Measuring diffusion in inhomogeneous systems in imaging mode using antisymmetric sensitizing gradients

Hong, Xiaole; Dixon, William J.

doi:10.1016/0022-2364(92)90210-x

Cited by 45 publications

(50 citation statements)

References 2 publications

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“…Others have suggested that water diffusion, as measured by NMR, could be anisotropic due to local susceptibility-difference-induced gradients in the nerves and white matter. 32,33 Myelin and axonal membranes…”

Section: Postulated Sources and Investigation Of Diffusion Anisotropymentioning

confidence: 99%

“…32,33 The first evaluation of its potential contribution to white matter was performed by Trudeau et al on excised porcine spinal cord at 4.7 T. 46 By varying the orientation of the fibre tracts parallel or perpendicular to the static magnetic field (B o ), the background gradients could be minimized or maximized, respectively. The ADCs parallel or perpendicular to the fibres measured with the standard PGSE diffusion sequence were independent of the fibre orientation relative to B o and hence the induced gradients do not play a role in the anisotropy of diffusion in white matter.…”

Section: Susceptibilitymentioning

confidence: 99%

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The basis of anisotropic water diffusion in the nervous system – a technical review

2002

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Anisotropic water diffusion in neural fibres such as nerve, white matter in spinal cord, or white matter in brain forms the basis for the utilization of diffusion tensor imaging (DTI) to track fibre pathways. The fact that water diffusion is sensitive to the underlying tissue microstructure provides a unique method of assessing the orientation and integrity of these neural fibres, which may be useful in assessing a number of neurological disorders. The purpose of this review is to characterize the relationship of nuclear magnetic resonance measurements of water diffusion and its anisotropy (i.e. directional dependence) with the underlying microstructure of neural fibres. The emphasis of the review will be on model neurological systems both in vitro and in vivo. A systematic discussion of the possible sources of anisotropy and their evaluation will be presented followed by an overview of various studies of restricted diffusion and compartmentation as they relate to anisotropy. Pertinent pathological models, developmental studies and theoretical analyses provide further insight into the basis of anisotropic diffusion and its potential utility in the nervous system.

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Section: Postulated Sources and Investigation Of Diffusion Anisotropymentioning

confidence: 99%

Section: Susceptibilitymentioning

confidence: 99%

The basis of anisotropic water diffusion in the nervous system – a technical review

2002

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“…4. It is worth noting that the nature of the diffusion gradients used in this pulse sequence make the resulting ADC measurements intrinsically insensitive to background and imaging gradient cross terms, according to the description by Hong and Dixon (25). The tetrahedral sampling of the diffusion tensor afforded maximal b-values for a given diffusion time and peak gradient amplitude, and provided sufficient information to compute the mean ADC (orientational mean, i.e, one-third the trace of the diffusion tensor), as originally described by Conturo et al (26).…”

Section: Mrimentioning

confidence: 99%

Oscillating gradient measurements of water diffusion in normal and globally ischemic rat brain

Does

Parsons

Gore

2003

Magnetic Resonance in Med

213

326

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Oscillating gradients were used to probe the diffusion-time/ frequency dependence of water diffusion in the gray matter of normal and globally ischemic rat brain. In terms of a conventional definition of diffusion time, the oscillating gradient measurements provided the apparent diffusion coefficient (ADC) of water with diffusion times between 9.75 ms and 375 s, an order of magnitude shorter than previously studied in vivo. Over this range, ADCs increased as much as 24% in vivo and 50% postmortem, depending on the nature of the oscillating gradient waveform used. Novel waveforms were employed to sample narrow frequency bands of the so-called diffusion spectrum. This spectral description of ADC includes the effects of restriction and/or flow, and is independent of experimental parameters, such as diffusion time. The results in rat brain were found to be consistent with restricted diffusion and the known microanatomy of gray matter. Differences between normal and postmortem data were consistent with an increase in water restriction and/or a decrease in flow, and tentatively suggest that physical changes following the onset of ischemia occur on a scale of about 2 m, similar to a typical cellular dimension in gray matter. Magn Reson Med 49:206 -215, 2003. © 2003 Wiley-Liss, Inc. Key words: MRI; diffusion; restriction; ischemia; oscillating gradientWhen the Brownian motion of a molecule is physically impeded or hindered, its effective rate of self-diffusion, or apparent diffusion coefficient (ADC), is reduced, and the diffusion is said to be restricted. In tissues, restriction is thought to be a significant mechanism affecting contrast in diffusion-weighted imaging, in both normal and pathological states. In particular, restriction of water by cell membranes is believed to be the dominant mechanism causing diffusion anisotropy in the nervous system (1), and it has also been postulated to play a central role in the marked decline of water ADC in neural tissue following the onset of ischemia (2-4). However, due to hardware limitations, there has been limited direct investigation in vivo of the effects of restrictions on diffusion measurements with MRI. Such observations can be made by measuring ADC as a function of diffusion time.In an NMR diffusion measurement, if the time allowed for spins to migrate is brief enough, restriction will have little impact on the measurement and the ADC will closely reflect the intrinsic diffusion coefficient. As the diffusion time increases and the molecules interact more often with the physical restrictions, the ADC declines toward an asymptotic value. Several such measurements have been made on excised tissue samples (5-9), typically using spectrometers equipped with strong gradients (50 -500 gauss/cm), which allow ADC measurements at relatively short diffusion times.In vivo measurements, however, have been limited by gradient strengths that are typically one or two orders of magnitude smaller, making the minimal diffusion time relatively long. In RIF-1 tumors, Helmer et al. (10) fou...

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“…Since the background gradients can be assumed to be symmetric about zero, the normalized echo intensity is equal to: (4) where odd orders of background gradients are zero as a result of the assumed symmetric distribution of the background gradients, and terms with orders higher than quadratic can be neglected. Furthermore, we assume that the spatial variation of background gradients in random structures can be modeled by a correlation function , and the normalized echo intensity is equal to: (5) In order to illustrate the effects of background gradients upon the diffusion measurements, we assume that the temporal correlation and PDF of the background gradients can be modeled by and , respectively, where ζ is the characteristic correlation time and σ is the variance of the background gradients. Because ζ depends on the spin displacement rate, characteristic pore size, as well as the local structure, its exact analytical form may not be known.…”

Section: Theorymentioning

confidence: 99%

“…Such background gradients vary with the susceptibility mismatch and local structure of the system, and also depend upon the magnetic field strength at which the experiment is being conducted [4][5][6][7]. For instance, in heterogeneous systems studied at high magnetic field strengths, background gradients can be comparable to or possibly stronger than laboratory gradients; if not properly accounted for, these background gradients may introduce severe errors when diffusion NMR techniques are used to characterize local structure.…”

Section: Introductionmentioning

confidence: 99%

Improved diffusion measurement in heterogeneous systems using the magic asymmetric gradient stimulated echo (MAGSTE) technique

Sun

2007

Journal of Magnetic Resonance

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A magic asymmetric gradient stimulated echo (MAGSTE) sequence was recently proposed to improve molecular diffusion measurements in the presence of spatially varying background gradients. Its effectiveness has been demonstrated previously with simulated background gradients and in phantoms that contain bulk susceptibility differences. In this study, we investigated the MAGSTE technique in microscopically heterogeneous systems, and compared it with the conventional bipolar pulsed gradient stimulated echo (bPGSTE) sequence. We demonstrated that the MASGTE measurements, compared to the bPGSTE method, varied significantly less when the diffusion encoding/decoding interval (δ) was changed. In addition, the MAGSTE technique provided good characterization of the surface area-to-volume ratio for heterogeneous systems investigated in this study. In sum, this study showed that the MAGSTE technique provided diffusion measurements superior to those of the bPGSTE sequence, especially in the presence of severe heterogeneous background gradients.

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Measuring diffusion in inhomogeneous systems in imaging mode using antisymmetric sensitizing gradients

Cited by 45 publications

References 2 publications

The basis of anisotropic water diffusion in the nervous system – a technical review

The basis of anisotropic water diffusion in the nervous system – a technical review

Oscillating gradient measurements of water diffusion in normal and globally ischemic rat brain

Improved diffusion measurement in heterogeneous systems using the magic asymmetric gradient stimulated echo (MAGSTE) technique

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