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.
Highlights
Magnetic susceptibility is one of the following major categories of tissue contrast mechanisms in proton MRI [1]: 1) spin thermal relaxation in a voxel of water; 2) water motion, including diffusion, perfusion, flow and tissue deformation; and 3) molecular electron cloud polarization by the main magnetic field B0
The field observed by a water proton is the sum of contributions from all surrounding susceptibility sources [their distribution defined by magnetization mðrÞ], excluding that from the proton’s own location: bðrÞ 1⁄4 m0 4p
quantitative susceptibility mapping (QSM) is feasible for applications in other body parts including the breast, extremity, and abdomen for studying hemorrhage, metabolic oxygen consumption, mineral distribution, and contrast agent kinetics [96]
Summary
Magnetic susceptibility is one of the following major categories of tissue contrast mechanisms in proton MRI [1]: 1) spin thermal relaxation in a voxel of water; 2) water motion, including diffusion, perfusion, flow and tissue deformation; and 3) molecular electron cloud polarization by the main magnetic field B0. A polarized molecule generates its own magnetic field, which is known as a chemical-shift shielding field for observer protons inside the molecule and as a magnetic-susceptibility inhomogeneity field for observer protons outside the molecule This field adds phase accumulation and causes intravoxel dephasing or magnitude T2* decay in the commonly available gradient echo (GRE) MRI. The GRE phase is equal to the magnetic field multiplied by the gyromagnetic ratio g and the echo time This phase may be used to further attenuate the signal for enhancing T2* image contrast, which is called susceptibility weighted imaging [2,3,4]. MRI provides plenty of information on tissue anatomical structures This information can serve as a prior in Bayesian regularization to overcome this ill-posed inverse problem, generating a reasonably accurate susceptibility map [17,18,19,20]. We review the mathematical relationships that link magnetization, field, and MRI signal, based on which the field can be estimated from the MRI signal
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