In gradient-echo images, not in spin-echo images, these local fields cause intravoxel dephasing of the water signal of the surrounding tissue. This description of the contrast enhancement has been confirmed by a series of in vitro blood sample experiments and image simulations.

Oct 22, 2018 large gradient encoding magnitudes as well as dephasing artefacts caused by subject motion, which is particularly challenging in fetal MRI.

Objective Characterization of magnetic susceptibility artefacts with assessment of the gradient-echo signal decay function of echo time, pixel size, and object geometry in the case of air-filled cylinders embedded in water. Materials and methods Signal loss due to flow is induced via intravoxel dephasing which, according to Eq. 1, is dependent on velocity profile and m1. For a certain velocity profile, the flow suppression capability of FSD-preparation is determined by m1. from dephasing effects and confoun d the microstructural information reflected in signal amplitude and relaxation rate measures derived from GRE data. As the mechanism underlying these dropouts is well understood, correction may be possible. In fact, t hrough - plane dephasing calculation and correction ( … Intravoxel dephasing related to turbulence Accumulation of positive phase shift over the course of the scan Fat-saturation pulses as used in time-of-flight imaging Inflow of unsaturated blood into the slice after the initial 90-degree pulse Flow-related enhancement is caused by 'naive' blood entering the slice.

Intravoxel dephasing

meta-DENSE complex acquisition for reduced intravoxel dephasing. / Aletras, Anthony; Arai, A E. In: Journal of Magnetic Resonance, Vol. 169, No. 2, 2004, p. 246-249. meta-DENSE complex acquisition for reduced intravoxel dephasing. Aletras, Anthony LU and Arai, A E In Journal of Magnetic Resonance 169 (2). p.246-249.

The cause of intravoxel dephasing is not yet well understood. This phenomenon is due to the spin dephasing when the voxel size is comparable to the residual lumen at level of stenosis ( 7 ). In addition to, previous studies were explained that turbulent flow in the axilla area of a small vessel may cause an intravoxel dephasing, especially at a broad bifurcation angle ( 8 , 9 ).

The RF pulse is designed with its phase response as the negative of the product of a chosen echo time and the Frahm, J., Merboldt, K. D., & Haenicke, W. (1995). The effects of intravoxel dephasing and incomplete slice refocusing on susceptibility contrast in gradient-echo MRI. Spin dephasing arising from intravoxel internal gradient lowers the signal-to-noise ratio (SNR) and therefore de-gradesthequalityofgradient-echoimages,especiallythose acquired at long TE. This dephasing effect can be lessened with increased in-plane spatial resolution and reduced Intravoxel dephasing causes signal loss and thus intensity attenuation in the images.

A technique is proposed to compensate for the slice dephasing artifact and improve the signal‐to‐noise ratio (SNR) of gradient‐echo images. This method is composed of two components: mapping of the internal gradient and design of the slice‐selective radiofrequency (RF) pulse. The RF pulse is designed with its phase response as the negative of the product of a chosen echo time and the

Phantom and human images are provided to from intravoxel dephasing in GRE acquisitions. Spin echo methods refocus the static field heterogeneities, and therefore do not have signal dropout. The relationships between contrast, artifacts and field strength are summarized in Table 1. At this time, 7 Tand higher field magnets are not in wide-spreaduse Se hela listan på academic.oup.com This paper addresses one of the major problems in interventional magnetic resonance imaging (MRI): the visualization of interventional devices. For visualization locally induced magnetic fields are used, which disturb the homogeneity of the main magnetic field of the MR scanner. This results in signal loss in the vicinity of the device due to intravoxel dephasing, and leads to a disturbance of Measurements of Inter- and Intravoxel Dephasing of Trabecular Bone at 7T Field Strength using a Chemical Shift-Selective.

This phenomenon is due to the spin dephasing when the voxel size is comparable to the residual lumen at level of stenosis ( 7 ). In addition to, previous studies were explained that turbulent flow in the axilla area of a small vessel may cause an intravoxel dephasing, especially at a broad bifurcation angle ( 8 , 9 ). From a finite and discrete Fourier transform point of view, we discuss the sources of localization errors in Fourier chemical-shift imaging, and demonstrate them explicitly by computer simulations for simple cases. Errors arise from intravoxel dephasing and the intravoxel asymmetry. The spectral leakage due to intravoxel dephasing is roughly 6-8% from one voxel to one of its nearest neighbors. Skip to main content. swedish website; Menu.
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Neighbors further away are influenced less significantly. The loss of localization due to intravoxel … The resultant intravoxel dephasing is a classic T2*-shortening effect most prominent near larger veins and accentuated by use of GRE sequences with echo times (TEs) close to T2*. The effect scales linearly with field strength (Bo) and is the dominant mechanism for BOLD contrast at 1.5T. In MRI, intravoxel dephasing causes signal loss in regions of magnetic field inhomogeneity and at interfaces of tissues having different magnetic susceptibilities. In MRA, signal loss due to intravoxel dephasing also occurs when nuclei having different velocities are present in the same voxel.

The intravoxel dephasing formulas in Equations (2) and (3) for complex voxel signal formation are linear due to the fact that a vector sum or a complex addition is linear (the linearity associated with multiresolution intravoxel dephasing summation has been described by “rebinning invariance” 16). The most effective method for reducing intravoxel dephasing is to shorten the echo time (TE); however, the amount that TE can be shortened in conventional sequences is limited. Signal loss due to flow is induced via intravoxel dephasing which, according to Eq. 1, is dependent on velocity profile and m1. For a certain velocity profile, the flow suppression capability of FSD-preparation is determined by m1.
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imation of intra-voxel dephasing. This approach allows for further improvement and precision of the calculated NMR signal and virtually avoids the problem related to an ﬁnite set of isochromats.

francois.de-guio@cemagref.fr ‘‘intravoxel dephasing,’’ but also from incomplete refocus-phase-encoding direction shift the gradient echo along corre-ing of the slice-selection gradient. Moreover, while intra-sponding directions in k space. Fourier transformation results voxel dephasing originates from an increase in broadness of Experiments were performed with a 0.2 T magnet on a network of small interacting air-filled cylinders along with Magnetic resonance imaging (MRI) simulations integrating intravoxel dephasing. Signal decay over echo time was assessed at different pixel sizes on real and simulated images. Each cylinder contributed to the intravoxel dephasing and associated signal loss occurring in the ROI ROI MGL decay on simulated and experimental images for the network of air-filled cylinders.

Efficient and Accurate Bloch-based Simulation of Intra-voxel Dephasing using Multiple Isochomats and Magnetization Spatial Gradients Zhipeng Cao 1, Christopher T. Sica, Giuseppe Carluccio 2, and Christopher M. Collins 1Radiology, The Pennsylvania State University, Hershey, PA, United States, 2Radiology, New York University, New York City, NY, United States

PDF. PDF. susceptibility, dipole effect, dipole inversion, intravoxel dephasing, nonlinear T2* magnitude image, T2* phase unwrapping, MRI transformations. INTRODUCTION. The concept of tomographic imaging refers to reproducing a digital image representation of object interior distribution (with respect to a physical property) and can be implemented by Voxel signal calculation by intravoxel dephasing integration. Exposed to an inhomogeneous fieldmap, a proton precesses with a phase angle Δϕ(x, y, z, T E) = γ · ΔB(x, y, z)·T E, where γ is the gyromagnetic ratio. It is noted that the phase angle is different from the field value by a constant factor γ ⋅ T E (Larmor law). At very low diffusion weighting the diffusion MRI signal is affected by intravoxel incoherent motion (IVIM) caused by dephasing of magnetization due to incoherent blood flow in capillaries or other sources of microcirculation.

Materials and methods Experiments were performed with a 0.2T magnet on a network of small interacting air-filled cylinders along with Magnetic resonance imaging (MRI) simulations integrating intravoxel dephasing. Signal decay over echo time was assessed at different pixel sizes on real and simulated images. Examples of the gradient echo (A-C) and spin-echo sequences (D-F) and the corresponding excited signals. The signals generated without intra-voxel dephasing exhibits false spurious echoes (B) and ‘‘intravoxel dephasing,’’ but also from incomplete refocus-phase-encoding direction shift the gradient echo along corre-ing of the slice-selection gradient. Moreover, while intra-sponding directions in k space. Fourier transformation results voxel dephasing originates from an increase in broadness of meta-DENSE complex acquisition for reduced intravoxel dephasing.