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Showing papers in "Physiological Measurement in 2001"


Journal ArticleDOI
TL;DR: This review article presents the theory and practice of these techniques using a tutorial approach and compares the relative merits of the scanning and full-field approaches to velocity map imaging and concludes with a review of reported applications ofThese techniques to blood perfusion mapping and imaging.
Abstract: Laser Doppler velocimetry uses the frequency shift produced by the Doppler effect to measure velocity. It can be used to monitor blood flow or other tissue movement in the body. Laser speckle is a random interference effect that gives a grainy appearance to objects illuminated by laser light. If the object consists of individual moving scatterers (such as blood cells), the speckle pattern fluctuates. These fluctuations provide information about the velocity distribution of the scatterers. It can be shown that the speckle and Doppler approaches are different ways of looking at the same phenomenon. Both these techniques measure at a single point. If a map of the velocity distribution is required, some form of scanning must be introduced. This has been done for both time-varying speckle and laser Doppler. However, with the speckle technique it is also possible to devise a full-field technique that gives an instantaneous map of velocities in real time. This review article presents the theory and practice of these techniques using a tutorial approach and compares the relative merits of the scanning and full-field approaches to velocity map imaging. The article concludes with a review of reported applications of these techniques to blood perfusion mapping and imaging.

706 citations


Journal ArticleDOI
TL;DR: A method is proposed to approximately standardize the reference of scalp EEG recordings to a point at infinity, based on the fact that the use of scalp potentials to determine the neural electrical activities or their equivalent sources does not depend on the reference.
Abstract: The effect of an active reference in EEG recording is one of the oldest technical problems in EEG practice. In this paper, a method is proposed to approximately standardize the reference of scalp EEG recordings to a point at infinity. This method is based on the fact that the use of scalp potentials to determine the neural electrical activities or their equivalent sources does not depend on the reference, so we may approximately reconstruct the equivalent sources from scalp EEG recordings with a scalp point or average reference. Then the potentials referenced at infinity are approximately reconstructed from the equivalent sources. As a point at infinity is far from all the possible neural sources, this method may be considered as a reference electrode standardization technique (REST). The simulation studies performed with assumed neural sources included effects of electrode number, volume conductor model and noise on the performance of REST, and the significance of REST in EEG temporal analysis. The results showed that REST is potentially very effective for the most important superficial cortical region and the standardization could be especially important in recovering the temporal information of EEG recordings.

415 citations


Journal ArticleDOI
TL;DR: A MATLAB package is written which can be used for two-dimensional mesh generation, solving the forward problem and reconstructing and displaying the reconstructed images (resistivity or admittivity).
Abstract: The EIDORS (electrical impedance and diffuse optical reconstruction software) project aims to produce a software system for reconstructing images from electrical or diffuse optical data. MATLAB is a software that is used in the EIDORS project for rapid prototyping, graphical user interface construction and image display. We have written a MATLAB package (http://venda.uku.fi/ vauhkon/) which can be used for two-dimensional mesh generation, solving the forward problem and reconstructing and displaying the reconstructed images (resistivity or admittivity). In this paper we briefly describe the mathematical theory on which the codes are based on and also give some examples of the capabilities of the package.

249 citations


Journal ArticleDOI
TL;DR: The system is able to visualize various states of the breast and it may be possible to apply it to breast cancer detection, and results of clinical evaluation of the system are presented.
Abstract: A medical device which allows imaging of the distribution of conductivity in 3D in regions below the skin surface has been developed and tested Its purpose is to enable early detection and preliminary diagnosis of breast tumours Design of the measuring system and software are described Results of clinical evaluation of the system are presented EIT images of healthy and cancerous breasts are presented and discussed The system is able to visualize various states of the breast and it may be possible to apply it to breast cancer detection

225 citations


Journal ArticleDOI
TL;DR: There is a strong relationship between PTT changes and RR interval changes, but these changes are not in phase.
Abstract: Pulse transit time (PTT) is a simple, non-invasive measurement, defined as the time taken from a reference time for the pulse pressure wave to travel to the periphery. PTT is influenced by heart rate, blood pressure changes and the compliance of the arteries, but few quantitative data are available describing the factors which influence PTT. The aim of this study was to investigate the relationship between the cardiac beat-to-beat interval (RR) and PTT, using paced respiration to generate changes in both variables. We analysed PTT and RR interval from 15 normal healthy subjects during paced breathing, and the cross-correlation function between PTT and RR was used to quantify their relationship. Over the 15 subjects, the maximum change in PTT ranged from 7 to 23 ms with a mean +/- standard deviation of 14 +/- 5 ms, and that in RR interval from 86 to 443 ms (241 +/- 102 ms). Examining changes over time, the best correlation (r = +0.69, p < 0.01) was obtained when PTT was advanced relative to RR, with a change in RR followed by a corresponding change in PTT 3.17 +/- 0.76 beats later. We conclude that there is a strong relationship between PTT changes and RR interval changes, but these changes are not in phase.

170 citations


Journal ArticleDOI
TL;DR: Results suggest that the Sheffield Mk3.5 EIT/EIS system which measures both the real and imaginary part of impedance at 30 frequencies between 2 kHz and 1.6 MHz is 10 dB better in absolute terms than the previous Sheffield (Mk3a) system.
Abstract: This paper describes the Sheffield Mk3.5 EIT/EIS system which measures both the real and imaginary part of impedance at 30 frequencies between 2 kHz and 1.6 MHz. The system uses eight electrodes with an adjacent drive/receive electrode data acquisition protocol. The system is modular, containing eight identical data acquisition boards, which contain DSPs to generate the drive frequencies and to perform the FFT used for demodulation. The current drive is in three sequentially applied packets, where each packet contains ten summed sine waves. The data acquisition system is interfaced to a host PC through an optically isolated high speed serial link (RS485) running at 2 Mbaud (2 Mbits s(-1)). Measurements on a saline filled tank show that the average signal to noise performance of the system is 40 dB measured across all frequencies and that this figure is independent of frequency of measurement. These results suggest that the current system is 10 dB better in absolute terms than the previous Sheffield (Mk3a) system.

159 citations


Journal ArticleDOI
TL;DR: It is shown that displaying planar two-dimensional maps of the currents detected at the breast's surface relate to the electric field distribution within the breast.
Abstract: In this paper we present the T-SCANTM technology and its use as a diagnostic tool for breast cancer detection. We show, using theoretical models with simplified geometries, that displaying planar two-dimensional maps of the currents detected at the breast's surface relate to the electric field distribution within the breast. This distribution is a manifestation of the bulk spatial inhomogeneities in the complex dielectric constant that represent the various tissue types. These differences may be used to discriminate between various pathological states. We furthermore illustrate a useful classifier, based on admittance data measured up to 2 kHz, and we argue that low frequency impedance measurements can be used successfully in breast cancer diagnosis.

137 citations


Journal ArticleDOI
TL;DR: On-line spectroscopy of tissue conductivity with low spatial resolution appears feasible, thus enabling applications such as non-invasive monitoring of brain oedema, according to a new MIT hardware developed consisting of a coil system with planar gradiometers and high-resolution phase detector.
Abstract: Magnetic induction tomography (MIT) is a contactless method for mapping the electrical conductivity of tissue. MIT is based on the perturbation of an alternating magnetic field by a conducting object. The perturbation is detected by a voltage change in a receiver coil. At physiologically interesting frequencies (10 kHz-10 MHz) and conductivities ( 100 kHz is possible. On-line spectroscopy of tissue conductivity with low spatial resolution appears feasible, thus enabling applications such as non-invasive monitoring of brain oedema.

134 citations


Journal ArticleDOI
TL;DR: The subject is so mature, and the investment in both hardware and software so significant, that there must be little to be gained from research at the margins of any new approaches and this book is stupendously good value-for-money.
Abstract: ed Al Bovic San Diego: Academic Press (2000) 891pp, price: £64.95, ISBN: 0 12 119790 5 If you want to know anything about image and video processing, this book is the place to begin and, probably, to end as well. Encyclopaedic in scale, 58 chapters by 97 contributors are packed into 891 pages. The subject is divided into 10 sections: an introduction, basic image processing techniques, image and video processing, image and video analysis, image compression, video compression, image and video acquisition, image and video rendering and assessment, image and video storage retrieval and communications, and applications of image processing. The boundaries of the coverage of the subject are quite sharply defined. First of all, an image is taken to mean a still picture that does not change with time, whereas a video evolves with time and generally contains moving and/or changing objects. There are two chapters on image and video acquisition; these are concerned with image capture, Fourier analysis, sampling rate, interpolation and reconstruction of continuous time-varying imagery. Neither image display nor perception are mentioned, even in the index. Image fusion is not included. Thus, the book is strictly limited to image and video processing and that is certainly not a criticism. As far as the depth of coverage is concerned, the objective is ambitious. This is to provide a resource that covers introductory, intermediate and advanced topics with equal clarity. I tested the extent to which this has been achieved by trying to find the answers to some questions about image and video compression. In the 277 pages of the 10 chapters devoted to this topic, there is a wealth of information extending from lossless coding, through other coding and quantization schemes, wavelet and JPEG compression, video and object-based coding, to MPEG video standards. I learned enough to realize that the subject is so mature, and the investment in both hardware and software so significant, that there must be little to be gained from research at the margins of any new approaches. It is the final section (134 pages) that contains the chapters that will be of most immediate interest to readers with a medical bias. This section is concerned with applications of image processing. The relevant chapters are on: computed tomography (x-ray, CT, PET and SPECT) with the emphasis on image reconstruction and three-dimensional topics; cardiac image processing, including the analysis of cardiac mechanics, perfusion and (perhaps oddly) electrocardiography; computer aided detection for screening mammography, concentrating on masses, calcifications and segmentation; and confocal microscopy. It is actually in these chapters that most information is to be found concerning the instrumentation for image acquisition. This is not an expensive book. Indeed, it is stupendously good value-for-money. Where else, in the field of image and video processing, can the knowledge of 97 contributors, all clearly expert and many deservedly famous, be so conveniently accessed? If there is a criticism, it has to be that the 55 pages of colour illustrations (which are additional to the 891 pages of text) are gathered together in four blocks, distributed through the book. These illustrations also appear in the text, but in grey scale. So, it is sometimes necessary to turn to the colour sections to appreciate the points that are being made. But this is only a small nuisance in exchange for what is presumably a substantial economy in the price of the book. Realistically, this is too big a book to be read from cover to cover. Consequently, it needs a good index and there is no criticism here. For example, I looked up 'ultrasound imaging' and found a reference to intravascular scanning and its combination with biplane angiography to create a map of the vessel in 3D space; this is a technique of which I was previously unaware. I also found a reference to echography, with an informative discussion of segmentation which, in this situation, is a far from trivial task. This handbook is the first in a new Academic Press series in communications, networking and multimedia, with Jerry Gibson of Southern Methodist University as the editor-in-chief. In editing and co-authoring the handbook, Al Bovik of the University of Texas at Austin has discharged a monumental assignment with spectacular success. If you are interesting in image and video processing, you must have a copy.

123 citations


Journal ArticleDOI
TL;DR: An image sequence analysis technique was developed to evaluate posture change and respiratory rate of a subject in bed without any physical contact and it evaluated 99.4% of the movements of subjects during the total monitoring time.
Abstract: An image sequence analysis technique was developed to evaluate posture change and respiratory rate of a subject in bed without any physical contact. Although the image sequence analysis requires many calculations, the system can perform them in real time. The system consisted of a CCD video camera and a PC equipped with a high-speed image processor. To evaluate the system, we tested it on five subjects at a nursing home. The system evaluated 99.4% of the movements of subjects during the total monitoring time (about 61 hours). The waveform was flat when the subject was out of view of the video camera. The system has the possibility of evaluating not only posture changes and respiratory rate. but also sleeping patterns.

123 citations


Journal ArticleDOI
TL;DR: This paper considers appropriate safety constraints and discusses how to find the optimal current patterns with those constraints.
Abstract: There are a number of constraints which limit the current and voltages which can be applied on a multiple drive electrical imaging system. One obvious constraint is to limit the maximum ohmic power dissipated in the body. Current patterns optimizing distinguishability with respect to this constraint are singular functions of the difference of transconductance matrices with respect to the power norm (the optimal currents of Isaacson). If one constrains the total current (L1 norm) the optimal patterns are pair drives. On the other hand if one constrains the maximum current on each drive electrode (an L(infinity) norm), the optimal patterns have each drive channel set to the maximum source or sink current value. In this paper we consider appropriate safety constraints and discuss how to find the optimal current patterns with those constraints.

Journal ArticleDOI
TL;DR: Analytically and experimentally the performance of a planar gradiometer as a sensing element in a system for magnetic induction tomography has less demanding requirements for the electronic system than a system using simple coils.
Abstract: We evaluated analytically and experimentally the performance of a planar gradiometer as a sensing element in a system for magnetic induction tomography. A system using an excitation coil and a planar gradiometer was compared against a system with two coils. We constructed one excitation coil, two different sensing elements and a high-resolution phase detector. The first sensor was a PCB square spiral coil with seven turns. The second sensor was a PCB planar gradiometer with two opposite square spirals of seven turns, with a distance between centres of 8 cm. Theoretical sensitivity maps were derived from basic equations and compared with experimental data obtained at 150 kHz. The experimental sensitivity maps were obtained measuring the perturbation produced by a brass sphere of 12 mm in empty space. The advantage of using a gradiometer is that it can be adjusted to give a minimum signal for homogeneous objects, while increasing the sensitivity to local perturbations of the conductivity. Results show that a system using a planar gradiometer as detector has less demanding requirements for the electronic system than a system using simple coils.

Journal ArticleDOI
TL;DR: A knowledge-based system using fuzzy logic for classifying plethysmogram pulses into two categories: valid and artefact is developed and can be implemented in its present from for real-time SaO2 monitoring in intensive care for detecting valid and distorted plethymogram pulses.
Abstract: Pulse oximetry is a useful, quick, non-invasive and widely used technology for monitoring oxygen saturation (SaO2) for neonates and paediatric patients. However, pulse oximetry is fraught with artefacts, causing false alarms resulting from patient or probe movement. The shape of the plethysmogram is a useful visual indicator for determining the reliability of SaO2 numerical readings. If certain features could be defined that tag valid plethysmogram pulses, then automatic recognition of valid SaO2 values can be attained. We observed that the systolic upstroke time (t1), the diastolic time (t2) and heart rate (HR) extracted from the plethysmogram pulse constitute features which can be used for detecting normal and distorted plethysmogram pulses. Therefore, we developed a knowledge-based system using fuzzy logic for classifying plethysmogram pulses into two categories: valid and artefact. A total of 22,497 pulse waveforms were used to define the system parameters. These were obtained from 13 patients with heart rates ranging between 62 and 209 beats min-1. A further 1420 waveforms obtained from another four patients were used for testing the system, and visually classified into 833 (59%) valid and 587 (41%) distorted segments. The system was able to classify 679 (82%) valid segments and 543 (93%) distorted segments correctly. The calculations of the system's performance showed 82% sensitivity, 86% accuracy and 93% specificity. We, therefore, conclude that the algorithm used in this system can be implemented in its present from for real-time SaO2 monitoring in intensive care for detecting valid and distorted plethysmogram pulses.

Journal ArticleDOI
TL;DR: A finite-difference model has been developed for simulating measurements in magnetic induction tomography (MIT) for biological tissues and the peak values in the modelled curves agreed to within 14% of practical measurements at 10 MHz on volumes of saline solution.
Abstract: 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.

Journal ArticleDOI
TL;DR: Based on physiological knowledge, and on an analysis of signals related to its dynamics, a model of the cardiovascular system consisting of coupled oscillators is proposed that describes one of the subsystems involved in the regulation of one passage of blood through the circulatory system.
Abstract: Based on physiological knowledge, and on an analysis of signals related to its dynamics, we propose a model of the cardiovascular system. It consists of coupled oscillators. Each of them describes one of the subsystems involved in the regulation of one passage of blood through the circulatory system. The flow of blood through the system of closed tubes—the blood vessels—is described by wave equations.

Journal ArticleDOI
TL;DR: The future use of a realistic head model in the reconstruction algorithm is likely to reduce the localization error in the human images due to the presence of the skull, which is not sufficient to explain the localization errors of up to 50% of the image diameter present in the images of some human subjects.
Abstract: Previous work has demonstrated that electrical impedance tomography can be used to image human brain activity during evoked responses, but two-thirds of the reconstructed images fail to localize an impedance change to the expected stimulated cortical area. The localization failure may be caused by modelling the head as a homogenous sphere in the reconstruction algorithm. This assumption may lead to errors when used to reconstruct data obtained from the human head. In this study a 3D reconstruction algorithm, based on a model of the head as a homogenous sphere, was characterized by simulating the algorithm model, the head shape and the presence of the skull in saline-filled tanks. EIT images of a sponge, 14 cm3 volume with a resistivity contrast of 12%, were acquired in three different positions in tanks filled with 0.2% saline. In a hemispherical tank, 19 cm in diameter, the sponge was localized to within 3.4-10.7% of the tank diameter. In a head-shaped tank, the errors were between 3.1 and 13.3% without a skull and between 10.3 and 18.7% when a real human skull was present. A significant increase in localization error therefore occurs if an algorithm based on a homogeneous sphere is used on data acquired from a head-shaped tank. The increased error is due to the presence of the skull, as no significant increase in error occurred if a head-shaped tank was used without the skull present, compared to the localization error within the hemispherical tank. The error due to the skull significantly shifted the impedance change within the skull towards the centre of the image. Although the increased localization error due to the skull is not sufficient to explain the localization errors of up to 50% of the image diameter present in the images of some human subjects, the future use of a realistic head model in the reconstruction algorithm is likely to reduce the localization error in the human images due to the presence of the skull.

Journal ArticleDOI
TL;DR: In the current study photoplethysmography was measured on toes and fingers of patients undergoing epidural anaesthesia and three parameters, related to the change in total and pulsatile tissue blood volume, were derived from the PPG baseline and amplitude.
Abstract: Epidural anaesthesia, used for pain relief, is based on blocking the sensory and the sympathetic nerves in the lower part of the body. Since the sympathetic nervous system regulates blood vessel diameter, the sympathetic block is also associated with several haemodynamic changes. In the current study photoplethysmography (PPG) was measured on toes and fingers of patients undergoing epidural anaesthesia. Three parameters, which are related to the change in total and pulsatile tissue blood volume, were derived from the PPG baseline and amplitude. All parameters showed statistically significant increase in the toes after the sympathetic block, indicating higher arterial and venous blood volume and higher pulsatile increase in the arterial blood volume (higher arterial compliance) in the toe. These haemodynamic changes originate from the lower tonus of the arterial and venous wall muscles after the sympathetic block. In the fingers the PPG parameters based on the change in PPG amplitude decreased after the sympathetic block, indicating lower compliance. The measurement of the haemodynamic changes by PPG enables the assessment of the depth of anaesthesia, and can help control the adverse effects of the blockade on the vascular system.

Journal ArticleDOI
Wang Feng1, Ding Haishu, Tian Fenghua, Zhao Jun, Xia Qing, Tang Xianwu 
TL;DR: The Monte Carlo simulation results showed that the sensitivity of the near infrared spectroscopy (NIRS) oximeter declined greatly with increasing overlying tissue thickness, and increasing the distance between the light source and the detector improved the sensitivity.
Abstract: In this paper, the influences of overlying tissue and detecting distance between the source and the detector on the measurement of a tissue oximeter were discussed. The signal-noise-ratio of the detector was also examined. A semi-infinite multi-layer Monte Carlo model was induced to simulate the migration of the photons in the skin, adipose and muscle. The thickness of the adipose layer and the separation between the source and the detector in the muscle were changed to simulate the clinical application. Partial pathlength was introduced as a characteristic parameter to evaluate the sensitivity of the oximeter. A two-wavelength (700 and 830 nm) tissue oximeter was developed to verify the results of the simulation. The Monte Carlo simulation results showed that the sensitivity of the near infrared spectroscopy (NIRS) oximeter declined greatly with increasing overlying tissue thickness. Increasing the distance between the light source and the detector improved the sensitivity. However, in order to achieve a sufficiently high signal-to-noise ratio, it was necessary to limit this distance. The results of a bicycle ergometer exercise verified the above results and showed that, for a special adipose thickness (AT), there may be a reasonable range of the emitter-detector separation.

Journal ArticleDOI
TL;DR: The reconstructed 2D EIT images showed reproducible impedance changes in the approximate region of the stimulated cortex in 7/16 visual and 5/18 somatosensory experiments, demonstrating that reproducible resistor changes can be measured during human brain activity.
Abstract: Previously, electrical impedance tomography (EIT) has been used to image impedance decreases in the exposed cortex of rabbits during brain activity. These are due to increased blood volume at the site of the stimulated cortex; as blood has a lower impedance than brain, the impedance decreases. During human brain activity similar blood flow changes have been detected using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). If blood volume also changes then the impedance of human cortex will change during brain activity; this could theoretically be imaged with EIT. EIT data were recorded from a ring of 16 scalp electrodes in 34 recordings in 19 adult volunteers before, during and after stimulation with (1) a visual stimulus produced by an 8 Hz oscillating checkerboard pattern or (2) sensory stimulation of the median nerve at the wrist by a 3 Hz electrical square wave stimulus. Reproducible impedance changes, with a similar timecourse to the stimulus, were seen in all experiments. Significant impedance changes were seen in 21 ± 5% (n = 16, mean ± SEM) and 19 ± 3% (n = 18) of the electrode measurements for visual and somatosensory paradigms respectively. The reconstructed 2D EIT images showed reproducible impedance changes in the approximate region of the stimulated cortex in 7/16 visual and 5/18 somatosensory experiments. This demonstrates that reproducible impedance changes can be measured during human brain activity. The final images contained spatial noise; the reasons for this and strategies to reduce this in future are discussed.

Journal ArticleDOI
TL;DR: The aim of this article is to review tools that have been developed to simulate the electrophysiology of cardiac cells and tissue, and to show how computational models have been used to gain insight into normal and abnormal action potential propagation.
Abstract: Computational models have the potential to make a huge impact on our understanding of normal and abnormal cardiac function. The aim of this article is to review tools that have been developed to simulate the electrophysiology of cardiac cells and tissue, and to show how computational models have been used to gain insight into normal and abnormal action potential propagation. Some of the practical problems experienced in the development and application of these models are described, and examples are given.

Journal ArticleDOI
TL;DR: It is demonstrated that conductivity changes caused by ventilation and perfusion in a human subject can be reconstructed from electrical impedance tomography data collected on a rectangular array of electrodes placed on a subject's chest.
Abstract: In this paper we demonstrate that conductivity changes caused by ventilation and perfusion in a human subject can be reconstructed from electrical impedance tomography data collected on a rectangular array of electrodes placed on a subject's chest. Currents are applied on the electrodes and the resulting voltages on the electrodes are measured. A 3D reconstruction algorithm is used to reconstruct the conductivity distribution in the region beneath the array. Time traces of the reconstructed conductivity distribution demonstrate the detected changes in conductivity due to ventilation and perfusion.

Journal ArticleDOI
TL;DR: A commercial tool is presented, which can be used to create FEM meshes for geometrical accurate FEM models, and is demonstrated by using segmented data from the human head to model impedance changes inside the head.
Abstract: If electrical impedance tomography is to be used as a clinical tool, the image reconstruction algorithms must yield accurate images of impedance changes. One of the keys to producing an accurate reconstructed image is the inclusion of prior information regarding the physical geometry of the object. To achieve this, many researchers have created tools for solving the forward problem by means of finite element methods (FEMs). These tools are limited, allowing only a set number of meshes to be produced from the geometric information of the object. There is a clear need for geometrical accurate FEM models to improve the quality of the reconstructed images. We present a commercial tool called IDEAS, which can be used to create FEM meshes for these models. The application of this tool is demonstrated by using segmented data from the human head to model impedance changes inside the head.

Journal ArticleDOI
TL;DR: It can be concluded that flow limitations in the iliac arteries in endurance athletes are associated with kinkings in the common and/or the external iliAC arteries.
Abstract: In endurance athletes, leg complaints upon maximal exercise caused by flow limitations in the iliac arteries are frequently encountered. We theorize that functional kinking of the vessels, which occurs especially during hip flexion, may be a cause for such flow limitations. Conventional diagnostic tests cannot demonstrate such kinkings. Using gadolinium-enhanced magnetic resonance angiography, a 3D dataset of the aorto-iliac arteries could be obtained with the hips flexed. An image processing procedure was developed using a new segmentation algorithm to be able to use standard surface rendering techniques to visualize the arteries with an improved 3D appearance. These techniques were applied in the current study in 42 endurance athletes with documented flow limitations in the iliac arteries. As a control group 16 national level competitive cyclists without flow limitations in the iliac arteries were studied. Forty-six affected legs were examined in 42 patients. In all patients and reference persons image quality was adequate and the segmentation algorithm could be applied. In 22 affected legs (48%) a kinking in the common iliac artery could be demonstrated, compared with one leg (3%) in the control group. In 13 affected legs (28%) a kinking in the external iliac artery could be demonstrated, compared with three legs (9%) in the control group. It can be concluded that flow limitations in the iliac arteries in endurance athletes are associated with kinkings in the common and/or the external iliac arteries. Magnetic resonance angiography with the hips flexed followed by this newly developed segmentation algorithm is effective to visualize and score these kinkings.

Journal ArticleDOI
TL;DR: It is found that the addition of noise also results in epochs of synchronization, as observed experimentally, and preliminary analysis suggests that there is a mixture of linear and parametric couplings, but that the linear coupling seems to dominate.
Abstract: A mathematical model of the cardiovascular system is simulated numerically. The basic unit in the model is an oscillator that possesses a structural stability and robustness motivated by physiological understanding and by the analysis of measured time series. Oscillators with linear couplings are found to reproduce the main characteristic features of the experimentally obtained spectra. To explain the variability of cardiac and respiratory frequencies, however, it is essential to take into account the rest of the system, i.e. to consider the effect of noise. It is found that the addition of noise also results in epochs of synchronization, as observed experimentally. Preliminary analysis suggests that there is a mixture of linear and parametric couplings, but that the linear coupling seems to dominate.

Journal ArticleDOI
TL;DR: A self-adaptive refinement algorithm based on an a posteriori error estimate has been developed and its results are shown in comparison with uniform mesh refinement for a simple head model.
Abstract: Adaptive mesh refinement techniques can be applied to increase the efficiency of electrical impedance tomography reconstruction algorithms by reducing computational and storage cost as well as providing problem-dependent solution structures. A self-adaptive refinement algorithm based on an a posteriori error estimate has been developed and its results are shown in comparison with uniform mesh refinement for a simple head model.

Journal ArticleDOI
TL;DR: This work shows that power spectral density (PSD) and phase images (derived from the Fourier transform) are easier to interpret and more useful tools for exploiting in vivo EIT data in healthy volunteers in order to explore the cardiovascular and respiratory systems.
Abstract: The aim of this study is to propose a useful method for exploring regional ventilation and perfusion in the chest. The paper describes two methods based on singular value decomposition (SVD) and Fourier transform (FT) respectively. This work shows that power spectral density (PSD) and phase images (derived from the Fourier transform) are easier to interpret and more useful tools for exploiting in vivo EIT data in healthy volunteers in order to explore the cardiovascular and respiratory systems.

Journal ArticleDOI
TL;DR: It is concluded that FA-MS is a powerful new method that, when combined with oral D2O loading, enables measurement of the dispersal kinetics of HDO and the calculation of total body water within 2 hours.
Abstract: The ability to measure total body water accurately, non-invasively and rapidly with results that are immediately available would represent an important advance in body composition research. Flowing afterglow mass spectrometry (FA-MS) has been developed to enable immediate measurement of deuterium content in breath water from single exhalations, which when combined with oral D2O loading enables measurement of total body water. We report here its first use in normal subjects, demonstrating the dispersal kinetics of deuterium in the body by monitoring deuterium in breath water following ingestion. Ten studies were performed in six normal subjects. After obtaining baseline samples, each subject ingested 0.3 g kg−1 of 99.9% pure D2O. Subsequently, breath samples were obtained every 3–5 minutes until equilibration had occurred. Three distinct phases in breath deuterium content were observed: an initial immediate peak due to HDO remaining in the oral cavity, a secondary peak reflecting gastrointestinal absorption and finally equilibration with the body water. The incremental increase in breath deuterium abundance between baseline and equilibration was used to calculate the total body water. Mathematical fitting of this final equilibration phase demonstrated that the measured deuterium abundance was within 0.2% of the ideal (asymptotic) value within 2 hours in all cases. We conclude that FA-MS is a powerful new method that, when combined with oral D2O loading, enables measurement of the dispersal kinetics of HDO and the calculation of total body water within 2 hours.

Journal ArticleDOI
TL;DR: The monitor was considered to be superior to conventional monitors and suitable for the successful conduct of the CHIME study and identified an additional 737 events where a human scorer did not detect bradycardia.
Abstract: A new physiologic monitor for use in the home has been developed and used for the Collaborative Home Infant Monitor Evaluation (CHIME). This monitor measures infant breathing by respiratory inductance plethysmography and transthoracic impedance; infant electrocardiogram, heart rate and R-R interval; haemoglobin O2 saturation of arterial blood at the periphery and sleep position. Monitor signals from a representative sample of 24 subjects from the CHIME database were of sufficient quality to be clinically interpreted 91.7% of the time for the respiratory inductance plethysmograph, 100% for the ECG, 99.7% for the heart rate and 87% for the 16 subjects of the 24 who used the pulse oximeter. The monitor detected breaths with a sensitivity of 96% and a specificity of 65% compared to human scorers. It detected all clinically significant bradycardias but identified an additional 737 events where a human scorer did not detect bradycardia. The monitor was considered to be superior to conventional monitors and, therefore, suitable for the successful conduct of the CHIME study.

Journal ArticleDOI
TL;DR: A DSP-based high-precision and high-performance wide-band (up to 10 MHz) bioimpedance analyser module for application in EIT or bioIMpedance spectroscopy and the number of data sampled per time interval is significantly reduced, the current consumption and the costs of the ADCs can be significantly reduced.
Abstract: In this paper we present the concept, the design and the test procedure for a DSP-based high-precision and high-performance wide-band (up to 10 MHz) bioimpedance analyser module for application in EIT or bioimpedance spectroscopy. The module implements a digital concept with appropriate signal conditioning hardware for voltage and current measurement, early signal digitization and subsequent digital signal processing in order to calculate the components of impedance (or admittance). At low frequencies, the module utilizes the conventional direct conversion method, whereas at high frequencies the undersampling technique is used. The advantages of the described system are the following: (a) the frequency range is extended to higher frequencies, (b) the number of data sampled per time interval is significantly reduced, and (c) the current consumption and the costs of the ADCs can be significantly reduced. The validation procedure is performed by comparing the measured and theoretical values of the magnitude and the phase of the impedance for a commonly used tissue model. The module offers an accuracy of better than 0.012% for the magnitude of impedance and better than 0.02 degrees for the phase.

Journal ArticleDOI
TL;DR: It is concluded that the current-arrow map and phase difference can be used to determine the T wave of an FMCG signal.
Abstract: To determine the T wave of a fetal magnetocardiogram (FMCG), we have evaluated the T/QRS ratio and obtained current-arrow maps that indicate weak currents. We measured FMCG signals for 52 normal fetuses and two abnormal fetuses with prolonged QT waves by using three superconducting quantum interference device (SQUID) systems: a nine-channel system, a 12-channel vector system and a 64-channel system. The T/QRS ratio was calculated for all the normal fetuses from the maximum magnitudes of the QRS complex and the T wave. Current-arrow maps of the QRS complex (R wave) and T wave were obtained by using the 64-channel system, and the phase differences of the total-current vectors were calculated by using the current-arrow maps. The results showed that the T/QRS ratio had a wide variability of 0.35 for the normal fetuses. However, the magnitude of the prolonged T wave was as weak as the detection limit of the SQUID magnetometer. Although the T/QRS ratios for the fetuses with QT prolongation were within the normal range (<0.35), the weak magnitude of the prolonged T wave could be evaluated. On the other hand, by comparing the current-arrow maps of the R and T waves for the normal fetuses, we found that the maximum-current arrows were indicated as either in the same direction or in opposite directions. These patterns could be identified clearly by the phase differences. Very weak prolonged T waves for the two abnormal fetuses could be determined by using these current-arrow maps and phase differences. Consequently, although the T/QRS ratios of FMCG signals have a wide distribution, we have concluded that the current-arrow map and phase difference can be used to determine the T wave of an FMCG signal.