Magnetic Field Distribution Measurement by the Modified FLASH Method

Weis, Ján; Frollo, Ivan; Budinský, Luboš

doi:10.1515/zna-1989-1203

Cited by 18 publications

(7 citation statements)

References 10 publications

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“…1a) were first measured to guide positioning of the SI slice and volume of interest (VOI). The SI technique with high spatial resolution was based on an RF spoiled gradient‐echo sequence (20). The slice thickness was chosen to be 15 mm.…”

Section: Methodsmentioning

confidence: 99%

Quantification of intramyocellular lipids in obese subjects using spectroscopic imaging with high spatial resolution

Weis

Johansson

Courivaud³

et al. 2006

Magnetic Resonance in Med

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Overweight and obesity are prevalent nutritional disorders that are becoming a worldwide epidemic (1). They are associated with an increase in total body fat, elevated blood pressure, insulin resistance, increased incidence of type 2 diabetes, etc. It is well known that in addition to over-accumulation of subcutaneous and visceral adipose tissue, extra-(EMCL) and intramyocellular (IMCL) lipids in skeletal muscle are important factors in the pathogenesis of these concomitant diseases (2,3). IMCL stores have been found to be a potent marker of insulin resistance in obese (4,5) and non-obese adults (6), as well as in nondiabetic offspring of type 2 diabetic subjects (7,8).Most previous attempts to quantify IMCL were based on the analysis of muscle biopsy samples (9 -11). However, measures of IMCL derived in this manner lack sufficient accuracy and sensitivity to distinguish between IMCL and EMCL content. Moreover, the samples in the above-cited studies were small, and it is technically difficult to assess fat biochemically (12). It was recently demonstrated that proton magnetic resonance spectroscopy ( 1 H MRS) is a convenient noninvasive technique that allows the discrimination and measurement of IMCL and EMCL pools in vivo (13,14). 1 H MRS has been used to measure the IMCL level, in most cases with single-voxel techniques (5)(6)(7)(8)(12)(13)(14) and a voxel volume greater than 1 cm 3 . In obese subjects, such large voxels often contain regions with increased EMCL, which contaminate the IMCL spectrum due to severe signal overlap. Single-voxel spectroscopy (SVS) is especially susceptible to this problem because postacquisition voxel repositioning is impossible and the selected voxel location may not yield spectra with acceptable IMCL and EMCL separation. Moreover, the problem of voxel location complicates inhomogeneous distribution of IMCL and EMCL within the same muscle and between muscles (15-17). This problem is often underestimated in studies that seek to measure IMCL in a predefined region (e.g., for correlations with other methods), and in longitudinal studies. To overcome these difficulties, MR spectroscopic imaging (MRSI) techniques with a measured matrix of up to 32 ϫ 32 (phase-encoding steps) have been used (15,16,18). Despite the smaller size of the voxels (0.25-0.5 cm 3 ), there are still severe overlaps of the IMCL and EMCL spectral components in many voxels. In addition, MRSI spectra are contaminated by a signal bleeding from bone marrow and subcutaneous and interstitial fat as a consequence of limited k-space sampling. Several methods have been reported to suppress fat signal bleeding with postacquisition processing (16,19), but none of these methods offers a definite solution. Standard SI with a large number of phase-encoding steps (e.g., 128 ϫ 128 or more) can be efficient in removing the signal bleeding. However, it requires unacceptably long acquisition times to acquire all k-space encodings.The purpose of the present study was to investigate whether high-resolution SI with a large number of pha...

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Section: Methodsmentioning

confidence: 99%

Quantification of intramyocellular lipids in obese subjects using spectroscopic imaging with high spatial resolution

Weis

Johansson

Courivaud³

et al. 2006

Magnetic Resonance in Med

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“…1. The main part of the sequence was derived from the radiofrequency (RF) spoiled gradient‐echo sequence with step increments of TE, while TR and bandwidth per pixel were kept constant (19–22). Slice selection was achieved by using the manufacturer's fat‐selective binomial 1 1 excitation pulse instead of the standard RF pulse.…”

Section: Methodsmentioning

confidence: 99%

Lipid content in the musculature of the lower leg: Evaluation with high‐resolution spectroscopic imaging

Weis

Courivaud

Hansen

et al. 2005

Magnetic Resonance in Med

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A novel spectroscopic imaging method with high spectral and spatial resolution was developed for the specific goal of assessing muscle fat. Sensitivity to the methylene and methyl protons of fatty acids was improved by the use of a binomial 1 1 excitation pulse instead of the standard radiofrequency (RF) pulse. Acceptable measurement time is achieved by using a narrow spectral bandwidth (6 ppm). The spectral resolution is sufficient to resolve extramyocellular (EMCL) and intramyocellular ( Key words: spectroscopic imaging; narrow bandwidth; muscular lipids; fat; intramyocellular triglyceridesThe first assessments of muscle fat were performed in biopsy specimens. However, because of the invasive nature of this technique, only a few data points can be obtained. Computed tomography (CT) has been used both to measure the total body fat content and to study different fat compartments, and provides a relatively accurate measurement of internal and subcutaneous fat (1,2). However, the low lipid concentrations in muscle cannot be quantified accurately from biopsy specimens or by CT because neither of these techniques is sufficiently sensitive to distinguish between extramyocellular (EMCL) and intramyocellular (IMCL) lipids. These problems can be overcome with the use of 1 H magnetic resonance spectroscopy (MRS) (3), which is a unique noninvasive method with high sensitivity. The main metabolites that are detectable in proton spectra of muscles include EMCL, IMCL, trimethylammonium, creatine/phosphocreatine, carnosine, and taurine (4).It is important to assess EMCL and IMCL stores in skeletal musculature in studies of the physiological and pathological aspects of lipid metabolism (5-7). Bulk fat (EMCL) is stored subcutaneously and in interstitial (adipose) tissue. Along the fasciae, muscle boundaries and fibers form macroscopic plate structures that run almost parallel with the long axes of the extremities. Intramyocellular lipids are stored in the form of liquid droplets in the cytoplasm of muscle cells (8). Different muscle groups contain different amounts of EMCLs and IMCLs (7,9), and these amounts are considerably greater in obese than in lean individuals (6,7). EMCLs serve as a long-term fat depot, whereas IMCLs are involved in lipid metabolism with a turnover of some hours. IMCLs vary in quantity during physical exercise (8) and also depend on food intake (10,11). The role of IMCL in diabetes is not fully understood; however, an inverse correlation between IMCL concentration and insulin sensitivity has been confirmed by several research groups (6,7,12,13).The spectral line positions of EMCL and IMCL depend on both the chemical shifts and the local bulk magnetic susceptibility effects. Since the IMCL droplets have a spherical shape and are stored in a susceptibility homogeneous (ϳwater) environment of muscle cells, IMCL signals are independent of the muscle orientation relative to the magnetic field, B 0 . IMCL always resonates at the same frequency: 1.3 ppm. For macroscopic EMCL structures distributed along muscle...

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“…using ratio processing of the NMR data [17]. The second methods require more measurements but the result is in very good agreement with the reality [10][11][12][13][14]16] without any correction. Both mentioned groups of methods require good stability of the static magnetic field of the scanner and the authors assumed it.…”

Section: Introductionmentioning

confidence: 59%

“…The subsequent optimization yielded values of the five variables ϕ 1shift to ϕ 5shift . Using the values the original five NMR data matrices were corrected and they were prepared for the calculations of magnetic field maps from the phase and from the position of the spectral line of water [13,16] (figure 4). Calculation of the spectrum from the five samples would provide very poor resolution; that is why the interpolation method of zero filling was used to improve the appearance of the result.…”

Section: Theory and Methodsmentioning

confidence: 99%

Optimized measurement of magnetic field maps using nuclear magnetic resonance (NMR)

Andris¹,

Frollo²

2011

Meas. Sci. Technol.

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A new method of measuring the relative maps of a nuclear magnetic resonance (NMR) scanner static magnetic field was developed. The knowledge of the static magnetic field distribution of a scanner is needed for several calculations but its absolute values are not always necessary. The magnetic field of magnets used for NMR scanners is not always absolutely stable.Measuring their absolute values is then very difficult; the results are usually only the values averaged over time. The averaging is also a complex task because if calculating the magnetic field from the phase of the measured NMR data the problems of unwrapping the phase must also be solved simultaneously. The paper presents a method for simultaneously solving both problems of averaging and unwrapping using an optimization method. The result is a relative map of the static magnetic field that can be used for subsequent calculations or testing.

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Magnetic Field Distribution Measurement by the Modified FLASH Method

Cited by 18 publications

References 10 publications

Quantification of intramyocellular lipids in obese subjects using spectroscopic imaging with high spatial resolution

Quantification of intramyocellular lipids in obese subjects using spectroscopic imaging with high spatial resolution

Lipid content in the musculature of the lower leg: Evaluation with high‐resolution spectroscopic imaging

Optimized measurement of magnetic field maps using nuclear magnetic resonance (NMR)

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