2008-06-25

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.

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. 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 ). 2021-04-03 Skip to main content.

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. It was found that a theta of 2 pi/3 radians has optimal S/N but that a theta of pi radians is a good compromise and that phase encoding of (O, pi, 2 pi) radians offer an advantage over the previous method, which used (-pi, O, pi) increments, in that a T2' (intravoxel susceptibility dephasing) image may be obtained in addition to the usual water, fat, and Bo images. intravoxel dephasing. phase difference between stationary nuclei and flowing nuclei in voxel, nuclei insame voxel are out of phase with each other, results inredirection of signal amplitude.

On the other hand, intravascular-dephasing effects scale with dHb concentration and are therefore most prominent in the draining veins. Both of these mechanisms of BOLD contrast are expected to contribute to the gradient-echo (GE) BOLD signal, while the static-dephasing mechanism makes effectively no contribution to the spin-echo (SE) BOLD signal, which is primarily sensitive to intravascular dephasing.

Moreover, while intra-sponding directions in k space. Fourier transformation results voxel dephasing originates from an increase in broadness of 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 Displacement encoding with stimulated echoes (DENSE) with a meta-DENSE readout and RF phase cycling to suppress the STEAM anti-echo is described for reducing intravoxel dephasing signal loss. 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.

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

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. / Aletras, Anthony; Arai, A E. In: Journal of Magnetic Resonance, Vol. 169, No. 2, 2004, p.

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. The new rephasing factor we introduce, which changes the intensity of a voxel in images during the registration, is used to improve the accuracy of the intensity-based nonrigid registration method and mitigate the intensity attenuation effect.
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Decreasing the voxel size (increasing matrix/decreasing slice thickness) can reduce the intravoxel dephasing caused by magnetic susceptibility. • Increasing the Intravoxel dephasing is the predominant cause of signal loss on GRE imaging resulting in a dark or black area (signal loss) around the metal on the processed leave a bodily area before the magnetization is measured.signal void. (2) In MRI, any loss of signal caused by combined turbulence and intravoxel dephasing . Oct 22, 2018 large gradient encoding magnitudes as well as dephasing artefacts caused by subject motion, which is particularly challenging in fetal MRI. Nov 30, 2017 At low spatial resolution in particular, the respective field gradients lead to an apparent increase in intravoxel dephasing, and subsequently, magnitude caused by intravoxel dephasing should remain due to microscopic field for intravoxel dephasing due to the neuronal magnetic fields (9).

Displacement encoding with stimulated echoes (DENSE) with a meta-DENSE readout and RF phase cycling to suppress the STEAM anti-echo is described for reducing intravoxel dephasing signal loss. 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.
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20 jan. 2012 — som båda minskar intravoxel dephasing med endast en liten minskning av FET känslighet. En högre bandbredd används även för att minska

francois.de-guio@cemagref.fr 2006-05-09 Materials and methods Experiments were performed with Keywords Magnetic susceptibility · Intravoxel dephasing · a 0.2 T magnet on a network of small interacting air-ﬁlled Signal loss · Artefacts · MRI simulation · cylinders along with Magnetic resonance imaging (MRI) Field inhomogeneity simulations integrating intravoxel dephasing. 2008-06-25 (1999) Chen, Wyrwicz. Magnetic Resonance in Medicine.