carolina is described. Diagnosis was made by special serologic studies performed at the Center for Disease Control, Atlanta. The light-microscopic and electron-microscopic findings are described. Organisms consistent with the etiologic agents of Legionnaires' disease were seen by electron microscopic examination. An eight-month follow-up in this case still showed the presence of some radiologic abnormalities of the pulmonary parynchema, as well as abnormal data on tests of pulmonary function.

Adult↗