To investigate whether high spatial resolution dual-venc 4D flow MRI acquired with k-t GRAPPA acceleration yields improved image quality and flow quantification for evaluating cerebral hemodynamics compared to a lower resolution scan with standard acceleration (GRAPPA with R=2 along the phase encoding direction).

Dual-venc 4D flow MRI was acquired at two different approximately isotropic spatial image resolutions, low resolution with (1.1mm)³ and high resolution with (0.8mm)³ voxel volumes, on a Siemens Magnetom 7T MRI scanner (SC72 gradients;16kW RF amplifier (Stolberg); 32RX/1TX head coil (Nova Medical)) in 6 healthy volunteers (mean age=29.3±13.5yrs, age range=21-56yrs, 1 female). The standard resolution scan included GRAPPA with acceleration R=2, while k-t GRAPPA with R=5 was used for the high resolution scan with the same volume coverage. Cardiac gating was performed with standard ECG or an acoustic cardiac triggering system (MRItools GmbH, Berlin). Sequence parameters are summarized in table 1. Both acquisitions covered the same 3D volume by adjusting the amount of slices to achieve the same slab thickness.

All data sets were corrected for eddy current induced phase offset errors and Maxwell terms (1). The unaliased high-venc data was used to correct the low-venc acquisition for velocity aliasing resulting in a combined dual-venc data set with intrinsically high dynamic velocity range as well as low velocity noise (2). Phase-contrast angiograms (PC-MRA) were calculated from the velocity and magnitude data and used to identify vessel boundaries and mask blood flow velocities (figure 1). Vessel sharpness was evaluated by determining the thickness at the location of the left or right middle cerebral arteries (lMCA, rMCA) in the axial PC-MRA maximum intensity projection (MIP) images. Further analysis included regional flow quantification at the left and right internal carotid arteries (lICA, rICA), lMCA, rMCA, left and right anterior cerebral arteries (lACA, rACA), basilar artery (BA), left and right posterior cerebral arteries (lPCA, rPCA) as well as the left and right transverse sinuses (lTS, rTS), the straight sinus (SS) and superior sagittal sinus (SagSin) (figure 2) to quantify net flow and peak velocity in commercial software (EnSight, CEI).

All dual-venc 4D flow MRI data were successfully acquired in all subjects with an average scan time of 21.4min (±2.9min) for the low resolution scan. Scan time for the high resolution and k-t GRAPPA accelerated scan was 16.4min (±2.0min). Assessment of vessel thickness in the PC-MRA images yielded smaller vessel diameters of the MCA when measured with high spatial resolution than at low resolution (3.97±0.8mm vs 4.22±0.83mm, Wilcoxon signed rank test, p = 0.03). The advantage of the high resolution scan can be clearly appreciated in figures 1 and 2A and B for the depiction of small intracranial vessels. Figure 1 gives an example of a velocity MIP masked with the PC-MRA for both acquisitions. In figure 2 an axial PC-MRA MIP for the two image resolutions is shown in panels A and B with the corresponding visualization of blood flow using streamlines in panels C and D. Spearman rank correlation analysis showed that peak velocities correlated well between low and high resolution scan (peak velocitylow_res = 0.61±0.35m/s vs 0.57±0.34m/s, R=0.81, P<0.001, figure 3). Net flow was also significantly correlated but was systematically overestimated by the low resolution scan (net flow_{low_res}=2.94±1.74m/cycle vs net flow_{high_res}=2.69±1.48m/cycle, Wilkoxon rank test: p=0.01, spearman correlation R=0.92, P<0.0001, figure 4).The findings of this feasibility study show that high spatially resolved k-t GRAPPA accelerated dual-venc 4D flow MRI provides the opportunity to resolve flow even in smaller vessels due to its intrinsically decreased noise. k-t acceleration at 7T resulted in both reduced scan time and improved image quality and the ability to quantify venous and arterial flow across a wide range of the velocity spectrum. The low resolution data systematically overestimated net flow, which is expected due to partial volume effects.In conclusion, dual-venc 4D flow MRI acquisition applied at 7T allows for high spatial resolution due to reduced velocity noise (high magnetic field plus lower venc value) while covering a large dynamic range. This is expected to be highly beneficial for quantification of hemodynamics in arteriovenous malformation or aneurysms with typically large velocity ranges.