Seong Dae Yun1, Patricia Pais-Roldán1, and N. Jon Shah1,2,3,4
1Institute of Neuroscience and Medicine 4, INM-4, Forschungszentrum Juelich, Juelich, Germany, 2Institute of Neuroscience and Medicine 11, INM-11, Forschungszentrum Juelich, Juelich, Germany, 3JARA - BRAIN - Translational Medicine, Aachen, Germany, 4Department of Neurology, RWTH Aachen University, Aachen, Germany
Synopsis
The
steady development of fMRI techniques has enabled the use of
submillimetre-resolution in recent fMRI studies. The use of
submillimetre-resolution allows the detection of cortical, depth-dependent
brain activation. Although there have been numerous attempts to perform
submillimetre-resolution fMRI, the level of spatial resolution in several
recent works is still around 0.7 mm. Moreover, most methods do not provide
whole-brain coverage. Therefore, this work aims to develop a novel
half-millimetre resolution fMRI technique capable of providing whole-brain
coverage. Here, the method was employed for exemplary finger-tapping fMRI at 7T
and the identification of cortical-depth dependent brain activation was
demonstrated.
Introduction
FMRI
techniques have been steadily developing to a point where activated brain
regions can now be revealed with precise spatial localisation. As a result,
submillimetre-resolution is now possible in recent fMRI applications enabling
the detection of cortical, depth-dependent brain activation. Although there
have been numerous attempts to perform submillimetre-resolution fMRI, the level
of spatial resolution in several recent works is still around 0.7 mm.1-6
Moreover, most methods are not dedicated to whole-brain fMRI applications.
Therefore, this work aims to provide a novel fMRI technique capable of providing
a half-millimetre resolution with whole-brain coverage. The proposed method was
developed based on two key elements: a TR-external navigator echo scheme7-9
and EPI with keyhole (EPIK).10-18 Here, finger-tapping fMRI was
performed at 7T to carefully examine cortical-depth dependent brain activation
in the motor cortex.Methods
FMRI applications require a particular
TE to derive strong blood-oxygenation-level-dependent (BOLD) contrast at each
field strength. However, this TE constraint often restricts the possibility of
increasing the imaging matrix size in EPI, as its echo train length is
relatively large. Moreover, in EPI, the navigator echoes, which are often
adopted as an internal part of the sequence for the elimination of the N/2
ghost artefacts,19 can also contribute significantly to increasing
the TE when the matrix size is relatively large.8,9 A TR-external
navigator echo scheme can effectively remove the contribution of navigator
echoes on the TE increase by allocating them in a separated low-flip angle
excitation (αPC) loop (see Fig. 1a). This scheme was employed here
to exploit the advantage it gives in terms of increasing the matrix size for
the given TE.
Another
key element employed in this work is EPIK, which accelerates the original
single-shot EPI, as depicted in Fig. 1b, with a similar acquisition strategy to
three-shot EPI. However, the segmented, interleaved sampling is only applied to
peripheral k-space and full Nyquist sampling is applied for the central
k-space, referred to here as “keyhole”. This ensures an optimum SNR and CNR for
each temporal frame. Here, the missing k-space lines in the peripheral k-space
were reconstructed with sliding-window reconstruction and 48 phase encoding
lines were configured as the keyhole region here. In this way, EPIK provides a
higher temporal resolution and less geometric distortions while maintaining
comparable performance in detecting BOLD signals.10-18
The EPIK readout was combined with the
TR-external scheme (see Fig. 1a) and the method was employed in block-based
finger-tapping fMRI. A healthy volunteer, screened with a standard safety form
with an informed consent, was recruited for the measurement on a Siemens Magnetom Terra 7T
scanner with following imaging parameters: TR/TE = 3500/22 ms, FOV = 210 × 210
mm2, matrix = 408 × 408 × 105 slices (0.51 × 0.51 × 1.0 mm3),
partial Fourier = 5/8, 3-fold in-plane/3-fold inter-plane (multi-band)
acceleration and αPC/αMain = 9°/90°.Results
Figure
2 shows three representative slices obtained from the proposed method. The
images are well reconstructed without any severe degradations and a detailed
spatial representation of microstructures (e.g. cortical ribbon) can be
observed. Figure 3 shows identified activated voxels at three representative
slice locations, obtained with a statistical threshold of a false-discovery-rate
(FDR) corrected p-value < 0.0001. The activated voxels are overlaid directly
on the reconstructed slices, showing that the identified functional voxels are
well localised along the cortical ribbon. Here, particularly for a selected ROI
(marked with a rectangle in Fig. 3), a further analysis was carried out to
inspect cortical-depth dependent brain activation. As shown in Fig. 4a, line
profiles of the t-value were examined with respect to the cortical depth. Here,
in total, 20 profiles were examined for the selected grey matter region and
their mean ± std plot was computed (see Fig. 4b). The mean plot shows that the
t-value was biggest at the pial surface, which was mainly due to the effect of
the large-vessel BOLD signals in the gradient-echo EPI sequences.20
However, it was also observed that the t-value, which decreases from the pial
surface, starts to increase at a deeper layer location (see ‘P1’)
and after reaching its peak point (see ‘P2’), the signal decreases
until the WM region. This observation is in line with the results presented in
previous cortical-depth dependent fMRI studies.4 Furthermore, for
the suppression of the large-vessel BOLD signals, a phase-based correction
method20,21 was applied to the acquired data. Figure 4d depicts the
corresponding mean ± std plot, showing that the effect of large-vessel BOLD
signals at the pial surface was reduced and the behaviour of signal increase at
the deeper layer location became more distinct.Discussion and conclusions
This work demonstrates the
identification of cortical-depth
dependent functional activation using the
proposed, half-millimetre, whole-brain imaging technique at 7T. The achieved
spatial resolution and brain coverage is substantially larger than the levels
presented in other recent high-resolution fMRI studies (see Fig. 5). Figure 5
reveals that the high spatial resolution in the present work was achieved with
a relatively large matrix size and a sufficiently large FOV, which is not the
case in the previous methods. In particular, the large brain coverage achieved
by the proposed method suggests its applicability for more general functional
studies such as resting-state fMRI.Acknowledgements
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