Jason Stockmann1,2, Nicolas S Arango3, Benedikt Poser4, Thomas Witzel1,2, Jacob White3, Lawrence L Wald1,2,5, and Jonathan R Polimeni1,2,5
1Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 2Harvard Medical School, Boston, MA, United States, 3Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 4Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands, 5Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States
Synopsis
Multi-coil ΔB0 shim arrays have recently
been used for zoomed imaging of target ROIs in mice, providing improved
acquisition efficiency without the high SAR, long RF pulses, and other
limitations of conventional selective excitation methods that rely solely on
linear gradients. We extend this work to
zoomed 3D EPI in humans using an integrated ΔB0/Rx array coil to
dynamically switch-on a static, spatially-tailored nonlinear ΔB0 pattern
during RF excitation. Proof-of-concept selective
excitations of the occipital visual cortex and the peripheral cerebrum are shown,
with strong correspondence to the target patterns. Four-fold in-plane undersampled EPI of
occipital visual cortex demonstrates the method’s potential for efficient
high-resolution neuroimaging.
Introduction
High-resolution functional and anatomical imaging provides improved
depiction of small brain structures. However, full encoding results in prolonged
acquisition times, introducing distortion and blurring artifacts in functional
imaging and motion vulnerability in anatomical imaging1. Methods such as Inner Volume Imaging and
Outer Volume Suppression have been proposed to overcome this limitation by only
exciting the target ROI2-6. But these methods typically require longer RF
pulse durations and/or higher SAR. Pulse
durations can be reduced using parallel transmit excitation7-8, but this
involves complex pulse optimization using subject-specific B1+
field distributions that include SAR and B0-robustness constraints.
An entirely different approach is to use
arrays of independently-driven multi-coil (MC) ΔB0 shim loops,
as recently demonstrated for selective excitation in mouse brain9. The MC ΔB0 array generates a nonlinear ΔB0 pattern
in the ROI during RF excitation, such that a given bandwidth of the RF pulse is
on-resonance for regions inside the target ROI, but not outside.
Here, we extend this approach to zoomed 3D
EPI acquisitions in humans using an integrated ΔB0/Rx array coil originally
developed for high spatial-order B0 shimming, in which the B0
shim and receive functions use the same set of wire loops on a close-fitting
helmet10. The array is well-suited
for selective excitation because (a) it has many degrees of freedom for field
control, (b) its coil currents can be switched quickly (i.e., energized
differently during the excitation and readout phase), and (c) it provides high receive
sensitivity and parallel imaging capability required for high-resolution imaging. For proof-of-concept, we excite two target
ROIs in a 3D EPI acquisition: occipital visual cortex and the outer peripheral
cerebrum. The choice of a peripheral cerebrum ROI is
motivated by recent findings that sparse images can be highly-undersampled with
reduced g-factor penalties11.
Methods
Two
volunteers were scanned on a Siemens Skyra 3T scanner using both conventional
slab excitation and MC-enabled selective excitation followed by 3D multi-shot
EPI (sixty 1.5mm partitions). Figure 1 shows the integrated ΔB0/Rx coil,
with 32 RF receive channels and 31 ΔB0 shim channels sharing the same
set of loops. Second-order scanner shims were applied and baseline ΔB0 field
maps were acquired. A min-max algorithm was
used to compute MC ΔB0 patterns based on an objective function enforcing
frequency separation between the target and excluded regions, subject to
constraints on the maximum currents (3.5A/coil or 40A/total). The RF pulse
bandwidth and center frequency were set to overlap with the B0
pattern in the target ROI. The shim hardware
was controlled from the 3D EPI pulse sequence (Figure 2) using 10μs TTL pulses applied 500μs
before each windowed-sinc RF excitation pulse, at the end of the RF pulse (during
which the negative current was applied to “re-wind” the spin phase, and one
half the pulse duration after the end (to end the re-wind); the MC shim coils
were turned off during the EPI readout.Results
Figure 3 shows the occipital visual cortex anatomic mask overlaid
on a reference image and the ΔB0 calculated by the optimizer to match
this ROI. While the agreement is not
perfect, the optimizer achieves spectral separation of the target and excluded
regions to prevent unwanted signal from aliasing into the target ROI. Figure 4 shows the corresponding raw images over a 9cm slab
acquired for both slab selection and MC-enabled zoom with R={1,2,4}
undersampling along the primary (in-plane) phase encode direction. Even without performing parallel image
reconstruction, very little aliasing is observed, even in images with a 5-fold
narrower display window. Figure
5 shows selective excitation of the peripheral cerebrum. The excited region follows the contours of the cerebral
surface with high fidelity over most of the slab, with some artifacts in the most superior and
inferior partitions. These artifacts may
be addressed using more sophisticated phase correction navigators12.Discussion
In the peripheral cerebrum acquisition, the most inferior and superior partitions show some artifacts and deviations
from the target shape, illustrating the limitations of this particular 31ch ΔB0 shim
coil geometry. Performance is especially
degraded in the deep brain/brainstem where the coil loops do not fully encircle
the head (see Fig. 1),
a limitation that could be overcome in future coil designs. In future work, standard
windowed-sinc RF pulses will be replaced by pulses with sharper transition
bands, providing clearer demarcation of the excited ROI.
We show MC-enabled zoomed 3D EPI using an
integrated ΔB0/Rx array that provides
both ΔB0 field control for selective excitation as well as high RF sensitivity
for high-resolution imaging. We anticipate
that the technique will be especially useful at ultra-high field (7T), where the improved SNR and contrast-to-noise ratio enable
very high-resolution zoomed imaging of targeted structures.
Acknowledgements
We
thank Ned Ohringer for assistance acquiring data on human volunteers. This work was supported in part by the NIH NIBIB
(grants K99-EB021349, P41-EB015896, R01-EB019437), by the BRAIN Initiative (NIH
NIMH grant R01-MH111419), and by the MGH/HST Athinoula A. Martinos Center for
Biomedical Imaging; and was made possible by the resources provided by NIH
Shared Instrumentation Grants S10-OD010364, S10-RR023401, S10-RR023043,
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