Off-resonance correction of MR images

Schomberg, H.

doi:10.1109/42.781014

Cited by 73 publications

(78 citation statements)

References 30 publications

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“…12, we used the formulation in Eq. [5] as part of an inverse problem approach to field-inhomogeneity-corrected image reconstruction (i.e., estimate the image f given the field map, ). We showed that in regions with large field inhomogeneity, the iterative reconstruction method results in more accurate images than the standard conjugate phase approach.…”

Section: Nonlinear Least-squares Joint Estimationmentioning

confidence: 99%

Dynamic field map estimation using a spiral‐in/spiral‐out acquisition

Sutton

Noll

Fessler

2004

Magnetic Resonance in Med

101

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The long readout times of single-shot acquisitions and the high field strengths desired for functional MRI (fMRI) using blood oxygenation level-dependent (BOLD) contrast make functional scans sensitive to magnetic field inhomogeneity. If it is not corrected during image reconstruction, field inhomogeneity can cause geometric distortions in the images when Cartesian k-space trajectories are used or blurring with spiral acquisitions. Many traditional methods to correct for field inhomogeneity distortions rely on a static field map measured with the use of images that are themselves distorted. In this work, we employ a regularized least-squares approach to jointly estimate both the undistorted image and field map at each acquisition using a spiral-in/spiral-out pulse sequence. Simulation and phantom studies show that this method is accurate and stable over a time series. Human functional studies show that the jointly estimated field map may be more accurate than standard field map estimates in the presence of respiration-induced phase oscillations, leading to better detection of functional activation. The proposed method measures a dynamic field map that accurately tracks magnetic field drift and respiration-induced phase oscillations during the course of a functional study.

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Section: Nonlinear Least-squares Joint Estimationmentioning

confidence: 99%

Dynamic field map estimation using a spiral‐in/spiral‐out acquisition

Sutton

Noll

Fessler

2004

Magnetic Resonance in Med

101

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“…3). The problem becomes increasingly severe at higher fields; therefore, methods for distortion corrections have found increasing interest (27)(28)(29)(30)(31)(32)(33)35). All of these approaches are based on measurements of local field variations by additional field map measurements integrated into the examination.…”

Section: Signal Behavior In Bold Fmrimentioning

confidence: 99%

Functional magnetic resonance imaging: A review of methodological aspects and clinical applications

Hennig¹,

Speck²,

Koch

et al. 2003

Magnetic Resonance Imaging

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This paper gives an overview of the recent literature on methodological developments of functional magnetic resonance imaging (fMRI) and recent trends in clinical applications. With the recent introduction of high-field systems and methodological developments leading to more robust signal behavior, fMRI is in a transition state from a research modality for use by experts to a standard procedure with useful applications in patient management. Compared to the use in neuroscientific research, which is often based on BOLD techniques alone, the application in patients is distinguished by a multiparametric characterization of the brain using a combination of several techniques. Neuronal fiber tracking based on diffusion anisotropy measurements, in particular, has already turned out to provide relevant supplementary information to the BOLD-based cortical activation maps.

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“…[10], if the regularization parameter values are fixed, then b ϭ (A WA ϩ R) Ϫ1 A Wd , where R ϭ I␤ ϩ ⌳. This analytical solution involves a matrix inversion, which is a computationally intensive O(N t 3 ) operation.…”

Section: Theorymentioning

confidence: 99%

“…Afterwards, the sample values are compensated for density variation in the trajectory. Some researchers have designed pulses by adopting the conjugate-phase (CP) approach from image reconstruction (9,10), which accounts for off-resonance effects and thus can correct for it to some extent (11,12).…”

mentioning

confidence: 99%

Iterative RF pulse design for multidimensional, small‐tip‐angle selective excitation

Yip

Fessler

Noll

2005

Magnetic Resonance in Med

114

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The excitation k-space perspective on small-tip-angle selective excitation has facilitated RF pulse designs in a range of MR applications. In this paper, k-space-based design of multidimensional RF pulses is formulated as a quadratic optimization problem, and solved efficiently by the iterative conjugate-gradient (CG) algorithm. Compared to conventional design approaches, such as the conjugate-phase (CP) method, the new design approach is beneficial in several regards. It generally produces more accurate excitation patterns. The improvement is particularly significant when k-space is undersampled, and it can potentially shorten pulse lengths. A prominent improvement in accuracy is also observed when large off-resonance gradients are present. A further boost in excitation accuracy can be accomplished in regions of interest (ROIs) if they are specified together with "don't-care" regions. The design of RF pulses for multidimensional, small-tipangle selective excitation is facilitated by the excitation k-space perspective developed by Pauly et al. (1) under the small-tip-angle approximation to the Bloch equation. kSpace-based selective excitation has been used in a range of MR applications, such as functional MRI (fMRI) artifact correction (2), brain imaging with reduced field of view (FOV) (3), blood velocity measurement (4), parallel excitation using multiple transmit coils (5,6), and excitation inhomogeneity correction (7). The k-space perspective is popular because it provides a fairly accurate linear Fourier relationship between the time-varying gradient and RF waveforms, and the resulting transverse excitation pattern. The Fourier relationship can also be established for rotation angles (possibly large), provided that certain symmetry conditions are satisfied (8).A common approach to small-tip-angle RF pulse design is to predetermine the gradient waveforms and thus the k-space trajectory, and then obtain the complex-valued RF waveform by sampling the Fourier transform of the desired excitation pattern along the trajectory. Afterwards, the sample values are compensated for density variation in the trajectory. Some researchers have designed pulses by adopting the conjugate-phase (CP) approach from image reconstruction (9,10), which accounts for off-resonance effects and thus can correct for it to some extent (11,12).These conventional approaches are nonideal in several regards. First, in terms of minimizing excitation error, they generally produce pulses that are suboptimal even with respect to the linear design model. This design suboptimality is an eradicable source of excitation error, on top of the intractable amount of error due to the small-tip-angle approximation underlying excitation k-space. The excitation error due to design suboptimality is particularly large when the trajectory undersamples k-space, or when large spatial variations of off-resonance are present (11). To compensate for the effects of off-resonance gradients to some extent, Noll et al. (13) suggested the use of a sophisticated density c...

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Off-resonance correction of MR images

Cited by 73 publications

References 30 publications

Dynamic field map estimation using a spiral‐in/spiral‐out acquisition

Dynamic field map estimation using a spiral‐in/spiral‐out acquisition

Functional magnetic resonance imaging: A review of methodological aspects and clinical applications

Iterative RF pulse design for multidimensional, small‐tip‐angle selective excitation

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