Towards high‐resolution 4D flow MRI in the human aorta using kt‐GRAPPA and B1+ shimming at 7T

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Purpose: To assess the performance of highly accelerated free-breathing aortic four-dimensional (4D) flow MRI acquired in under 2 minutes compared to conventional respiratory gated 4D flow. Methods: Eight k-t accelerated nongated 4D flow MRI (parallel MRI with extended and averaged generalized autocalibrating partially parallel acquisition kernels [PEAK GRAPPA], R ¼ 5, TRes ¼ 67.2 ms) using four k y -k z Cartesian sampling patterns (linear, center-out, out-center-out, random) and two spatial resolutions (SRes1 ¼ 3.5 Â 2.3 Â 2.6 mm 3 , SRes2 ¼ 4.5 Â 2.3 Â 2.6 mm 3 ) were compared in vitro (aortic coarctation flow phantom) and in 10 healthy volunteers, to conventional 4D flow (16 mm-navigator acceptance window; R ¼ 2; TRes ¼ 39.2 ms; SRes ¼ 3.2 Â 2.3 Â 2.4 mm 3 ). The best k-t accelerated approach was further assessed in 10 patients with aortic disease. Results: The k-t accelerated in vitro aortic peak flow (Qmax), net flow (Qnet), and peak velocity (Vmax) were lower than conventional 4D flow indices by 4.7%, 11%, and 22%, respectively. In vivo k-t accelerated acquisitions were significantly shorter but showed a trend to lower image quality compared to conventional 4D flow. Hemodynamic indices for linear and out-center-out k-space samplings were in agreement with conventional 4D flow (Qmax 13%, Qnet 13%, Vmax 17%, P > 0.05). Conclusion: Aortic 4D flow MRI in under 2 minutes is feasible with moderate underestimation of flow indices. Differences in k-space sampling patterns suggest an opportunity to mitigate image artifacts by an optimal trade-off between scan time, acceleration, and k-space sampling. Magn Reson Med 79:195-207, 2018. V C 2018 International Society for Magnetic Resonance in Medicine.

Section: Discussionmentioning

confidence: 99%

k‐t accelerated aortic 4D flow MRI in under two minutes: Feasibility and impact of resolution, k‐space sampling patterns, and respiratory navigator gating on hemodynamic measurements

Bollache

Barker

Dolan

et al. 2017

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“…4D flow MRI allows measuring temporally and 3D spatially resolved three‐directional velocity vector fields in vivo based on gradient echo phase‐contrast imaging. Consequently, it has attracted increasing interest in recent years, especially in cardiovascular research and in clinical care . Yet, application in clinical routine is still hindered by long acquisition times of 10–20 min .…”

Section: Introductionmentioning

confidence: 99%

“…Yet, application in clinical routine is still hindered by long acquisition times of 10–20 min . To address this issue, substantial effort has been devoted to accelerate 4D flow MRI by a variety of different spatial and temporal undersampling and reconstruction techniques, such as kt‐GRAPPA, compressed sensing, principal component analysis, non‐Cartesian sampling, and UNFOLD…”

Section: Introductionmentioning

confidence: 99%

4D flow imaging with 2D‐selective excitation

Wink

Ferrazzi

Bassenge

et al. 2019

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Purpose: 4D flow MRI permits to quantify non-invasively time-dependent velocity vector fields, but it demands long acquisition times. 2D-selective excitation allows to accelerate the acquisition by reducing the FOV in both phase encoding directions. In this study, we investigate 2D-selective excitation with reduced FOV imaging for fast 4D flow imaging while obtaining correct velocity quantification. Methods: Two different 2D-selective excitation pulses were designed using spiral kspace trajectories. Further, their isophase time point was analyzed using simulations that considered both stationary and moving spins. On this basis, the 2D-selective RF pulses were implemented into a 4D flow sequence. A flow phantom study and seven 4D flow in vivo measurements were performed to assess the accuracy of velocity quantification by comparing the proposed technique to non-selective and conventional 1D slab-selective excitation. Results: The isophase time point for spiral 2D-selective RF pulses was found to be located at the end of excitation for both stationary and moving spins. Based on that, 2D-selective excitation with reduced FOV allowed us to successfully quantify velocities both in a flow phantom and in vivo. In a flow phantom, the velocity difference Δv = (0.8 ± 5.3) cm/s between the smaller reduced FOV and the reference scan was similar to the inter-scan variability of Δv = (−1.0 ± 2.3) cm/s. In vivo, the differences in flow (P = 0.995) and flow volume (P = 0.469) between the larger reduced FOV and the reference scan were non-significant. By reducing the FOV by twothirds, acquisition time was halved. Conclusion: A reduced field-of-excitation allows to limit the FOV and therefore shorten 4D flow acquisition times while preserving successful velocity quantification. K E Y W O R D S 4D flow, fast imaging, reduced FOV, reduced FOX, spatially selective 2D RF excitation | 887 WINK et al.

“…1 Therefore, 4D flow MRI has been accelerated by various spatial and temporal undersampling techniques. [6][7][8][9][10][11] Another promising acceleration approach restricts the field of excitation (FOX) in 2D by using 2D spatially selective excitation (2DRF), which allows reducing the FOV in 2D to the target region only. [12][13][14] Spatial selectivity is achieved by applying gradients along both selective directions during RF application using, for example, a spiral excitation-k-space trajectory.…”

Section: Introductionmentioning

confidence: 99%

4D flow imaging with UNFOLD in a reduced FOV

Wink

Bassenge

Ferrazzi

et al. 2019

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Purpose Two‐dimensional selective excitation (2DRF) allows shortening 4D flow scan times by reducing the FOV, but the longer 2DRF pulse duration decreases the temporal resolution, yielding underestimated peak flow values. Multiple k‐space lines per cardiac phase, nl ≥ 2, are commonly applied in 4D flow MRI to shorten the inherent long scan times. We demonstrate that 2DRF 4D flow with nl ≥ 2 can be easily combined with UNFOLD (UNaliasing by Fourier‐encoding the Overlaps using the temporaL Dimension), a technique that allows regaining nominally the temporal resolution of the respective acquisition with nl = 1, to assure peak flow quantification. Methods Two different 2DRF pulses with spiral k‐space trajectories were designed and integrated into a 4D flow sequence. Flow phantom experiments and 7 healthy control 4D flow in vivo measurements, with and without UNFOLD reconstructions, were compared with conventional reconstruction and 1D slab‐selective excitation (1DRF) by evaluating time‐resolved flow curves, peak flow, peak velocity, blood flow volume per cardiac cycle, and spatial aliasing. Results Applying UNFOLD to 4D flow imaging with 2DRF and reduced FOV increased the quantified in vivo peak flow values significantly by 3.7% ± 2.3% to 5.2% ± 2.4% (P < .05). Accordingly, the peak flow underestimation of 2DRF scans compared with conventional 1DRF scans decreased with UNFOLD. Finally, 2DRF combined with UNFOLD accelerated the 4D flow acquisition 3.5 ± 1.4 fold by reducing the FOV and increasing the effective temporal resolution by 6.7% compared with conventional 1D selective excitation, with 2 k‐space lines per cardiac phase. Conclusion Two‐dimensional selective excitation combined with UNFOLD allows limiting the FOV to shorten 4D flow scan times and compensates for the loss in temporal resolution with 2DRF (Δt = 64.8 ms) compared with 1DRF (Δt = 43.2 ms), yielding an effective resolution of Δteff = 40.5 ms to enhance peak flow quantification.