Abstract
Two-dimensional selective excitation (2DRF) allows shortening 4Dflow 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 4Dflow 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. Two different 2DRF pulses with spiral k-space trajectories were designed and integrated into a 4Dflow sequence. Flow phantom experiments and 7 healthy control 4Dflow 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. Applying UNFOLD to 4Dflow 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 4Dflow 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. Two-dimensional selective excitation combined with UNFOLD allows limiting the FOV to shorten 4Dflow 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.
Highlights
Resolved, complex, 3D velocity vector fields can be measured noninvasively by 4D flow imaging[1] to characterize hemodynamics, for example in intracranial aneurysms or feeding arteries of arteriovenous malformations.[2,3] In stenotic diseases, it has been shown that, in particular, the peak flow decreases significantly and may serve as a biomarker.[4,5] Still, long acquisition times of 10-20 minutes prevent the routine clinical application of 4D flow imaging,[1] even though multiple k-space lines per cardiac phase, nl ≥ 2, are typically acquired.[1]
We demonstrate that 2DRF 4D flow with nl ≥ 2 can be 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
We investigate the benefit of an UNFOLD2DRF combination for 4D flow imaging
Summary
Resolved, complex, 3D velocity vector fields can be measured noninvasively by 4D flow imaging[1] to characterize hemodynamics, for example in intracranial aneurysms or feeding arteries of arteriovenous malformations.[2,3] In stenotic diseases, it has been shown that, in particular, the peak flow decreases significantly and may serve as a biomarker.[4,5] Still, long acquisition times of 10-20 minutes prevent the routine clinical application of 4D flow imaging,[1] even though multiple k-space lines per cardiac phase, nl ≥ 2, are typically acquired.[1] 4D flow MRI has been accelerated by various spatial and temporal undersampling techniques.[6,7,8,9,10,11]. The temporal resolution is, critical for correct peak flow quantification.[16,17,18] the reduced FOV (rFOV) needs to be oversampled in both phase-encoding directions[19] (i.e., rFOV > FOX), to prevent static spatial aliasing,[19] which prolongs the scan time
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