H. McCann

The University of Manchester, Manchester, ENG, United Kingdom

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Publications (53)48.33 Total impact

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    ABSTRACT: We report on a pilot study of dynamic lung electrical impedance tomography (EIT) at the University of Manchester. Low-noise EIT data at 100 frames per second were obtained from healthy male subjects during controlled breathing, followed by magnetic resonance imaging (MRI) subsequently used for spatial validation of the EIT reconstruction. The torso surface in the MR image and electrode positions obtained using MRI fiducial markers informed the construction of a 3D finite element model extruded along the caudal-distal axis of the subject. Small changes in the boundary that occur during respiration were accounted for by incorporating the sensitivity with respect to boundary shape into a robust temporal difference reconstruction algorithm. EIT and MRI images were co-registered using the open source medical imaging software, 3D Slicer. A quantitative comparison of quality of different EIT reconstructions was achieved through calculation of the mutual information with a lung-segmented MR image. EIT reconstructions using a linear shape correction algorithm reduced boundary image artefacts, yielding better contrast of the lungs, and had 10% greater mutual information compared with a standard linear EIT reconstruction.
    Physiological Measurement 04/2014; 35(5):863-879. · 1.50 Impact Factor
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    ABSTRACT: We report on a pilot study with healthy subjects who had an MR scan in addition to EIT data acquired with the Manchester fEITER system. The MR images are used to inform the external shape of a 3D EIT reconstruction model of the thorax, and small changes in the boundary that occur during respiration are addressed by incorporating the sensitivity with respect to boundary shape into a robust reconstruction algorithm. A quantitative comparison of the image quality for different EIT reconstructions is achieved through calculation of their mutual information with a segmented MR image. A shape corrected reconstruction algorithm reduces boundary artefacts relative to a standard reconstruction, and has a greater mutual information of approximately 4% with the segmented MR image.
    Journal of Physics Conference Series 04/2013; 434(1):2082-.
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    ABSTRACT: A novel opto-electronic scheme for line-of-sight Near-IR gas absorption measurement based on direct absorption spectroscopy (DAS) is reported. A diode-laser-based, multiwavelength system is designed for future application in nonintrusive, high temporal resolution tomographic imaging of H<sub>2</sub>O in internal combustion engines. DAS is implemented with semiconductor optical amplifiers (SOAs) to enable wavelength multiplexing and to induce external intensity modulation for phase-sensitive detection. Two overtone water transitions in the Near-IR have been selected for ratiometric temperature compensation to enable concentration measurements, and an additional wavelength is used to account for nonabsorbing attenuation. A wavelength scanning approach was used to evaluate the new modulation technique, and showed excellent absorption line recovery. Fixed-wavelength, time-division-multiplexing operation with SOAs has also been demonstrated. To the best of our knowledge this is the first time SOAs have been used for modulation and switching in a spectroscopic application. With appropriate diode laser selection this scheme can be also used for other chemical species absorption measurements.
    Applied Optics 12/2012; 51(34):8057-67. · 1.69 Impact Factor
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    ABSTRACT: A novel and effective methodology of combining images obtained by both electrical impedance tomography (EIT) and magnetic resonance imaging (MRI) data is described. Co-registered and fused image results for an example laboratory phantom based on the polysaccharide gel TX151 are presented. Bespoke software was used to convert an EIT dataset into a form consistent with 3D-Slicer, a powerful software package used for visualisation and image analysis. The data fusion provides a highly effective method of directly comparing EIT images, which typically have good temporal resolution and relatively poor spatial resolution, with those obtained with MRI, which has relatively poor temporal resolution but excellent spatial resolution.
    Electronics Letters 01/2012; 48(11):617-618. · 1.04 Impact Factor
  • N. Terzija, H. McCann
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    ABSTRACT: We introduce a new wavelet-based hard-field image reconstruction method that is well suited for data inversion of limited path-integral data obtained from a geometrically sparse sensor array. It is applied to a chemical species tomography system based on near-IR spectroscopic absorption measurements along an irregular array of only 27 paths. This system can be classified as producing severely limited data, where both the number of viewing angles and the number of measurements are small. As shown in our previous work, the Landweber iteration method allows stable solution of this tomography problem by incorporating suitable a priori information. In the new method, a 2-D discrete wavelet transform has been used as a smoothing function. We present a method of designing the optimal wavelet-based smoothing function, depending on a priori knowledge of the subject. The significance of the particular wavelet filter selected is considered in terms of the accuracy of reconstruction of the spatial location and shape of the gas distribution. Results are presented for simulated phantoms using different sensor arrays and for experiments with propane plumes, showing excellent spatial localization and quantification. The computational time of the iterative algorithm is significantly reduced by applying the wavelet transform method. Some of our conclusions are applicable to other hard-field tomographic modalities in applications where similar constraints may be encountered.
    IEEE Sensors Journal 10/2011; · 1.48 Impact Factor
  • Sandip Pal, Hugh McCann
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    ABSTRACT: In many dynamic gas-phase reaction processes, there is great interest to measure the distribution of minor constituents, i.e. <10−3 by volume (1000 ppm). One such case is the after-treatment of automotive gasoline engine exhaust by catalytic conversion, where a characteristic challenge is to image the distribution of 10 ppm (average) of carbon monoxide (CO) at 1000 frames per second across a 50 mm diameter exhaust pipe; this particular problem has been pursued as a case study. In this paper, we present a novel electronic scheme that achieves the required measurement of around 10−3 absorption with 10−4 precision at kHz bandwidth. This was not previously achievable with any known technology. We call the new scheme Auto-Digital Gain Balancing. It is amenable to replication for many simultaneous measurement channels, and it permits simultaneous measurement of multiple species, in some circumstances. Experimental demonstrations are presented in the near-infrared. In single scans of a tunable diode laser, measurements of both CO and CO2 have been made with 20 dB signal-to-noise ratio at peak absorption. This work paves the way for chemical species tomography of minor constituents in many dynamic gas-phase systems.
    Measurement Science and Technology 09/2011; 22(11):115304. · 1.44 Impact Factor
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    ABSTRACT: Electrical Impedance Tomography (EIT) can resolve dynamic physiological information deep within human subjects [1], but its sensitivity is challenged in the case of imaging the head [2]. Here, we report a new system called fEITER that has been designed and built to enable functional imaging of the human brain using EIT via scalp-mounted electrodes, integrated with stimulation of evoked responses. Using Field-Programmable Gate Array (FPGA) technology, it provides excellent flexibility in terms of current-pattern excitation and signal processing. The instrument operates at 100 frames/second (fps) with noise of 1 μV on the rms voltage measurements. Clinical trials have been authorized by the UK MHRA and example data from human subjects are presented.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2011; 2011:7029-32.
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    ABSTRACT: The concept of Agile Tomography is introduced and exemplified by reviewing the progress in tomography sensors and systems which can be deployed in situ. Agile tomography capabilities are examined across a number of electromagnetic and electrical modalities, ranging from gamma-rays to low-frequency electrical measurements. The recent achievements in already established areas are highlighted, as well as emerging technology and new modalities.
    IEEE Sensors Journal 01/2011; 11(12):3086-3105. · 1.48 Impact Factor
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    ABSTRACT: We report here the first application of chemical species tomography (CST) in a multi-cylinder automotive engine. This technique offers high-speed continuous imaging of hydrocarbon fuel distribution and mixing within the combustion chamber and is therefore of interest to both engine designers and combustion scientists. Many of the methods described are equally applicable to chemically selective imaging of other highly dynamic mixing and reaction processes.A measurement grid consisting of 27 dual-wavelength optical paths has been implemented in one cylinder of an otherwise standard four-cylinder port-injected gasoline engine, using a unique OPtical Access Layer (OPAL) carrying embedded optical fibres and collimators. The OPAL provided adequate performance on many beams for more than 2 h of fired engine operation. To improve sensitivity and to cope with fuel spray injection directly into the cylinder (in other engine types), a low-noise opto-electronic system has been developed, offering laser intensity modulation at frequencies up to 1 MHz. Dual-wavelength measurements are recorded on each channel at 100 kSPS, prior to off-line processing that typically reduces the effective frame rate to 3000–4000 frames/s, dependent upon engine speed. The performance of the system is assessed, using running conditions chosen to provide a qualitatively known (homogeneous) fuel distribution for validation purposes. Examples of measured data and processing schemes are discussed. Sample tomographic images, obtained using a novel quality-based approach to data selection, are presented.
    Chemical Engineering Journal. 01/2010;
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    ABSTRACT: We report on human tests of the new EIT-based system fEITER (functional Electrical Impedance Tomography of Evoked Responses), targeted principally at functional brain imaging. It is designed and built to medical standard BS EN 60601-1:2006 and clinical trials have been approved by the MHRA in the UK. fEITER integrates an EIT sub-system with an evoked response sub-system capable of providing visual, auditory or other stimuli, and the timing of each stimulus is recorded within the EIT data to a resolution of 500 microseconds. The EIT sub-system operates at 100 frames per second using 20 polar/near-polar current patterns distributed among 32 scalp electrodes that are arranged in a 3-dimensional array on the subject. Presently, current injection is fixed in firmware at 1 mA pk-pk and 10 kHz. Performance testing on inanimate subjects has shown voltage measurement SNR better than 75 dB, at 100 frames per second. We describe the fEITER system and give example topographic results for a human subject under no-stimulus (i.e. reference) conditions and on application of auditory stimuli. The system's excellent noise properties and temporal resolution show clearly the influence of basic physiological phenomena on the EIT voltages. In response to stimulus presentation, the voltage data contain fast components (~100 ms) and components that persist for many seconds.
    Journal of Physics Conference Series 01/2010; 224(1).
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    ABSTRACT: Since the early 1980s, a number of electrical imaging techniques based on capacitance, resistance, or inductance measurement at low frequencies have been developed for the monitoring of industrial processes, such as oil- and gas-multiphase flows. In principle, microwave tomography would produce higher resolution images than these low-frequency techniques. But it has mainly been studied for medical applications over the past two decades and is less developed for industrial applications. In this paper, the development of an experimental microwave-tomography system intended for oil- and gas-flow measurements is described, which includes the hardware for data acquisition and the numerical algorithm for image reconstruction. The investigation of the system for the imaging of static–dielectric phantoms modelling oil- and gas-flow distributions is reported together with the images obtained at two different microwave frequencies: 2.5 GHz and 4 GHz. It has been demonstrated that images of the dielectric phantoms can be reconstructed using the system, with the images obtained at 4 GHz having better quality and higher resolution.
    Measurement Science and Technology 09/2009; 20(10):104026. · 1.44 Impact Factor
  • S. Pal, P. Wright, H. McCann
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    ABSTRACT: Ultra-sensitive detection of minor species is important in many fields. The requirement of direct online measurement of gas species in process engineering motivates fast measurements over short pathlengths. The resulting high bandwidth and low absorptions reduce the measurement Signal-to-Noise Ratio (SNR). Our target is to detect CO in engine exhaust with sensitivity of the order of 0.05 ppm-m at several kHz. In this paper, a novel electronic scheme is discussed and presented with simulation results. A generic approach is followed, to use it with either a pulsed or a continuous wave (CW) laser source. A detailed noise analysis is presented for improved understanding of the SNR of the overall scheme.
    Electronics, Circuits and Systems, 2008. ICECS 2008. 15th IEEE International Conference on; 10/2008
  • S Pal, K B Ozanyan, H McCann
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    ABSTRACT: In order to develop a chemical species tomography system, this paper carries out various computational steps to address the problem of measuring minor species concentration using single-pass, short path-length absorption techniques in the mid-infrared. It focuses on the imaging of carbon monoxide (CO) in combustion exhaust as a case study, with an average concentration of 10 ppm over a 50 mm diameter cross-section, taking account of the presence of other absorbing species. CO absorption transitions R6, R7, R9 and R10 are identified as possible measurement targets. The joint effects of spectral absorption linewidth and laser source linewidth are considered at length, resulting in recommendations for laser linewidth to achieve appropriate levels of CO absorption signal purity. Measurement strategies are considered for achievement of the necessary sensitivity, noise and bandwidth performance. A feasible beam arrangement for tomographic imaging is discussed, providing 48 measurements of path concentration integral. Representative phantom reconstructions are presented, with encouraging results for application to such dynamic gaseous subjects.
    Measurement Science and Technology 07/2008; 19(9):094018. · 1.44 Impact Factor
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    ABSTRACT: High-speed tomographic imaging of hostile engineering processes using absorption-based measurements presents a number of difficulties. In some cases, these challenges include severe limitations on the number of available measurement paths through the subject and the process of designing the geometrical arrangement of these paths for best imaging performance. This paper considers the case of a chemical species tomography system based on near-IR spectroscopic absorption measurements, intended for application to one cylinder of a multi-cylinder production engine. Some of the results, however, are also applicable to other hard-field tomographic modalities in applications where similar constraints may be encountered. A hitherto unreported design criterion is presented for optimal beam geometry for imaging performance, resulting in an irregular array with only 27 measurement paths through the subject for the engine application. Image reconstruction for this severely limited geometry is considered at length, using both simulated and experimental phantom data. Novel methods are presented for the practical generation of gaseous phantoms for calibration and testing of the system. The propane absorption coefficient at 1700 nm is measured. Quantitative imaging of propane plumes in air is demonstrated, showing good localization of circular plumes with diameter as small as 1/5 of the subject diameter and excellent imaging of multiple plumes.
    Measurement Science and Technology 07/2008; 19(9):094007. · 1.44 Impact Factor
  • M Rafiei-Naeini, H McCann
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    ABSTRACT: Electrical impedance tomography (EIT) has many potential applications in medicine. The sensitivity and spatial resolution of EIT can be enhanced significantly by increasing the measurement signal-to-noise ratio (SNR), to which a significant contribution is made by the SNR of the current excitation sub-system. In this paper we present a prototype EIT current excitation sub-system with 80 dB SNR, exploiting both digital and analogue techniques. It uses digital waveform synthesis, a 16-bit DAC and subsequent reconstruction filter, to drive an enhanced Howland current source. Detailed analysis and testing of the current sub-system are presented. Its output impedance is 10 MOmega for different load impedances, varying slowly over the frequency range from 10 kHz up to 4 MHz.
    Physiological Measurement 07/2008; 29(6):S173-84. · 1.50 Impact Factor
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    ABSTRACT: Electrical impedance tomography has been used in the medical environment for many years. Recently it has been used for brain function imaging. This type of technology uses a large number of electrodes on the scalp, examples in the literature ranging from 8 to 64. In this paper, the composite impedance of the electrode in contact with tissue is investigated. An experimental method has been developed to enable the use of a commercial impedance analyzer in compliance with medical safety standards. The magnitude and long-term stability of the composite impedance of the electrode-tissue system is found to be greatly dependent on the type of electrodes used.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 02/2008; 2008:1171-4.
  • S Pal, K B Ozanyan, H McCann
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    ABSTRACT: Tomographic imaging of minor species in combustion exhaust is an important requirement for combustion engineering. Only one group has tried carbon monoxide imaging in laminar flames in the mid-infrared region. A number of attempts have been made by different groups for single-channel multi-pass or long path-length measurement of CO. Single-pass measurement is being assessed in our work for tomographic measurement of the distribution of minor species. We report here the spectroscopic study of CO in different absorption lines of the strong fundamental bands in the mid-IR range. The criteria for selection of a light source in this context are also considered. Although strong spectral lines are available for CO in the mid-infrared region, strong water vapour and carbon dioxide interferences are also present. This addresses the choice of such useful spectral lines under interference by major species. The HITRAN 2004 database is used to predict the spectral lines of H2O and CO2 near the fundamental spectrum of CO in the mid-IR range. The exhaust condition in respect of temperature and pressure is considered in order to study the broadening of the spectral lines. The percentages of absorption for different species for different types of light sources are reported on the basis of computations. The concept can be readily extended to further species of interest in the presence of other interfering species. Different laser sources and spectroscopic methods are considered to design a strategy suitable to this application.
    Journal of Physics Conference Series 11/2007; 85(1):012020.
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    ABSTRACT: Current density distributions have been calculated using a high resolution model of the human head on application of low direct currents. The 3D internal electric fields and current densities have been calculated for electrode pair combinations for dc current levels from 1 to 10 mA. For current levels at 1 mA, the results show maximum electric fields of approximately 1.5 V/m and current densities less than 500 mA/m2 for the outermost regions of the brain. To our knowledge, this is the first study to describe current distributions in the brain using a 3D anatomically realistic model of the head in a manner which is pertinent to Electrical Impedance Tomography (EIT).
    12/2006: pages 376-379;
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    ABSTRACT: Abstract—EIT has unique potential to provide fast imaging of impedance changes in the deep brain. Both EIT sensitivity and spatial resolution can be greatly improved by increased measurement SNR. In this paper we present a prototype EIT channel with better than 70 dB SNR at excitation frequencies from 10 kHz to 100 kHz. The system is divided into three major sub-systems: digital waveform generation and demodulation, current excitation and voltage measurement. The digital sub-system, presently implemented on a Xilinx Spartan 3-1000 FPGA, provides 16- bit excitation waveform data to a DAC and subsequent reconstruction filter, which in turn provides the input to a Howland current source. The SNR of the resulting excitation current is better than 70dB. Voltage measurement is accomplished using a buffered differential amplifier followed by an anti-aliasing filter and ADC. Phase-sensitive demodulation is performed within the FPGA and the resulting phase and amplitude information returned to a PC via a USB interface.
    12/2006: pages 324-327;
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    ABSTRACT: Chemcial Species Tomography (CST) can be used to provide information on the spatial distribution of a target species by reconstruction from multiple independent optical absorption measurements taken in the measurement space. In this paper, we describe how CST can be used to capture the spatial and temporal distribution of the fuel-vapour mixture within an internal combustion engine, thereby providing invaluable data to engine designers in their quest for reduced emissions and improved fuel economy. We further report on the first application of CST to a standard, multi-cylinder production engine, which involved the design and fabrication of a custom OPtical Access Layer (OPAL), which provided 27 independent, collimated optical channels in one cylinder of a 4 cylinder, 2.0 litre gasoline engine
    10/2006;