High-resolution macaque MRI at 3T using implanted multichannel array coils.
Thomas
Janssens1, 2,
John
T.
Arsenault1, 2,
Jonathan
R.
Polimeni2, 3,
Boris
Keil2, 3, 4,
Reza
Farivar2, 3,
Jennifer
A.
McNab2, 3,
Annelies
Gerits1, 2, 3,
Lawrence
L.
Wald2, 3, 4 and
Wim
Vanduffel1, 2, 3*
-
1
KU Leuven, Belgium
-
2
Massachusetts General Hospital, A.A. Martinos Center for Biomedical Imaging, United States
-
3
Harvard Medical School, Department of Radiology, United States
-
4
Massachusetts Institute of Technology, Harvard-MIT Division of Health Science and Technology, United States
In the last decade diffusion and functional MRI have proven highly valuable tools for studying the anatomy and sensory as well as cognitive processing in awake monkeys. However, the smaller monkey brains compared to those of humans require higher spatial resolution imaging in order to reveal equivalent anatomical detail. In turn, increased signal-to-noise (SNR) ratio is needed to support the smaller voxel sizes. Although conventional phased-array receive coil technology (Roemer, 1990) offers a significant increase in SNR, the arrays never reach their full potential due to the distance between the coil and the monkey brain. In monkey imaging the coil-brain distance is relatively large compared to the brain size due to the large lateral yaw muscles and the head fixation post. In the past, this coil-brain distance has been minimized by implanting a single loop coil on top op the skull for use in anesthetized monkeys (Logothetis, 2002). However, single-shot EPI acquisitions used in DTI and fMRI benefit greatly from accelerated image encoding that mitigates geometric distortions. In this work we describe the design and use of implanted multichannel phased-array coils on top of the monkey’s skull, embedded inside the acrylic headpost. This improves the SNR of monkey imaging without having to rely on high field scanners, which are expensive and increase the sensitivity to susceptibility artifacts. Moreover, this approach does not require additional invasive procedures and has the advantage that, across days, the imaged tissue is always at a fixed position relative to the receive coils.
4 subjects received an implanted array consisting of a central loop surrounded by 6-7 loops and fits tightly around the skull of monkey. Except for the copper wires and MR compatible MCX connectors, no electrical components were used on the implanted part of the arrays. The arrays were implanted on top of the skull of the monkey underneath the headpost and were covered with dental acrylic. The external matching and detuning circuit components were positioned on detachable circuit boards. Non of the subjects show any distress from the implant and after more than one year all the arrays are still functional. In an anesthetized macaque, we compared the SNR performance of the implanted array to that of several external coils (1-, 4-, and 8-channels). The data show a 3.8-, 3.0, and 2.7-fold increase in SNR in the brain compared to an external 1, 4, and 8-channel coil, respectively. Furthermore, awake fMRI with 500 μm isotropic voxels, structural imaging (MPRAGE) with 200 μm isotropic voxels, and diffusion-weighted imaging with 700 μm isotropic voxels was performed and confirmed the superior SNR obtained with implanted coils relative to monkey imaging at 3 Tesla using 4 or 8-channel external coils.
In conclusion, significant improvements in SNR were obtained compared to external coils. We have shown that the implant coils are suited for both anesthetized (anatomical and diffusion-weighted imaging) and awake (fMRI) macaque MRI experiments. The use of the implanted array coils for macaque MRI allows very high spatial resolution imaging (an improvement of roughly 1 log unit), while retaining the SNR in structural imaging and functional fMRI at 3T relative to previous approaches.
References
Roemer PB et al. MRM 16: 192-225 (1990)
Logothetis N et al. Neuron 35(2): 227-242 (2002)
Keywords:
fMRI,
macaque,
parallel imaging,
Implanted Coil,
high resolution
Conference:
Belgian Brain Council, Liège, Belgium, 27 Oct - 27 Oct, 2012.
Presentation Type:
Poster Presentation
Topic:
Other basic/clinical neurosciences topic
Citation:
Janssens
T,
Arsenault
JT,
Polimeni
JR,
Keil
B,
Farivar
R,
McNab
JA,
Gerits
A,
Wald
LL and
Vanduffel
W
(2012). High-resolution macaque MRI at 3T using implanted multichannel array coils..
Conference Abstract:
Belgian Brain Council.
doi: 10.3389/conf.fnhum.2012.210.00067
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Received:
30 Aug 2012;
Published Online:
12 Sep 2012.
*
Correspondence:
Prof. Wim Vanduffel, KU Leuven, Leuven, 3000, Belgium, wim@nmr.mgh.harvard.edu