Characterization and suppression of stripe artifact in velocity‐selective magnetization‐prepared unenhanced MR angiography

et al. 2020

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Purpose: Most existing non-contrast-enhanced methods for abdominal MR arteriography rely on a spatially selective inversion (SSI) pulse with a delay to null both static tissue and venous blood, and are limited to small spatial coverage due to the sensitivity to slow arterial inflow. Velocity-selective inversion (VSI) based approach has been shown to preserve the arterial blood inside the imaging volume at 1.5 T.Recently, velocity-selective saturation (VSS) pulse trains were applied to suppress the static tissue and have been combined with SSI pulses for cerebral MR arteriography at 3 T. The aim of this study is to construct an abdominal MRA protocol with large spatial coverage at 3 T using advanced velocity-selective pulse trains. Methods: Multiple velocity-selective MRA protocols with different sequence modules and 3D acquisition methods were evaluated. Sequences using VSS only as well as SSI+VSS and VSI+VSS preparations were then compared among a group of healthy young and middle-aged volunteers. Using MRA without any preparations as reference, relative signal ratios and relative contrast ratios of different vascular segments were quantitatively analyzed. Results: Both SSI+VSS and VSI+VSS arteriograms achieved high artery-to-tissue and artery-to-vein relative contrast ratios above aortic bifurcation. The SSI+VSS sequence yielded lower signal at the bilateral iliac arteries than VSI+VSS, reflecting the benefit of the VSI preparation for imaging the distal branches. Conclusion: The feasibility of noncontrast 3D MR abdominal arteriography was demonstrated on healthy volunteers using a combination of VSS pulse trains and SSI or VSI pulse. K E Y W O R D S abdominal MRA, arteriography, non-contrast-enhanced MRA, velocity-selective pulse train 1174 | ZHU et al.

Section: Discussionsupporting

confidence: 56%

Non‐contrast‐enhanced abdominal MRA at 3 T using velocity‐selective pulse trains

Zhu

et al. 2020

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“…The sensitivities are shown as the subtracted errors of label and control affected by ECs normalized to the equilibrium signal. Note that severe B 1 offsets are known to generate stripe artifacts due to imperfect refocusing, as characterized in previous velocity‐selective MR angiography (VSMRA) studies 37,38 . To suppress this periodic banding of high spatial‐frequency, the final response Mz was displayed as the average of Mz in 4 mm intervals, to be consistent with the spatial resolution of the ASL experiments.…”

Section: Resultsmentioning

confidence: 87%

“…Note that severe B 1 offsets are known to generate stripe artifacts due to imperfect refocusing, as characterized in previous velocity-selective MR angiography (VSMRA) studies. 37,38 To suppress this periodic banding of high spatial-frequency, the final response Mz was displayed as the average of Mz in 4 mm intervals, to be consistent with the spatial resolution of the ASL experiments. The effect of ECs, which should be zero at the isocenter and increase with the distance to the center, is more severe when B + 1 offset is present and mostly within the range of time constants of 10-3-10-1 s. Notably, a baseline artifact primarily associated with B + 1 offset is also observed, which is not dependent on either the spatial distance or the time constants of ECs.…”

Section: Discussionmentioning

confidence: 99%

Improved velocity‐selective‐inversion arterial spin labeling for cerebral blood flow mapping with 3D acquisition

et al. 2020

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Purpose To further optimize the velocity‐selective arterial spin labeling (VSASL) sequence utilizing a Fourier‐transform based velocity‐selective inversion (FT‐VSI) pulse train, and to evaluate its utility for 3D mapping of cerebral blood flow (CBF) with a gradient‐ and spin‐echo (GRASE) readout. Methods First, numerical simulations and phantom experiments were done to test the susceptibility to eddy currents and B1 field inhomogeneities for FT‐VSI pulse trains with block and composite refocusing pulses. Second, the choices of the post‐labeling delay (PLD) for FT‐VSI prepared 3D VSASL were evaluated for the sensitivity to perfusion signal. The study was conducted among a young‐age and a middle‐age group at 3T. Both signal‐to‐noise ratio (SNR) and CBF were quantitatively compared with pseudo‐continuous ASL (PCASL). The optimized 3D VSI‐ASL was also qualitatively compared with PCASL in a whole‐brain coverage among two healthy volunteers and a brain tumor patient. Results The simulations and phantom test showed that composite refocusing pulses are more robust to both eddy‐currents and B1 field inhomogeneities than block pulses. 3D VSASL images with FT‐VSI preparation were acquired over a range of PLDs and PLD = 1.2 s was selected for its higher perfusion signal. FT‐VSI labeling produced quantitative CBF maps with 27% higher SNR in gray matter compared to PCASL. 3D whole‐brain CBF mapping using VSI‐ASL were comparable to the corresponding PCASL results. Conclusion FT‐VSI with 3D‐GRASE readout was successfully implemented and showed higher sensitivity to perfusion signal than PCASL for both young and middle‐aged healthy volunteers.

“…A different four-cycling scheme has previously been demonstrated to be able to effectively remove the stripe artifact for VS-MRA. 12 This method added time-variant phase waveforms to the FT-VSS pulse trains of each of the four dynamics in order to generate excitation profiles spatially shifted by a quarter of the primary period of the stripes. Note that this technique could not remove the DC component or spatial stripes with more than two periods.…”

Section: Discussionmentioning

confidence: 99%

Ensuring both velocity and spatial responses robust to field inhomogeneities for velocity‐selective arterial spin labeling through dynamic phase‐cycling

et al. 2020

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To evaluate both velocity and spatial responses of velocity-selective arterial spin labeling (VS-ASL), using velocity-insensitive and velocity-compensated waveforms for control modules, as well as a novel dynamic phase-cycling approach, at different B 0 /B + 1 field inhomogeneities. Methods: In the presence of imperfect refocusing, the mechanism of phase-cycling the refocusing pulses through four dynamics was first theoretically analyzed with the conventional velocity-selective saturation (VSS) pulse train. Numerical simulations were then deployed to compare the performance of the Fourier-transform based velocityselective inversion (FT-VSI) with these three different schemes in terms of both velocity and spatial responses under various B 0 /B + 1 conditions. Phantom and human brain scans were performed to evaluate the three methods at B + 1 scales of 0.8, 1.0, and 1.2. Results: The simulations of FT-VSI showed that, under nonuniform B 0 /B + 1 conditions, the scheme with velocity-insensitive control was susceptible to DC bias of the static spins as systematic error, while the scheme with velocity-compensated control had deteriorated velocity-selective labeling profiles and, thus, reduced labeling efficiency. Through numerical simulation, phantom scans, and brain perfusion measurements, the dynamic phase-cycling method demonstrated considerable improvements over these issues. Conclusion: The proposed dynamic phase-cycling approach was demonstrated for the velocity-selective label and control modules with both velocity and spatial responses robust to a wide range of B 0 and B + 1 field inhomogeneities. K E Y W O R D S B + 1 field inhomogeneity, arterial spin labeling, B 0 field inhomogeneity, cerebral blood flow, velocity-selective inversion 2724 | LIU et aL.