Rou Li1, Jingqiang Peng2, Xiao Chen1, Xiaoqing Hu1, Xiaoliang Zhang3,4, Ye Li1, Xin Liu1, Hairong Zheng1, and Zheng Wang2
1Paul C. Lauterbur Research Center for Biomedical Imaging, Shenzhen Institutes of Advanced Technology, CAS, Shenzhen, China, People's Republic of, 2institute of neuroscience, shanghai institutes for biological sciences, CAS, Shanghai, China, People's Republic of, 3Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, United States, 4UCSF/UC Berkeley Joint Graduate Group in Bioengineering, San Francisco, CA, USA, San Francisco, CA, United States
Synopsis
Macaque monkey is a critical model for fMRI to investigate large scale functional network. In this work, an 8 channel loop array with shield birdcage transmit coil is proposed to achieve high resolution homogeneous macaque brain images at 3T MRI. Both phantom and in-vitro experiments demonstrate the capability of the proposed design of achieving homogeneous transmit field and high SNR in the whole brain region, which provides the possibility to perform in-vivo macaque fMRI experiments to investigate large-scale functional network.Introduction
Macaque monkey is a critical model
for fMRI to investigate large scale functional network [1, 2].Implanted and
high density arrays [3, 4] have been developed to obtain high resolution macaque
brain images. However these designs yield extremely high signal-to-noise ratio
(SNR) in the surface region of macaque brain, which may lead to inhomogeneous
images when large scale network is investigated. In this work, we propose an 8
channel loop array with shield birdcage transmit coil to achieve high resolution
homogeneous macaque brain images at 3T. Both phantom and
in-vitro imaging experiments
are performed to evaluate the SNR and homogeneity of the design.
Methods and Materials
The proposed macaque coil (MC) consisted of a
16-rung shielded birdcage for transmit and an 8 loop array for receive. The
birdcage coil and 8-loop array were fabricated on two acrylic tubes with 200-mm
and 140-mm diameter respectively as shown in Fig. 1. The birdcage coil with 130-mm
length was tuned to 123.2 MHz. The birdcage was driven in quadrature to improve
the homogeneity and efficiency. The 8 loops of the array were placed into two
rows with 4 loops on each row. The element size was ~55×65 mm
2.
The decoupling of array elements was achieved by using overlapping and low
input impedance preamplifiers.
Phantom
study: A cylindrical saline phantom from Siemens with 115-mm diameter and
200-mm length is employed for all phantom studies. Double angle method (DAM) [5]
with the following parameters is used to compare the transmit homogeneity
between the shield birdcage and body coil of the 3T TIM Trio system: TR = 2000 ms,
TE = 12 ms, bandwidth = 130 Hz/pixel, FOV = 150×150 mm
2, slice
thickness = 3 mm, acquisition matrix = 128×128, flip angle = 60 and 120 degree. A
gradient echo (GRE) sequence with the following parameters is utilized to
compare SNR maps between 8-channel array and the small flexible array (FA) of
the 3T Trio system: TR = 300ms, TE = 10ms, acquisition matrix = 256×256, flip
angle = 60 degree. Other parameters are the same as the DAM. Transverse and sagittal
images are acquired. Noise images are acquired with no transmit power. The covariance root sum of squares [6] method is used to evaluate
the SNR.1/g-factor
maps with acceleration factor = 2 have been calculated to evaluate the parallel
imaging performance of the proposed coil.
In-vitro study: The T1-weighted 3D MARAGE
sequence is utilized for in-vitro high resolution macaque brain imaging with the
following imaging parameters: TR/TE/TI = 2300 ms /3.69 ms/ 1000 ms, bandwidth = 130
Hz/pixel, Echo Spacing = 11.6, FOV = 90×90×64 mm
3, matrix = 256×256×160,
acceleration factor = 2 (mSENSE), reference lines = 24, number of average = 1.
Results
Fig. 2 shows the B1 map
comparison between the proposed 16-rung shielded birdcage coil and body coil.
The similar B1 maps of the two coils demonstrate the capability of the proposed
birdcage coil to provide homogeneous transmit field. The SNR maps and 1-D profile of SNR of the proposed MC and the FA are
shown in Fig. 3.The black dash lines indicate the location of the 1-D profile. Compared
with the FA, the SNR of the MC is dramatically improved in the macaque brain
region. The 1/g-factor maps with acceleration factor = 2 in transverse and sagittal
planes are shown in Fig. 4.The maximum and mean values of g-factor in marked
black dotted region are shown below the maps. The results indicate the parallel
imaging capability of the proposed MC. As shown in Fig. 5, high spatial resolution (0.35×0.35×0.4 mm
3)
in-vitro macaque brain images have been
achieved. The scan time is totally 5 min and 21 sec. The results demonstrate
the potential of the proposed RF coil for large scale network investigation in
terms of SNR, homogeneity and parallel imaging capability.
Discussion/Conclusion
The proposed 8-channel receive array with
shielded birdcage macaque coil is capable of providing homogeneous transmit
field and high SNR in the whole brain region. By using this coil, high resolution whole macaque brain images can be obtained in relatively short scan time, which provides the possibility
to perform
in-vivo macaque fMRI experiments
to investigate large-scale functional network.
Acknowledgements
This work is supported in part by
national grants No. 51307171, 61571433, 61401450 and
81527901, provincial grants No. 2014A030313691, 2015B020214006 and 2014A030310200,
city grant No. JCYJ20140610152828673, and internal grant No. 201314.References
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Stollberger, et al., MRM. 1996, 35: 246-251.[6] B. Keil et al., JMR 2013, 229,
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