Understanding phase maps in MRI: a new cutline phase unwrapping method

Chavez, Sofia; Xiang, Qing; Liu, An

doi:10.1109/tmi.2002.803106

Cited by 150 publications

(117 citation statements)

References 15 publications

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“…The integration path of phase unwrapping is prevented from crossing the "branch cuts." Properly placed "branch cuts" will thus avoid enclosing unbalanced poles and ensure the uniqueness of the phase unwrapping (9,10). The other approach of the path-following methods does not aim to generate explicit "branch cuts."…”

Section: Path-following Methodsmentioning

confidence: 99%

“…It is, therefore, conceivable that minimizing the total length of the branch cuts alone may not always lead to the correct placement of the branch cuts. On the basis of their study of some phase maps in MRI, Chavez et al (10) postulated that poles that are spatially close together are noise related and that longer-range poles are from true undersampling. Under this assumption, Chavez et al designed a fringeline tracking algorithm to distinguish between the two types of the poles based on their relative fringeline lengths.…”

Section: Path-following Methodsmentioning

confidence: 99%

“…Regardless of the underlying causes, violation of the condition in Equation [14] generates some singularities called residues or poles in a phase map (9,10). A residue or pole is defined as a 2 ϫ 2 pixel loop around which an integration of the wrapped phase difference is not zero and is equal to a multiple of 2 .…”

Section: Phase Unwrappingmentioning

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: Path-following Methodsmentioning

confidence: 99%

Section: Path-following Methodsmentioning

confidence: 99%

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Dixon techniques for water and fat imaging

2008

Magnetic Resonance Imaging

513

442

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“…(4) and (5). The displacement of the target can readily be extracted from cos ψ and sin ψ using the phase unwrapping method, which is a powerful tool to resolve the phase ambiguity problem in a variety of applications [3,12,13]. Specifically, the displacement ∆x of the target at time t n , n = 0, 1, 2, .…”

Section: Model Of Two-probe Measurementsmentioning

confidence: 99%

A Way to Improve the Accuracy of Displacement Measurement by a Two-Probe Implementation of Microwave Interferometry

Doronin¹,

Gorev²,

Kodzhespirova³

et al. 2013

PIER M

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Abstract-This paper addresses the possibility of displacement measurement by microwave interferometry at an unknown reflection coefficient with the use of as few as two probes. The case of an arbitrary interpobe distance is considered. The measurement error as a function of the interprobe distance is analyzed with the inclusion of variations of the detector currents from their theoretical values. The analysis has shown that as the interprobe distance decreases, the maximum measurement error passes through a minimum for reflection coefficients close to unity and increases monotonically for smaller reflection coefficients. Based on the results of the analysis, the interprobe distance is suggested to be one tenth of the guided operating wavelength λ g . In comparison with the conventional interprobe distance of λ g /8, the suggested one offers a marked reduction in the maximum measurement error for reflection coefficients close to unity, while for smaller ones this error remains much the same (for a detector current error of 3%, the maximum measurement error in percent of the operating wavelength is 2.2% and 1.0% at λ g /10 as against 4.8% and 2.7% at λ g /8 for a reflection coefficient of 1 and 0.9, respectively, and 2.9% at λ g /10 as against 2.4% at λ g /8 for a reflection coefficient of 0.1).

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“…The range of phase values is limited to [-, ϩ ], so phase wrapping may occur, making it difficult to obtain the correct B 0 map. Numerous studies have addressed the challenging task of robust phase unwrapping, not only for B 0 map generation, but also for MR venography (7)(8)(9)(10)(11). Although all the techniques described have shown promising results, they have not led to a widespread application of field mapping and appropriate correction methods in current fMRI studies.…”

mentioning

confidence: 99%

Robust field map generation using a triple‐echo acquisition

Windischberger

Robinson

Rauscher

et al. 2004

Magnetic Resonance Imaging

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Purpose: To establish a fast and robust technique for generating magnetic field maps for the correction of geometric distortions in echo-planar magnetic resonance (MR) images. Materials and Methods:Multislice gradient-echo (GE) images were acquired at echo times of 6, 6.5, and 7.5 msec in order to cover a field shift range of Ϯ666 Hz in the resulting B 0 maps. To account for possible phase wrap scenarios, seven phase triples were calculated for each pixel. Linear regression of the phase vs. echo time was performed for each set. The slope of the set with the minimum fitting error was taken as the true magnetic field in the respective pixel.Results: Based on the fitting error distribution, the technique is shown to be feasible and effective for assessing the field distribution in the brain at 3 T, especially in inferior brain areas (amygdalae, hippocampus). Examples of echoplanar images distortion corrected using the calculated field maps are shown. Conclusion:The approach presented yields robust estimation of magnetic field maps and requires under a minute of additional acquisition time and only seconds of computational time. As such, it is easily possible to apply image distortion correction in routine functional MR imaging (fMRI) studies, enabling improved coregistration of brain activation maps with structures on anatomical images. MOST FUNCTIONAL MAGNETIC RESONANCE imaging (fMRI) studies today employ echo-planar or spiral imaging sequences to achieve the high image repetition rates required to sufficiently sample the hemodynamic response following a stimulus. These fast data-acquisition sequences, however, are vulnerable to inhomogeneities in the static magnetic field, leading to signal dropouts via intravoxel dephasing effects and geometric distortions (for echo-planar images) or image blurring (for spiral imaging) (1). Some of those field nonuniformities, so-called concomitant field changes, are caused by gradients that occur in addition to the imaging gradients due to intrinsic properties of the magnetic field. Compensation techniques have been proposed (2,3). Others originate from susceptibility differences between tissue and bone/air in the sample, leading to distortions of an initially homogeneous B 0 magnetic field. The calculation of these B 0 changes is the focus of this study.In the case of echo-planar imaging (EPI), local B 0 deviations lead to simple spatial shifts of the affected voxels. These shifts increase with higher field strength, as the susceptibility-induced field changes scale linearly with B 0 , while imaging gradient strengths are approximately constant due to hardware limitations. With receiver bandwidths of 100 -200 kHz in the readout direction, B 0 changes of up to 200 Hz at 3 T do not lead to a significant spatial shift along this axis. The bandwidth per voxel in the phase-encoding direction, however, is lower by a factor of the matrix size, commonly leading to spatial shifts of several voxels. Early studies showed that it is possible to elegantly compensate for these geometric dis...

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Understanding phase maps in MRI: a new cutline phase unwrapping method

Cited by 150 publications

References 15 publications

Dixon techniques for water and fat imaging

Dixon techniques for water and fat imaging

A Way to Improve the Accuracy of Displacement Measurement by a Two-Probe Implementation of Microwave Interferometry

Robust field map generation using a triple‐echo acquisition

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