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      Quantitative susceptibility mapping (QSM): Decoding MRI data for a tissue magnetic biomarker

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          Abstract

          In MRI, the main magnetic field polarizes the electron cloud of a molecule, generating a chemical shift for observer protons within the molecule and a magnetic susceptibility inhomogeneity field for observer protons outside the molecule. The number of water protons surrounding a molecule for detecting its magnetic susceptibility is vastly greater than the number of protons within the molecule for detecting its chemical shift. However, the study of tissue magnetic susceptibility has been hindered by poor molecular specificities of hitherto used methods based on MRI signal phase and T2* contrast, which depend convolutedly on surrounding susceptibility sources. Deconvolution of the MRI signal phase can determine tissue susceptibility but is challenged by the lack of MRI signal in the background and by the zeroes in the dipole kernel. Recently, physically meaningful regularizations, including the Bayesian approach, have been developed to enable accurate quantitative susceptibility mapping (QSM) for studying iron distribution, metabolic oxygen consumption, blood degradation, calcification, demyelination, and other pathophysiological susceptibility changes, as well as contrast agent biodistribution in MRI. This paper attempts to summarize the basic physical concepts and essential algorithmic steps in QSM, to describe clinical and technical issues under active development, and to provide references, codes, and testing data for readers interested in QSM. Magn Reson Med 73:82–101, 2015. © 2014 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance. This is an open access article under the terms of the Creative commons Attribution License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited.

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          SENSE: Sensitivity encoding for fast MRI

          New theoretical and practical concepts are presented for considerably enhancing the performance of magnetic resonance imaging (MRI) by means of arrays of multiple receiver coils. Sensitivity encoding (SENSE) is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. Thus, by using multiple receiver coils in parallel scan time in Fourier imaging can be considerably reduced. The problem of image reconstruction from sensitivity encoded data is formulated in a general fashion and solved for arbitrary coil configurations and k-space sampling patterns. Special attention is given to the currently most practical case, namely, sampling a common Cartesian grid with reduced density. For this case the feasibility of the proposed methods was verified both in vitro and in vivo. Scan time was reduced to one-half using a two-coil array in brain imaging. With an array of five coils double-oblique heart images were obtained in one-third of conventional scan time. Magn Reson Med 42:952-962, 1999. Copyright 1999 Wiley-Liss, Inc.
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            Inverse Problem Theory and Methods for Model Parameter Estimation

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              Susceptibility weighted imaging (SWI).

              Susceptibility differences between tissues can be utilized as a new type of contrast in MRI that is different from spin density, T1-, or T2-weighted imaging. Signals from substances with different magnetic susceptibilities compared to their neighboring tissue will become out of phase with these tissues at sufficiently long echo times (TEs). Thus, phase imaging offers a means of enhancing contrast in MRI. Specifically, the phase images themselves can provide excellent contrast between gray matter (GM) and white matter (WM), iron-laden tissues, venous blood vessels, and other tissues with susceptibilities that are different from the background tissue. Also, for the first time, projection phase images are shown to demonstrate tissue (vessel) continuity. In this work, the best approach for combining magnitude and phase images is discussed. The phase images are high-pass-filtered and then transformed to a special phase mask that varies in amplitude between zero and unity. This mask is multiplied a few times into the original magnitude image to create enhanced contrast between tissues with different susceptibilities. For this reason, this method is referred to as susceptibility-weighted imaging (SWI). Mathematical arguments are presented to determine the number of phase mask multiplications that should take place. Examples are given for enhancing GM/WM contrast and water/fat contrast, identifying brain iron, and visualizing veins in the brain. Copyright 2004 Wiley-Liss, Inc.
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                Author and article information

                Journal
                Magn Reson Med
                Magn Reson Med
                mrm
                Magnetic Resonance in Medicine
                John Wiley & Sons, Ltd (Chichester, UK )
                0740-3194
                1522-2594
                January 2015
                17 July 2014
                : 73
                : 1
                : 82-101
                Affiliations
                [1 ]Radiology, Weill Medical College of Cornell University New York, New York, USA
                [2 ]Biomedical Engineering, Cornell University Ithaca, New York, USA
                [3 ]Biomedical Engineering, Kyung Hee University Seoul, South Korea
                [4 ]MedImageMetric, LLC New York, New York, USA
                Author notes
                Correspondence to: Yi Wang, Ph.D., 515 East 71st St, Suite 102, New York, NY, 10022. E-mail: yiwang@ 123456med.cornell.edu

                Correction added after online publication 19 August 2014. Magnetic moment was updated to due to a font error in the text on page 3, equation 1, in Figure 2, and in the online supporting information file. In column one paragraph two of page 2, the inline expression for was corrected. In the caption of Figure 1, “three-dimensional” was removed. In the text on page 14 and in equation 16, was changed to and was change to . Equation 16 was also updated to change “ ” in the first exponent to “ ” and to lower “‖” to appear in line with the equation. In the text on page 14 column two, “3.4 ppm” was changed to “-3.4 ppm.”

                Article
                10.1002/mrm.25358
                4297605
                25044035
                6b74e602-330c-4787-bede-c212a89845bc
                © 2014 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                History
                : 13 March 2014
                : 13 June 2014
                : 18 June 2014
                Categories
                Imaging Methodology—Reviews

                Radiology & Imaging
                qsm,quantitative susceptibility mapping,gradient echo,metabolism,iron,oxygen consumption,ferritin,hemoglobin,hemorrhage,calcification,myelin,contrast agent,quantification,dipole field,dipole kernel,morphology enabled dipole inversion,bayesian

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