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ABSTRACT: Magnetic induction tomography (MIT) has been proposed for the detection of cerebral oedema and haemorrhagic stroke. Achieving the required phase measurement precision for these applications is however a major technical challenge. A critical component within an MIT system is the detector amplifier and for this role an ultra-phase-stable, low noise instrumentation amplifier has been developed. The design of the amplifier is described and (i) the results of simulations and measurements of the amplifiers phase stability versus temperature and (ii) measurements of the phase noise and drift performance of the amplifier within a single-channel magnetic induction spectroscopy system are provided and discussed. For a 10 MHz signal the amplifier, with a gain of 21, displayed an average change in the measured phase of its output of just -0.1 ± 0.6 m° °C(-1) as the ambient temperature was varied between 35 and 50 °C, demonstrating a level of phase stability approaching that required for potential biomedical applications such as the detection of cerebral haemorrhage.
Physiological Measurement 07/2011; 32(7):917-26. · 1.68 Impact Factor
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ABSTRACT: Magnetic induction tomography (MIT) allows the reconstruction of conductivity distributions inside a given volume for a wide
variety of industrial and medical applications. Philips is interested in using the MIT technique for acquiring information
of conductivity distribution and conductivity changes in human tissue. The advantage of this technique is the contactless
and non-invasive way of collecting information on the tissue. An MIT system consists of excitation coils that produce a primary
magnetic field that causes eddy currents in a conductive object. The eddy current produces a secondary magnetic field that
can be detected by an array of receiving coils. Improved Philips MIT setups based on IF down-conversion and high-speed sampling
are compared. MIT demands an accurate measure of phase and the new setups are found to offer improvements in noise, linearity
and drift over our previous system.
KeywordsMIT-Magnetic induction tomography-high-speed sampling-medical instrumentation
01/2010: pages 4-6;
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ABSTRACT: Magnetic Induction Tomography is a relatively new non-invasive modality for the imaging of the electrical properties of materials
which is currently under investigation for a variety of industrial and biomedical applications, in particular the detection
and monitoring of cerebral haemorrhage. The speed of FFT-based phase measurement algorithms employed in some current MIT systems
is however a major limit to higher data acquisition rate and precision.
This paper describes an effective approach for parallel FFT processing implemented on an Nvidia Geforce 9800 GX2 Graphics
Processing Unit (GPU). Acquired signals are processed in the GPU using the CUDA FFT library. The processed data is then passed
by the GPU to a Labview-based measurement software tool on the host PC for further processing and image reconstruction.
The FFT algorithm was accelerated on the GPU by up to 9.5 times compared to the processing time for the same data set on a
single processor of an Intel Core2 Duo (E6750) PC. This compares to a maximum speed-up of 2.2 achieved for a parallel implementation
using four cores of an Intel Quad-core (Q9300) PC with the software written in Labview using the same definition and data
sets.
We show that the use of GPUs can be an effective solution to accelerate FFT-based signal measurement algorithms. Faster data
acquisition may then allow imaging of dynamic processes, compensation of artifacts associated with head movement and/or improved
measurement precision through greater data averaging.
12/2009: pages 1889-1892;
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ABSTRACT: Magnetic Induction Tomography (MIT) is a relatively new contactless imaging modality which aims at reconstructing conductivity and permittivity distributions within objects. One of MIT's main challenges is the computational intensity required for image reconstruction in potential industrial and medical applications.
Image Processing (ICIP), 2009 16th IEEE International Conference on; 12/2009
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ABSTRACT: Ryo et al (2005 Diabetes Care 28 451-3) reported a new method for measuring the visceral fat area (VFA) by combining abdominal bioelectrical impedance analysis (BIA) with measurement of waist circumference (WC), but very few methodological details were provided. Furthermore, the study did not test the use of WC alone as an indicator of VFA even though others had previously reported a strong correlation. We sought to determine the optimal measurement technique and analysis for measuring VFA by abdominal BIA and WC. 18 volunteers (age 23-64 years) underwent measurement of WC, abdominal impedance (Bodystat 500 four-electrode system) and a single cross-sectional CT scan at the umbilicus. VFA derived using WC(3) and measurements of abdominal impedance from electrode pairs sited at the flank predicted the value of VFA measured by CT with correlation r = 0.904 (p < 0.0001); the optimizing power of WC was 3.3 (r = 0.905). However, the use of WC(1.9) alone, without involving BIA at all, provided a similar correlation (r = 0.923). Our small preliminary study shows that abdominal BIA is potentially a practicable non-invasive technique for measurement of VFA but casts doubt on whether it adds any value to the use of WC alone. Larger studies are now required to test this finding.
Physiological Measurement 05/2009; 30(7):N53-8. · 1.68 Impact Factor
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ABSTRACT: This paper describes the design and performance of a 5-channel multi-frequency Magnetic Induction Tomography (MIT) data acquisition
system operating in the frequency range 0.5 – 14MHz. A novel phase stabilization scheme is employed to reduce phase drift
of the system. The signals detected by the sensors are amplified and digitized using a 60MS/s high speed data acquisition
system. The amplitude and phase are measured using an FFT-based algorithm.
Assuming that the time delay between measurement and reference data sets is kept to within 15 minutes, the phase noise obtained
is less than 1m° for operating frequencies of 1MHz or higher using a measurement time constant of 1s. The maximum drift over
a 12 hour period was less than 20m° for all channels. Details of the sensors, circuits and algorithms employed and the results
of performance measurements using a saline phantom are provided.
12/2008: pages 744-747;
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ABSTRACT: A novel method for increasing the phase measurement stability of Magnetic Induction Tomography (MIT) systems is described.
The principle of the method is discussed and practical implementations of the method for (i) an MIT system in which the sensors
are sensitive to the primary magnetic field, and (ii) an MIT system employing gradiometer sensors, are described. The results
of measurements of the phase noise and long term phase stability of MIT systems employing the new method are given. The method
was found to provide very significant improvements in the measurement stability of MIT systems, in particular for the long
term amplitude and phase drift performance of systems of type (i). The new method potentially enables high precision / low
frequency MIT system to be implemented without the use of gradiometers and may be particularly suitable for use in long-term
monitoring applications.
12/2008: pages 748-751;
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ABSTRACT: In an ideal magnetic induction tomography (MIT) system, the coupling between the coils and the sample is entirely by the magnetic field. In a practical system, unwanted electric-field (capacitive) coupling can also exist and cause large errors in the MIT measurements unless the hardware is designed carefully. A series of tests was carried out to assess the magnitude of capacitive coupling present in a 10 MHz MIT system designed for biomedical use and other applications involving low-conductivity samples (<or=10 S m(-1)). The tests indicated that, even with the individual coils left unscreened, the signal contamination from capacitive coupling was very small compared with the true MIT signal. Because the contamination was small, it was demonstrated possible to derive the permittivity of the sample from the real part of the MIT signal. This was shown to work well when the conductivity of the sample was less than about 0.5 S m(-1), but for higher conductivities, when the skin depth became comparable with the width of the sample, the commonly used theoretical expression for the MIT signal began to break down. This implies that the measurement of permittivity (and permeability) in real biological tissues (which have conductivities of up to 2 S m(-1)) will require a more detailed derivation taking into account both the real and imaginary parts of the signals.
Physiological Measurement 07/2007; 28(7):S301-11. · 1.68 Impact Factor
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ABSTRACT: In this study the performance of a planar array for magnetic induction tomography (MIT) was investigated and the results of measurements to determine the precision and sensitivity of the sensor were undertaken. A planar-array MIT system utilizing flux-linkage minimization for the primary field has been constructed and evaluated. The system comprises 4 printed excitation coils of 4 turns which were shielded, 8 surface-mount inductors of inductance 10 microH as sensor, mounted such that in principle no primary-field flux threads them, and a calibration coil to produce a strong primary field. The excitation current was multiplexed via relays to drive the excitation and reference coils. The noise values were similar in real and imaginary components in the lower frequencies and the factor to which the primary field could be reduced was greatest in the nearest coil. Methods for determining the true real and imaginary components and for flux-linkage minimization for the primary field for variations in channel sensitivities are described and the results of measurements of the system's noise and drift are given. A SNR of 47 dB was observed at 4 MHz when a 0.3 Sm-1 saline filled tank of dimensions 20 cmx20 cmx10 cm was placed centrally over the array. Finally, images were reconstructed from measurements of saline samples in a free space background, with the samples moved past the array in 21 1 cm steps to emulate mechanical scanning of the array. The image reconstruction characteristics of the planar array in conjunction with the reconstruction technique employed are discussed.
Physiological Measurement 05/2005; 26(2):S263-78. · 1.68 Impact Factor
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ABSTRACT: In magnetic induction tomography reducing the influence of the primary excitation field on the sensors can provide a significant improvement in SNR and/or allow the operating frequency to be reduced. For the purposes of imaging, it would be valuable if all, or a useful subset, of the detection coils could be rendered insensitive to the primary field for any excitation coil activated. Suitable schemes which have been previously suggested include the use of axial gradiometers and coil-orientation methods (Bx sensors). This paper examines the relative performance of each method through computer simulation of the sensitivity profiles produced by a single sensor, and comparison of reconstructed images produced by sensor arrays. A finite-difference model was used to determine the sensitivity profiles obtained with each type of sensor arrangement. The modelled volume was a cuboid of dimensions 50 cmx50 cmx12 cm with a uniform conductivity of 1 S m-1. The excitation coils were of 5 cm diameter and the detection coils of 5 mm diameter. The Bx sensors provided greater sensitivity than the axial gradiometers at all depths, other than on the surface layer of the volume. Images produced using a single-planar array were found to contain distortion which was reduced by the addition of a second array.
Physiological Measurement 05/2005; 26(2):S319-31. · 1.68 Impact Factor
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ABSTRACT: 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.
Physiological Measurement 03/2004; 25(1):271-9. · 1.68 Impact Factor
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ABSTRACT: 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.
Physiological Measurement 06/2003; 24(2):555-64. · 1.68 Impact Factor
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ABSTRACT: 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.
Physiological Measurement 02/2002; 23(1):189-94. · 1.68 Impact Factor
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ABSTRACT: Magnetic Induction Tomography (MIT) is a technique for imaging the electromagnetic properties of materials. Excitation coils are used to induce eddy currents within the sample volume which are then sensed by receiver coils. The technique has attracted interest for biomedical application due to the non-contacting nature of the measurements, which may provide advantages over electrode based impedance tomography in certain applications. The paper describes a transceiver designed for use in a prototype biomedical MIT system operating with a single excitation frequency of 10 MHz. To improve channel isolation and phase stability during signal distribution, the received signals undergo heterodyne downconversion to 10 kHz, filtering and limiting at the transceiver. Direct phase measurement between the downconverted reference and received signals is then undertaken to measure the signal perturbation due to the induced conduction eddy currents.
Engineering in Medicine and Biology Society, 2001. Proceedings of the 23rd Annual International Conference of the IEEE; 02/2001
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Meas Sci Technol. 19:045501 11.
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International Conference on Electrical Bioimpedance, Journal of Physics: Conference Series 224;
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ABSTRACT: A hemispherical MIT helmet coil array for imaging cerebral haemorrhage has been designed using a realistic 12-tissue finite-difference model of the head including a large peripheral haemorrhage (volume 49 ml). The coil array was first optimised by reaching a compromise between the quality of the reconstructed images and the financial cost of the digital detection system. The practical implementation of the helmet is partially complete.
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ABSTRACT: A 16-channel magnetic induction tomography (MIT) system has been constructed for imaging samples with low conductivities (<10 S m−1) such as biological tissues or ionized water in pipelines. The system has a fixed operating frequency of 10 MHz and employs heterodyne downconversion of the received signals, to 10 kHz, to reduce phase instabilities during signal distribution and processing. The real and imaginary components of the received signal, relative to a synchronous reference, are measured using a digital lock-in amplifier. Images are reconstructed using a linearized reconstruction method based on inversion of a sensitivity matrix with Tikhonov regularization. System performance measurements and images of a pipeline phantom and a human leg in vivo are presented. The average phase precision of the MIT system is 17 millidegrees. Engineering and Physical Sciences Research Council (grants EP/E009832/1 and EP/E009697/1).
