There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
Background
Recently, 3D CINE phase-contrast MRI (4D Flow) has demonstrated potential for quantitative
measurement of blood flow of the cardiovascular systems in the entire thoracic trunk
[1]. However, the needs of 4D flow for high resolution, large volume coverage & respiratory
gating render very long scan time. This work intended to improve the data acquisition
efficiency of free-breathing cardiac 4D flow.
Methods
k-t Acceleration: kat ARC [2], a spatiotemporal-correlation-based autocalibrating
parallel imaging method, was used for accelerating 4D flow. The acquisition & reconstruction
were optimized for high acceleration. As shown in Figure 1, data was collected with
a variable density random (VDR) k-t sampling scheme [3] to improve overall reconstruction
accuracy and reduce coherent artifacts.
Figure 1
VDR kt sampling at t=8 (upper) & 9 (middle). Bottom: variable density NEX factor.
3 colored lines indicate 3 successive trajectories.
In reconstruction, a static tissue removal scheme [4] was adapted for 4D flow to reduce
residual aliasing artifacts. Specifically, from initial view-sharing reconstruction,
static tissues (e.g. chest wall, spine, etc) with no flow or motion were identified.
Next, static tissue signal was removed from the original data to reduce aliasing in
undersampled data. After kat ARC reconstruction of dynamic signal, the static tissue
signal was added back to generate the final image.
Variable density signal averaging: signal averaging of multiple excitations is a commonly
used strategy for free-breathing MRI, but it requires multiple-fold increase in scan
time. This work used a variable density number of excitations (NEX) scheme for improving
scan efficiency. Figure 1 demonstrates the acquisition scheme. The NEX factor is the
highest at center k-space and decreases toward outer k-space for an optimal compromise
between motion artifacts and scan time. Furthermore, a radial golden angle vieworder
was used to minimize the adverse effects of residual motion artifacts [5].
To evaluated the proposed method, 5 healthy adult volunteers were scanned on GE 3T
(MR750) using 32-channel cardiac coil without contrast. 4D flow MRI was performed
covering the entire chest. Imaging parameters were: 380x250 mm2 FOV, 2x2mm2 resolution,
72 slices, 2.5mm thickness, 60ms temporal resolution, 8× acceleration.
Results
As shown in Fig. 2, compared to acquisition without variable density NEX (b), the
proposed acquisition scheme (a) effectively reduced respiratory motion artifacts.
On all subjects, we were able to obtain 4D flow images with only minor residual motion
artifacts and perform offline visualization and measurement of blood flow in an arbitrary
reformatting. The average scan time was ~6.5 & ~5.5 min for scans with & without variable
density NEX, respectively.
Figure 2
Comparison of acquisition without (a) and with (b) variable density NEX. Magnitude
(c), LR (d), AP (ef) and SI (f) flow of descending aorta at a reformatted plane.
Conclusions
This work developed a variable density NEX scheme for cardiac 4D flow MRI with high
scan efficiency. Combined with kat ARC, we were able to perform whole-chest 4D flow
under 8min. Further clinical evaluation will be performed to evaluate the proposed
4D flow method.
Funding
N/A.
This article is published under license to BioMed Central Ltd. This is an Open Access
article distributed under the terms of the Creative Commons Attribution License (
http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided
the original work is properly cited. The Creative Commons Public Domain Dedication
waiver (
http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.