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Signal Processing Magazine, IEEE

Issue 6 • Date Nov. 2001

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Displaying Results 1 - 6 of 6
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  • Emerging medical imaging modalities

    Page(s): 12 - 13
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  • Electromagnetic brain mapping

    Page(s): 14 - 30
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    There has been tremendous advances in our ability to produce images of human brain function. Applications of functional brain imaging extend from improving our understanding of the basic mechanisms of cognitive processes to better characterization of pathologies that impair normal function. Magnetoencephalography (MEG) and electroencephalography (EEG) (MEG/EEG) localize neural electrical activity using noninvasive measurements of external electromagnetic signals. Among the available functional imaging techniques, MEG and EEG uniquely have temporal resolutions below 100 ms. This temporal precision allows us to explore the timing of basic neural processes at the level of cell assemblies. MEG/EEG source localization draws on a wide range of signal processing techniques including digital filtering, three-dimensional image analysis, array signal processing, image modeling and reconstruction, and, blind source separation and phase synchrony estimation. We describe the underlying models currently used in MEG/EEG source estimation and describe the various signal processing steps required to compute these sources. In particular we describe methods for computing the forward fields for known source distributions and parametric and imaging-based approaches to the inverse problem View full abstract»

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  • Electrical impedance tomography

    Page(s): 31 - 43
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    Electrical impedance tomography (EIT) is an imaging modality that estimates the electrical properties at the interior of an object from measurements made on its surface. Typically, currents are injected into the object through electrodes placed on its surface, and the resulting electrode voltages are measured. An appropriate set of current patterns, with each pattern specifying the value of the current for each electrode, is applied to the object, and a reconstruction algorithm uses knowledge of the applied current patterns and the measured electrode voltages to solve the inverse problem, computing the electrical conductivity and permittivity distributions in the object. This article focuses on the type of EIT called adaptive current tomography (ACT) in which currents are applied simultaneously to all the electrodes. A number of current patterns are applied, where each pattern defines the current for each electrode, and the subsequent electrode voltages are measured to generate the data required for image reconstruction. A ring of electrodes may be placed in a single plane around the object, to define a two-dimensional problem, or in several layers of such rings, to define a three-dimensional problem. The reconstruction problem is described and two algorithms are discussed, a one-step, two-dimensional (2-D) Newton-Raphson algorithm and a one-step, full three-dimensional (3-D) reconstructor. Results from experimental data are presented which illustrate the performance of the algorithms View full abstract»

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  • Imaging the body with diffuse optical tomography

    Page(s): 57 - 75
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    Diffuse optical tomography (DOT) is an ongoing medical imaging modality in which tissue is illuminated by near-infrared light from an array of sources, the multiply-scattered light which emerges is observed with an array of detectors, and then a model of the propagation physics is used to infer the localized optical properties of the illuminated tissue. The three primary absorbers at these wavelengths, water and both oxygenated and deoxygenated hemoglobin, all have relatively weak absorption. This fortuitous fact provides a spectral window through which we can attempt to localize absorption (primarily by the two forms of hemoglobin) and scattering in the tissue. The most important current applications of DOT are detecting tumors in the breast and imaging the brain. We introduce the basic idea of DOT and review the history of optical methods in medicine as relevant to the development of DOT. We then detail the concept of DOT, including a review of the tissue's optical properties, modes of operation for DOT, and the challenges which the development of DOT must overcome. The basics of modelling the DOT forward problem and some critical issues among the numerous implementations that have been investigated for the DOT inverse problem, with an emphasis on signal processing. We summarize with some specific results as examples of the current state of DOT research View full abstract»

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  • Imaging myocardial strain

    Page(s): 44 - 56
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    Measuring the local mechanical activity of the heart has lagged behind the measurement of electrical activity due to a lack of measurement tools. Myocardial wall motion abnormalities have been studied for years in the context of regional ischemia. Implanted beads and screws have been used to measure the mechanical activity of the heart in a few isolated regions. Over the past decade, precise and accurate methods for measuring local three-dimensional (3-D) myocardial motion with magnetic resonance imaging (MRI) have been developed using presaturation tagging patterns, velocity encoded phase maps, and displacement encoded phase maps. Concurrently, the quality of cardiac MRI images improved greatly with the use of customized receiver coils and the speed of acquisition has increased dramatically with the advent of undersampling techniques and new generations of MR machines with faster switching gradient coils. The use of these cardiac MRI techniques to produce an image of the local deformation of the heart in the form of a myocardial strain image is described. Using these images, the “mechanical activation” of the heart are defined, that is, the time of onset of contraction. A map of the mechanical activation over the heart is a direct analogy to an electrical activation map of the heart View full abstract»

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IEEE Signal Processing Magazine publishes tutorial-style articles on signal processing research and applications, as well as columns and forums on issues of interest.

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Editor-in-Chief
Min Wu
University of Maryland, College Park
United States 

http://www/ece.umd.edu/~minwu/