Manushka V. Vaidya1,2,3, Mariana Lazar2, Cem M. Deniz1,2, Gillian G. Haemer1,2,3, Gang Chen1,2,3, Mary Bruno2, Daniel K. Sodickson1,2,3, Riccardo Lattanzi1,2,3, and Christopher M. Collins1,2,3
1Center for Advanced Imaging Innovation and Research (CAI2R), Department of Radiology, New York University School of Medicine, New York, NY, United States, 2Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States, 3Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States
Synopsis
To
improve cerebellar fMRI using a commercial head coil at 7T, we arranged High Permittivity
Material (HPM) pads against the inferior, posterior region of the head. We
first simulated the change in Specific energy Absorption Rate (SAR) with HPM
and then examined the effects on experimental Signal to Noise Ratio (SNR), flip
angle maps, and measured fMRI activation throughout the brain (including the
cerebellum) during a finger-tapping task in eight subjects. Both SNR and fMRI BOLD
contrast were improved in the cerebellum with no potential safety detriments.
PURPOSE:
Functional
magnetic resonance imaging (fMRI) has facilitated non-invasive in vivo studies
of brain function and connectivity. There is growing interest in studying the
functional role of cerebro-cerebellar circuits (1-4), which requires robust
sensitivity to blood oxygenation level dependent (BOLD) contrast throughout the
whole brain. Ultra-high field MRI (>4 Tesla) is promising for fMRI, because of
improved BOLD contrast (5). However, whole brain fMRI remains challenging, because commonly used
head coils at 7 Tesla (T) have poor coverage in inferior regions of the brain
and particularly in the cerebellum. Previous work has shown that appropriately
placed HPM pads can extend the coil sensitivity into inferior regions of the
brain and neck (6). In this work, we tested whether
this extension in coil sensitivity can translate to an improvement in the
detection of functional activation in the cerebellum for a finger tapping motor
task in eight subjects.METHODS:
Simulations: Numerical simulations were performed
to assess the Specific energy Absorption Rate (SAR) with and without HPM pads
(εr=110 and σ=0.08 S/m) (Fig 1). A head-sized quadrature-driven
birdcage coil, similar to the commercial transmit head coil used in the
experiments (Fig. 1), was modeled using CST Microwave Studio 2015 (Computer
Simulation Technology, Darmstadt, Germany), and loaded with a human model
(“Duke,” Virtual Population, IT’IS Foundation, Zurich, Switzerland) (7) with a voxel resolution of 5x5x5mm3.
Each port was tuned to 297.2 MHz, and matched to 50 Ohms in the absence of HPM.
Simulations were performed with an accuracy of -30 dB to
ensure convergence, and approximately 14 million mesh cells were used. The simulated 10 g local maximum SAR
values were then scaled (8) to estimate SAR for the gradient-echo
EPI sequence used in experiments. Experiments:
Two identical HPM pads (12x14x2cm3) constructed from calcium
titanate and deuterium oxide (9) were positioned inside a commercial
head coil (Nova Medical 1Transmit/24Receive, Wilmington, MA) (Fig. 1). All
studies involving human subjects were performed in accordance with our institution’s
IRB. Eight healthy volunteers were enrolled in a prospective motor fMRI study,
performed both with and without HPM. Volunteers were scanned on a whole-body 7 T
scanner (Magnetom, Siemens Healthineers, Erlangen, Germany) using the
commercial head coil. For each volunteer, the transmit reference voltage was
set, with no HPM in the coil, to achieve a 90o flip angle at the
center of the brain, using a turbo-FLASH based technique (10), and kept
constant in the presence of HPM pads. To separate the receive-only coil
contribution to SNR, SNR maps (11) were normalized
by the sine of the flip angle maps (10), which were
separately obtained. Functional activation maps were obtained using a gradient
echo EPI sequence (101 measurements, slices=55, voxel size=2x2x2.5mm3,
TR=3s, TE=23ms, bandwidth = 1930 Hz/pixel, flip angle = 15deg) for a right hand
finger tapping task. fMRI data was analyzed using FEAT FSL (fMRI Expert
Analysis Tool, version 6.0, FMRIB’s software Library, version 5.0.10) for each
subject, followed by a group level analysis using paired t-test of acquisitions
with and without HPM.RESULTS AND DISCUSSION:
SAR
values (Table) were well below the corresponding limits recommended by the FDA
and IEC (12,13). Normalized experimental SNR maps demonstrated
that the SNR improvement in the cerebellum (+26.79%) observed
in the presence of HPM pads was primarily due to a larger achieved flip angle (+38.67%).
The flip angle averaged across all subjects
showed a mean improvement of 33% in the cerebellum and a decrease of 3.4% at
the center, while the average flip angle in the whole brain remained
approximately the same (Fig. 2). Functional MRI showed contralateral
somatosensory motor cortical activation and activation of the superior and
inferior axial slices of the ipsilateral cerebellar cortex (Fig. 3). In the presence of
HPM pads a statistically significant improvement in the signal as well as the
functional activation was observed for the inferior axial slices of the
cerebellum (Fig. 3). This confirmed that an improvement in the flip angle (Fig. 2) can translate to an improvement in the detection of BOLD fMRI in the
cerebellum. Group analysis results showed a statistically significant increase
in activation in the cerebellar cortex with the HPM (Fig. 4), adjacent to the HPM
pads (Fig. 1), and with no considerable loss elsewhere in the brain (Fig. 4).CONCLUSIONS:
HPM
pads can improve cerebellar fMRI at 7T with a commonly used commercial head
coil without compromising RF safety. Acknowledgements
This work was supported by the Center for
Advanced Imaging Innovation and Research (www.cai2r.net), a NIBIB Biomedical
Technology Resource Center (NIH P41 EB017183) and NIH R01 EB021277.References
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