Tijl van der Velden1, Mark Gosselink1, Ingmar Voogt2, Martijn Froeling1, Hans Hoogduin1, Dennis Klomp1, Bart Steensma1, and Alexander Raaijmakers1,3
1UMC Utrecht, Utrecht, Netherlands, 2Wavetronica, Utrecht, Netherlands, 3Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Netherlands
Synopsis
In this study we investigate the parallel imaging
performance of a 72 channel body array for 7 tesla. G-factor maps and T2w prostate
images have been acquired in a healthy volunteer. Accelerations of 3x3 and 4x1
are feasible without detrimental g-factor penalties.
Introduction
Ultrahigh field MRI can profit from high SNR levels that can
be interchanged with reduced scan time. However, this requires sufficient
acceleration potential of the receive array setup. In fact, the potential for
acceleration is larger anyway at 7T because the higher B1 frequency results in
lower noise correlations and more distinct sensitivity patterns [1]. This calls
for an exploration of acceleration performance with a massively parallel
receive array. Several studies have been presented on massively parallel
receive arrays and parallel imaging performance [2]. However, these studies
have focused predominantly at the brain. In this study, we explore parallel
imaging potential for body imaging at 7T using a 8 channel transceiver setup
combined with a 64 channel receiver array (figure 1) [3]. The setup consists of
8 fractionated dipole antennas and 64 loops. In this study we have investigated
the parallel imaging performance when using all 72 elements simultaneously for prostate
imaging at 7T.Methods
The coil setup was connected to a 7 tesla whole body MR
system (Philips, Best, NL; Software release 5.4), equipped with 8x2kW RF
amplifiers, and 72 digital receivers for 1H. All of the experiments
were performed in accordance with the local ethical committee, and written
informed consent was obtained from a 33 year healthy male volunteer.
To get an impression of the limits on the acceleration of
parallel imaging, a multi-slice 2D PDw image was acquired to calculate
1/g-factor maps. Values below 0.5 in these maps indicate too much noise amplification.
Based on the noise preparation data of the same scan, a noise correlation
matrix was calculated to determine the coupling between all elements; the 8
antennas as well as the 64-channel receiver array.
To visualize the prostate and its structures, three 2D T2w
TSE images were acquired, with SENSE accelerations of 1, 2 and 4 (RL direction).
In addition, 2 3D T1w images were acquired, with SENSE accelerations of 1x1 and
3x3 (RLxFH direction) to demonstrate imaging performance on a larger area of
the lower torso.Results
Figure 2 shows the noise correlation matrix, where a
correlation of typically 0.3 or lower is shown, indicating good decoupling
between elements. Although several elements show a correlation between 0.4 and
0.5, little effect on the total performance is expected, due to the high number
of receiver channels.
This is reflected by the 1/g-factor maps, shown in figure 3 in
transversal and coronal orientations. Accelerations up to a factor of 3 in any
direction is expected to be feasible with little noise amplification.
Figure 4 shows the T2w images with SENSE acceleration 1, 2
and 4, and a close-up of the prostate. From an acceleration of 4 small unfolding
artefacts can be observed, which was expected when analyzing the 1/g-factor
maps. On top on that, the reduced SNR as a result of SENSE acceleration results
in reduced contrast between tissue types.
Similar results can be seen in figure 5, which shows a video
of the 3D T1w acquisition with SENSE accelerations of 1x1 and 3x3. Again, in
accordance with g-factor maps, unfolding artefacts can be observed, predominately
in peripheral regions of the body.Conclusions
In this study we have explored the parallel imaging
performance for prostate imaging with a 72-channel setup for body imaging at
7T. Accelerations up to 9 (3D acquisitions) and 4 (2D acquisitions) are shown
to be feasible.Acknowledgements
No acknowledgement found.References
1. Wiesinger et al. MRM 2004,
https://doi.org/10.1002/mrm.20183
2. Auerbach et al. ISMRM 2017, #1218
3. Voogt et al. ISMRM 2016, #0173