Jing Liu1, Yan Wang1, Farshid Faraji1, Sarah Kefayati1, Henrik Haraldsson1, and David Saloner1,2
1University of California San Francisco, San Francisco, CA, United States, 2VA Medical Center, San Francisco, CA, United States
Synopsis
A highly accelerated 4D flow
MRI method with a high tempospatial resolution has been validated in healthy volunteers
by comparing to the conventional method. The proposed method been demonstrated to be very
promising for imaging the patients with intracranial aneurysms, by achieving an isotropic resolution of 1.3mm and a temporal resolution of
26ms within a 5-minute scan time (R=12). INTRODUCTION
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.
MATERIALS
AND METHODS
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=15o, 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=6o, 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.
RESULTS AND DISCUSSION
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.
CONCLUSIONS
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.
Acknowledgements
NIH K25 EB014914 (JL), NIH R01 NS059944 (DS), a VA
MERIT Review grant (DS)References
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