High tempospatial 4D flow MR imaging is desirable but the requirement of excessively long scan time limits its practical use in patients. We have developed a highly accelerated 4D flow MRI sequence based on novel variable density pseudo-random undersampling scheme and advanced image reconstruction method combing k-t, compressed sensing and parallel imaging techniques. In this study, we first show comparable qualitative and quantitative image quality comparisons between the 4D flow images acquired with the proposed method and those with a conventional acceleration method, and then demonstrate the feasibility of applying the accelerated 4D flow MRI in patients with intracranial aneurysms.

We have applied the developed 4D flow MRI to 5 healthy subjects (1 female, 31.6±3.6 years) and 7 patients with intracranial aneurysms (5 ICA, 1 MCA, 1 vertebral). Data was acquired on a 3.0T Siemens Skyra scanner with a 20-ch head coil. For volunteers, images in the axial plan were acquired to cover the Circle of Willis, with scan settings: VENC=100cm/s, FOV=18x18cm2, slice thickness=1.4 mm, matrix=128x128x24, FA=15^o, and TR/TE=6.0/3.5ms. Our proposed variable-density pseudo-random undersampling scheme, CIRcular UnderSampling (CIRCUS) [1] and advanced image reconstruction method combing k-t, compressed sensing and parallel imaging techniques [2, 3] were applied with an acceleration factor of R=4 with a temporal resolution of 72 ms. Scan time was 4.1±1.0 mins. With CIRCUS acquisition, a flexible temporal resolution could be retrospectively selected. We reconstructed images of a higher temporal of 24 ms (corresponding to R=12) with the same data as CIRUCS4. Clinical 4D flow protocol iPAT2 (R=2, effective R=1.6) was also applied as the reference for comparison, with the same imaging settings as CIRUCS but a longer scan time of 9.9±1.9 mins. We compared qualitative image quality (artifact level, vessel depiction, flow patterns, and so on) as well as quantitative measurements (per-pixel flow velocities and flow-waveforms within selected vessels) between the reference images and the ones acquired with CIRCUS. The qualitative evaluation uses a 5-point scale: 4 = excellent, 3 = slightly limited but good, 2 = suboptimal, 1 = minimally perceived, and 0 = not perceived. We report the bias and confidence intervals for quantitative comparisons. For patients, we acquired images in the oblique planes covering the vessels of interest (with aneurysms). The scan settings are VENC=100cm/s, FOV=24x18cm2, slice thickness=1.3 mm, matrix=192x144x26, FA=6^o, TR/TE=6.4/3.7ms, and scan time of 4.7±0.7 mins. Images with a high temporal resolution of 26 ms (R=12) were reconstructed.

Images have been successfully acquired from all subjects. Table 1 demonstrates that the highly accelerated 4D flow method with CIRCUS acquisition could provide comparable image quality and velocity measurements compared to those acquired with the conventional iPAT method. CIRCUS allowed a high temporal resolution of 24ms and even achieved the highest overall image quality score (3.3 in Table 1) with a high acceleration factor of R=12. This encourages us to apply the proposed highly accelerated 4D flow MRI in patients, to achieve a high tempospatial resolution within a relatively short scan time. Seven patients with aneurysms have been imaged, achieving an isotropic spatial resolution of 1.3mm3 and a temporal resolution of 26ms, within 5 minutes scan time. Figures 1-3 show the images acquired from 3 representative patients with ICA and MCA aneurysms. The magnitude images (including the reference and three velocity encodings), phase-contrast MR angiography (PC-MRA) maximum intensity projection (MIP) images and velocity maps in three directions, obtained with CIRCUS (R=12), show reasonable image quality. The streamline images from these 3 patients are shown in Figure 4, where the flow patterns were nicely captured.

A highly accelerated 4D flow MRI method with a high tempospatial resolution has been validated in healthy volunteers by comparing to the conventional method, and been demonstrated to be very promising for imaging the patients with intracranial aneurysms.