Rapid <i>in vivo</i> proton shimming

Schneider, Erika; Glover, Gary H.

doi:10.1002/mrm.1910180208

Cited by 211 publications

(185 citation statements)

References 6 publications

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“…In addition to water and fat imaging, a byproduct of many Dixon techniques is the magnetic field map, which is directly related to the phase map after phase unwrapping and useful for field shimming and other purposes (12). Some Dixon techniques may also be helpful in the assessment of the tissue iron concentration through the determination of transverse relaxation times (8,59,70).…”

Section: Discussionmentioning

confidence: 99%

“…A more practical minimum-norm approach is to unwrap the phase through some empirical modeling of the true phase. For Dixon imaging, a polynomial model or truncated Taylor series is reasonable in some situations and has been used to represent the spatial phase variations (12,13). By imposing a specific model, phase unwrapping in the minimum-norm sense amounts to some parameter estimation.…”

Section: Minimum-norm Methodsmentioning

confidence: 99%

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Dixon techniques for water and fat imaging

2008

Magnetic Resonance Imaging

513

442

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In 1984, Dixon published a first paper on a simple spectroscopic imaging technique for water and fat separation. The technique acquires two separate images with a modified spin echo pulse sequence. One is a conventional spin echo image with water and fat signals in-phase and the other is acquired with the readout gradient slightly shifted so that the water and fat signals are 180°out-of-phase. Dixon showed that from these two images, a water-only image and a fat-only image can be generated. The wateronly image by the Dixon's technique can serve the purpose of fat suppression, an important and widely used imaging option for clinical MRI. Additionally, the availability of both the water-only and fat-only images allows direct imagebased water and fat quantitation. These applications, as well as the potential that the technique can be made highly insensitive to magnetic field inhomogeneity, have generated substantial research interests and efforts from many investigators. As a result, significant improvement to the original technique has been made in the last 2 decades. The following article reviews the underlying physical principles and describes some major technical aspects in the development of these Dixon techniques. MR IMAGES ACQUIRED IN VIVO usually contain signals from both water and fat. In many pulse sequences, fat appears hyperintense. However, the contribution from water is often of the primary interest for many practical applications. Without suppression, the bright fat signal may result in aggravated motion-related artifacts and, more importantly, reduce the underlying lesion conspicuity. Because of its chemical shift, fat is also responsible for the well-known spatial misregistration artifacts, which can appear both along the frequency encode direction and along the slice select direction. For a few other applications such as diagnosis of bone marrow diseases or hepatic steatosis, detection rather than suppression of the fat signal can also be valuable.Several approaches have been proposed and developed for achieving fat suppression. Perhaps the most popular is the chemical shift selective saturation (1) or its variants. In this approach, a frequency selective radiofrequency (RF) pulse and a spoiler gradient pulse are used in conjunction to first excite and then saturate the fat magnetization before water is excited for imaging. Alternatively, a frequency selective RF pulse can be used to directly excite only the water magnetization and leave the fat magnetization alone along the longitudinal axis. These techniques, particularly the selective saturation technique, are easy to implement and have been widely used with great success. However, both selective saturation pulses and selective excitation pulses increase the scan time. Another limitation for fat suppression by the selective saturation pulses is that it typically requires an accurate 90°flip angle, and as a result, its performance can be dependent on B1 homogeneity. Perhaps most importantly, fat suppression using the frequency selective approach...

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Section: Discussionmentioning

confidence: 99%

Section: Minimum-norm Methodsmentioning

confidence: 99%

Dixon techniques for water and fat imaging

2008

Magnetic Resonance Imaging

513

442

View full text Add to dashboard Cite

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“…[4] or [5], by calculating the Jacobian term from differentiation of either Eq. [1] or [2], respectively.…”

Section: Theorymentioning

confidence: 99%

“…In order for Eqs. [4] and [5] to be meaningful, the Jacobian must be greater than zero. Calculating the Jacobian term directly by differentiating Eqs.…”

Section: Theorymentioning

confidence: 99%

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Correction of spatial distortion in EPI due to inhomogeneous static magnetic fields using the reversed gradient method

Morgan

Bowtell

McIntyre

et al. 2004

Magnetic Resonance Imaging

174

177

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Purpose: To derive and implement a method for correcting spatial distortion caused by in vivo inhomogeneous static magnetic fields in echo-planar imaging (EPI). Materials and Methods:The reversed gradient method, which was initially devised to correct distortion in images generated by spin-warp MRI, was adapted to correct distortion in EP images. This method provides point-by-point correction of distortion throughout the image. EP images, acquired with a 3 T MRI system, of a phantom and a volunteer's head were used to test the correction method.Results: Good correction was observed in all cases. Spatial distortion in the uncorrected images ranged up to 4 pixels (12 mm) and was corrected successfully. Conclusion:The correction was improved by the application of a nonlinear interpolation scheme. The correction requires that two EP images be acquired at each slice position. This increases the acquisition time, but an improved signal-tonoise ratio (SNR) is seen in the corrected image. The local SNR gain decreases with increasing distortion. In many EPI acquisition schemes, multiple images are averaged at each slice position to increase the SNR; in such cases the reversed gradient correction method can be applied with no increase in acquisition duration.

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Spatial distribution of deoxy myoglobin in human muscle: an index of local tissue oxygenation

Tran

Sailasuta²,

Hurd³

et al. 1999

NMR Biomed.

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The proximal histidyl NδH signal of myoglobin is detectable in 1H NMR spectra of myocardial and skeletal muscle, and its intensity reflects the intracellular oxygenation. At 1.5 Tesla (T), the typical field strength of clinical magnetic resonance imaging (MRI) magnets, the paramagnetic relaxation contribution decreases sufficiently to permit the implementation of chemical shift imaging technique to map the spatial distribution of the deoxy Mb NδH signal from human gastrocnemius muscle. One and two‐dimensional chemical shift imaging experiments reveal clearly the localized deoxy Mb signal in muscle and consequently the spatial distribution of the cellular oxygenation. The results indicate the feasibility to assess the pO2 in tissue regions and to directly study the regulation of oxidative metabolism in human tissue. Copyright © 1999 John Wiley & Sons, Ltd.

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Rapid in vivo proton shimming

Cited by 211 publications

References 6 publications

Dixon techniques for water and fat imaging

Dixon techniques for water and fat imaging

Correction of spatial distortion in EPI due to inhomogeneous static magnetic fields using the reversed gradient method

Spatial distribution of deoxy myoglobin in human muscle: an index of local tissue oxygenation

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