Matthew Finnerty1, Derick Petrey1, Paul Taylor1, Luke Beery1, Tsinghua Zheng1, Xiaoyu Yang1, Hiroyuki Fujita1,2,3,4, Se-Hong Oh5, Ken Sakaie5, and Mark Lowe5
1Quality Electrodynamics, LLC, Mayfield Village, OH, United States, 2Department of Physics, Case Western Reserve University, Cleveland, OH, United States, 3Department of Radiology, University Hospitals of Cleveland, Cleveland, OH, United States, 4School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, Australia, 5Imaging Institute, Cleveland Clinic, Cleveland, OH, United States
Synopsis
While fMRI
at 7-Tesla can provide clinically relevant increases in functional sensitivity
over 3-Tesla, it also typically uses visual and audio stimulation devices that
require additional space accommodations inside the RF coil. In order to accommodate a wider range of
stimulus devices than possible with high filling factor designs, a head array coil utilizing a volume transmitter and 32 receive elements
for 7-Tesla was constructed inside a versatile mechanical package to support
fMRI and other applications.Introduction
Despite challenges, fMRI at 7-Tesla can
provide clinically relevant increases in functional sensitivity over 3-Tesla.
1 In some examples
of previous studies,
2-4 receive surface coil arrays with high
filling factors were combined with a separate local transmitter to take advantage of
the higher signal-to-noise ratio (SNR) available at 7T. However, fMRI studies typically employ visual
and audio stimulation devices that require additional space to fit inside the
coil. In order to accommodate a wider range
of stimulus devices than possible with high filling factor designs, a head array coil utilizing a volume transmitter and 32 receive elements
for 7-Tesla was constructed. The coil
utilizes a mechanical package that has an inner profile to support headphone
audio stimulus devices and a z-position adjustable mirror for fMRI. Furthermore, as imaging at 7T has become more
commonplace, there has been a need for RF coils that support different
applications, research, and faster workflow.
To that end, the mechanical package was designed with an outer diameter
that can fit within higher-order B0 shim coils, a superior-side access opening
for EEG leads, and a split-top design for easier patient setup.
Methods
The
32-channel receive-only head array spans an S-I length of approximately 21cm
and consists of two separate halves. The
posterior half contains 20 elements of approximately 5x7cm arranged along 3
rows with the loops in a 7-6-7 configuration.
The anterior half contains 12 elements, of which 10 are approximately 5x7cm
(6 along the dome, 4 along the sides), and 2 larger elements (approximately 12x11cm)
around eye openings (Fig 1). The receive
elements are still sample noise dominated , so the SNR reduction attributable to
coil losses in these larger receive element sizes is negligible per the
definition of coil noise figure, given by $$$NF_{coil}=10\log_{10}{\frac{R_s+R_c}{R_s}}$$$,
where Rs
and Rc are the noise resistances due to the sample and coil
respectively.5 Overlaps were
adjusted to minimize the coupling between neighboring elements at the 7T 1H
frequency of 297.18 MHz. A partially
shielded, high-pass, circularly polarized birdcage transmitter was used.
The internal dimensions and profile
of the mechanical formers were designed to accommodate audio stimulus
headphones (Avotec) (Fig 2). An opening
at the dome region of approximately 3.5cm diameter was added to allow access
from the superior side, such as for running EEG leads. The outer diameter of the coil is
approximately 37cm to allow mechanical compatibility with higher-order B0 shim
coils (Resonance Research), while fitting onto a detachable base that fits the system
table under standard use. To assist with
workflow and patient comfort, the coil has a split-top design and includes a
z-position adjustable mirror (Fig 3). With
limited space for the electronics, an ultra-compact, low noise, pre-amplifier 6
developed for 297.18 MHz was used and properly positioned relative to the B0
field as to maximize its performance.7
A Siemens MAGNETOM 7T system at the
Cleveland Clinic was used for testing.
Results
High resolution T1-weighted images were obtained
from a healthy volunteer scanned with the 7T 32-channel head array (Fig 4). A visual task fMRI (flashing checker board)
using a BOLD EPI sequence was also performed (Fig 5).
Conclusion
A
7T transmit with 32-channel receive-only head array was successfully
constructed in a mechanical package accommodating a wider range of stimulus
devices for fMRI. Initial results from
healthy volunteers were obtained and a visual task fMRI (flashing checker
board) was performed. Future work will
involve comparing the performance of the coil to other known coils and investigating
a design that supports parallel transmit.
Acknowledgements
This
work was in part supported by the National Institutes of Health and National
Institute of Biomedical Imaging and Bioengineering, Grant Award 2R44EB007094-2
and the 2010 Ohio Third Frontier Medical Imaging Program Award.References
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