Magnetic susceptibility: Further insights into macroscopic and microscopic fields and the sphere of Lorentz

Durrant, C. J.; Hertzberg, Mark P.; Kuchel, Philip W.

doi:10.1002/cmr.a.10067

Cited by 69 publications

(98 citation statements)

References 17 publications

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“…The calculation of the frequency perturbation due to this form of anisotropy in the susceptibility is detailed in SI Text. The calculation involves forming the gradient of the magnetic scalar potential, followed by the addition of an (isotropic) sphere of Lorentz correction (21). Expressions describing the frequency perturbation due to the different mechanisms are listed in Table 1.…”

Section: Resultsmentioning

confidence: 99%

Fiber orientation-dependent white matter contrast in gradient echo MRI

Wharton

Bowtell

2012

Proc. Natl. Acad. Sci. U.S.A.

299

611

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Recent studies have shown that there is a direct link between the orientation of the nerve fibers in white matter (WM) and the contrast observed in magnitude and phase images acquired using gradient echo MRI. Understanding the origin of this link is of great interest because it could offer access to a new diagnostic tool for investigating tissue microstructure. Since it has been suggested that myelin is the dominant source of this contrast, creating an accurate model for characterizing the effect of the myelin sheath on the evolution of the NMR signal is an essential step toward fully understanding WM contrast. In this study, we show by comparison of the results of simulations and experiments carried out on human subjects at 7T, that the magnitude and phase of signals acquired from WM in vivo can be accurately characterized by (i) modeling the myelin sheath as a hollow cylinder composed of material having an anisotropic magnetic susceptibility that is described by a tensor with a radially oriented principal axis, and (ii) adopting a two-pool model in which the water in the sheath has a reduced T 2 relaxation time and spin density relative to its surroundings, and also undergoes exchange. The accuracy and intrinsic simplicity of the hollow cylinder model provides a versatile framework for future exploitation of the effect of WM microstructure on gradient echo contrast in clinical MRI. G radient echo (GE) MRI is widely used in imaging the human brain, because both the phase and magnitude of the complex NMR signal measured with GE sequences can be used to create high-resolution images that show strong contrast between different types of brain tissue (1). Recent studies have shown that there is a direct link between the orientation of the nerve fibers in white matter (WM) with respect to the magnetic field and the contrast observed in magnitude and phase images (2-6). Although the origin of this link is currently not fully understood, orientation-dependent contrast is of great interest because it could offer researchers access to a new diagnostic tool for investigating tissue microstructure using MRI.It has recently been suggested that the myelin sheaths that surround axons are the dominant source of WM contrast in GE MRI (7,8). Creating an accurate model for characterizing the effect of the myelin sheath on the evolution of the magnitude and phase of the NMR signal is consequently an essential step toward fully understanding WM contrast and its relationship to fiber orientation. Such a model must incorporate two main features: (i) a representation of the microscopic spatial variation of resonant frequency, due to the myelin compartment-isotropic and anisotropic magnetic susceptibility effects (2, 9, 10) and chemical exchange of protons between water and macromolecules (11, 12), have been proposed as mechanisms through which myelin could perturb the resonant frequency in WM; (ii) a signal-weighting scheme to account for the reduced T 2 relaxation time constant of the myelin water relative to that of water found outs...

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Section: Resultsmentioning

confidence: 99%

Fiber orientation-dependent white matter contrast in gradient echo MRI

Wharton

Bowtell

2012

Proc. Natl. Acad. Sci. U.S.A.

299

611

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“…Accumulation of fat between the muscle fibers where it is constrained to form elongated cylinders results in separate 1 H MR spectral resonances from the IMCL and EMCL. The physical basis of the effect resides in the altered local magnetic field strength in a geometrical body when the magnetic susceptibility is different inside (in this case lipid) and outside (water); for spheres (IMCL) there is of course no dependence of this internal field on orientation relative to the main magnetic field, but there is a marked effect on the magnetic field in elongated cylinders (12). The emergence of separate 1 H MRS signals from TGs posits this as an early marker of lipid accumulation with mild cardiac hypertrophy associated with obesity and/or prediabetic conditions.…”

Section: Discussionmentioning

confidence: 99%

Cardiac function and lipid distribution in rats fed a high-fat diet: in vivo magnetic resonance imaging and spectroscopy

Nagarajan

Gopalan

Kaneko

et al. 2013

American Journal of Physiology-Heart and Circulatory Physiology

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Nagarajan V, Gopalan V, Kaneko M, Angeli V, Gluckman P, Richards AM, Kuchel PW, Sendhil Velan S. Cardiac function and lipid distribution in rats fed a high-fat diet: in vivo magnetic resonance imaging and spectroscopy. Am J Physiol Heart Circ Physiol 304: H1495-H1504, 2013. First published March 29, 2013 doi:10.1152/ajpheart.00478.2012.-Obesity is a major risk factor in the development of cardiovascular disease, type 2 diabetes, and its pathophysiological precondition insulin resistance. Very little is known about the metabolic changes that occur in the myocardium and consequent changes in cardiac function that are associated with high-fat accumulation. Therefore, cardiac function and metabolism were evaluated in control rats and those fed a high-fat diet, using magnetic resonance imaging, magnetic resonance spectroscopy, mRNA analysis, histology, and plasma biochemistry. Analysis of blood plasma from rats fed the high-fat diet showed that they were insulin resistant (P Ͻ 0.001). Our high-fat diet model had higher heart weight (P ϭ 0.005) and also increasing trend in septal wall thickness (P ϭ 0.07) compared with control diet rats. Our results from biochemistry, magnetic resonance imaging, and mRNA analysis confirmed that rats on the high-fat diet had moderate diabetes along with mild cardiac hypertrophy. The magnetic resonance spectroscopy results showed the extramyocellular lipid signal only in the spectra from high-fat diet rats, which was absent in the control diet rats. The intramyocellular lipids in high-fat diet rats was higher (8.7%) compared with rats on the control diet (6.1%). This was confirmed by electron microscope and light microscopy studies. Our results indicate that lipid accumulation in the myocardium might be an early indication of the cardiovascular pathophysiology associated with type 2 diabetes.

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“…[51] in the accompanying article (22), for which g ϭ Ᏸ s Ϫ 1). The sphere and capillary are rarely of perfectly ideal shape, so the term (g cyl Ϫ g sph ) ϭ G is determined as a single calibration factor using the known susceptibilities of benzene and 2 H 2 O (21).…”

Section: Analysis: Examplementioning

confidence: 99%

Magnetic susceptibility: Solutions, emulsions, and cells

Kuchel

Chapman

Bubb

et al. 2003

Concepts Magnetic Resonance

Self Cite

105

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Differences in magnetic susceptibility between various compartments in heterogeneous samples can introduce unanticipated complications to NMR spectra. On the other hand, an understanding of these effects at the level of the underlying physical principles has led to the development of several experimental techniques that provide data on cellular function that are unique to NMR spectroscopy. To illustrate some key features of susceptibility effects we present, among a more general overview, results obtained with red blood cells and a recently described model system involving diethyl phthalate in water. This substance forms a relatively stable emulsion in water and yet it has a significant solubility of ϳ5 mmol L Ϫ1 at room temperature; thus, the NMR spectrum has twice as many resonances as would be expected for a simple solution. What determines the relative intensities of the two families of peaks and can their frequencies be manipulated experimentally in a predictable way? The theory used to interpret the NMR spectra from the model system and cells was first developed in the context of electrostatics nearly a century ago, and yet some of its underlying assumptions now warrant closer scrutiny. While this insight is used in a practical way in this article, the accompanying article deals with the mathematics and physics behind this new analysis.

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Magnetic susceptibility: Further insights into macroscopic and microscopic fields and the sphere of Lorentz

Cited by 69 publications

References 17 publications

Fiber orientation-dependent white matter contrast in gradient echo MRI

Fiber orientation-dependent white matter contrast in gradient echo MRI

Cardiac function and lipid distribution in rats fed a high-fat diet: in vivo magnetic resonance imaging and spectroscopy

Magnetic susceptibility: Solutions, emulsions, and cells

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