Vincent Gras1, Alexandre Vignaud1, Cécile Rabrait-Lerman1, Denis Le Bihan1, Tony Stoecker2, Ruediger Stirnberg2, and Nicolas Boulant1
1NeuroSpin, CEA, Saclay, France, 2DZNE, Bonn, Germany
Synopsis
The volumetric 3D-EPI
sequence has already shown great promise for fMRI studies due to potentially increased
temporal SNR (tSNR) and lower energy demands compared to 2D multi-slice
acquisition schemes. For whole-brain studies the tSNR yet can suffer from
sub-optimal flip angles due to RF inhomogeneity. Parallel transmission (pTx)
has been shown to be a very powerful technology to mitigate these effects but
has been barely used in routine due to a cumbersome calibration procedure.
Here, we use Universal Pulses to skip entirely the latter step and increase
locally the tSNR at 7T in the 3D-EPI GE sequence.
Introduction
The volumetric 3D-EPI sequence is particularly
attractive for whole-brain fMRI studies due to increased SNR compared to
multi-slice 2D acquisition schemes1. Temporal SNR (tSNR) as an
indicator of sensitivity for detection of neuronal activation yet can suffer locally
from sub-optimal flip angle excitations2. Here we demonstrate the
application of pTx universal pulses3,4,5 to restore the tSNR in B1+-deprived
regions at 7T, without the usual and cumbersome parallel transmission (pTx)
calibration.Methods
Measurements were performed at 7T on a Magnetom
scanner (Siemens Healthcare, Erlangen, Germany) and with the 8Tx-8Rx Rapid-
Biomed head coil (Rapid biomedical, Rimpar, Germany). The sequence used was a
custom 3D-EPI6 sequence with sagittal slice orientation, TE=21 ms, 1.5×1.5×1.5
mm3 resolution, matrix 112×140×140, iPAT acceleration factor of 2×2 along
the phase and partition encoding directions respectively, PF=6/8 along the PE
direction, leading to a volume TR of 2.4s with whole-brain coverage. The
universal, non-selective, pulse was designed on a previously acquired database
of 10 B1 and ΔB0 maps4 with a kT-point
pulse parametrization7 and with target flip angle (FA) of 12° corresponding
to the Ernst angle for gray matter. The pulse design algorithm consisted of
minimizing the FA Normalized Root Mean Square Error (NRMSE) averaged over the
database subjects, under explicit hardware and SAR constraints, and with
simultaneous optimization of the k-space trajectory8. The total
duration of the kT-points UP was varied in a prior simulation study
over the database subjects to achieve water selective excitation and potentially
provide fat artefact-free images9. Two series of 100 volumes were
acquired with the 3D-EPI sequence on four different healthy volunteers by
blindly applying the pre-computed kT-points UP and a single CP-mode 2ms
square pulse, respectively. tSNR was computed for each voxel in the
brain by calculating the mean magnitude signal of the time-series divided by
its standard deviation, after correcting for a linear drift and after volume
realignment using FSL10. Finally, B1 and ΔB0 maps were acquired for each subject for retrospective control of the universal
pulse performance with Bloch simulations.Results
Figure 1 illustrates
the gain of increasing the pulse duration from 2 ms (9 kT-points ~ 0.2 ms/sub-pulse) to 5 ms (5 kT-points
~ 1.0 ms/sub-pulse) to remove
the chemical shift artefact. This was confirmed by our simulations which
indicated that for a 5 ms pulse duration, the mean FA over the database
subjects for fat was 8 times smaller than for water, while they were comparable
for a 2 ms pulse. Figure 2 reports the computed FA maps for the CP and UP
excitations, based on the measured subject-specific field maps.
Retrospectively, it could be thereby determined that the UP achieved 13.6% NRMSE
compared to 28.6% in CP mode. Due to B0 inhomogeneity however, the 5
ms pulse duration for water selective excitation came at the cost of slightly poorer
FA uniformity compared to a 2 ms kT-point pulse (13.6% versus 10.5%).
The shorter solution on the other hand was not retained for the experiments
because of the more pronounced chemical-shift artefact. Figure 3 shows for one
subject two series of 25 sagittal slices obtained with the UP and CP modes
respectively, clearly illustrating the signal gain obtained in the cerebellum
and the temporal lobes with the UP. Figure 4 shows for the same volunteer the
resulting measured versus predicted tSNR gain (in the thermal noise dominated regime), the latter being computed by
taking the FA maps and GRE signal equation with uniform T1. A tSNR
improvement of 20 % on average over the whole brain and up to a factor of 4
locally when using UPs could be observed experimentally. Similar results were
obtained for the three other volunteers.Discussion and conclusion
A gain by up to a
factor of 4 in tSNR in the cerebellum and temporal lobes with the 3D-EPI
sequence was obtained through the use of pre-computed pTx universal pulses. The
pulses were designed prior to the acquisitions to be robust against
intersubject field map variability and thus could be blindly applied potentially
on any subject, i.e. without any subject-specific pTx calibration. To minimize
chemical-shift artefacts, a recently proposed water excitation technique9 based on a single square pulse was successfully integrated into the pTx UP.
Apart from the cerebellum and temporal lobes, the tSNR gain compared to the CP
mode was found to be insignificant and can be explained by the flattening of
the signal around the Ernst angle for small interpulse repetition times. More
widespread gains of tSNR are expected for acquisitions with higher FA and
larger repetition times such as multi-slice GRE-EPI sequences. Acknowledgements
The research leading
to these results has received funding from the European Research Council under
the European Union’s Seventh Framework Program, Proof of Concept Grant
Agreement n. 700812.References
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Joint design of kT-points trajectories and RF pulses under explicit SAR and power constraints in the large flip angle regime
Joint design of kT-points trajectories and RF pulses under explicit SAR and power constraints in the large flip angle regime