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A 16-Channel RF Receive Array with Integrated B0-shim Capability for Anesthetized Monkey Whole Brain Imaging at 7T1Interdisciplinary Institute of Neuroscience and Technology, Qiushi Academy for Advanced Studies, College of Biomedical Engineering & Instrument Science, Zhejiang University, Hangzhou, China, 2Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 3MR Collaboration Northeast Asia, Siemens Healthcare, Hangzhou, China, 4Harvard Medical School, Boston, MA, United States, 5Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States
Synopsis
To mitigate signal dropout in EPI images due to local B0 inhomogeneities inevitably caused by craniotomies and chamber implantations needed for multimodal studies, we designed a custom 16-channel tight-fit receive array, with each receive element integrated with DC pathway for multi-coil B0 shimming. The feasibility of constructing high-density receive array (5cm diameter loop elements) integrated with B0-Shim has been demonstrated. With DC-enabled higher-order local B0 shimming, obvious improvement in recovering local signal drop out and image distortion around brain surgery region, implying its promising application in high-resolution multi-modal monkey neuroimaging.
Introduction
Close-fit and conformal geometry is optimal in RF receive coil design [1]. For anesthetized monkey studies, craniotomies and chamber implantations needed for multimodal studies often lead to local B0 inhomogeneities [2-3]; such inhomogeneities can be particularly pronounced at higher fields. In such cases, a higher order (＞2) shim can help to mitigate local B0 inhomogeneity and be useful for recovering signal loss around the implantation site [4]. Recently, combined RF-receive/B0-shim coil has been shown to be effective for improving local B0 homogeneity and maintaining RF performance in 3T and 7T human brain imaging [5-7]. Here, to improve macaque monkey brain imaging at 7T, we designed a custom 16-channel tight-fit receive array, with each receive element integrated with DC pathway for multi-coil B0 shimming. We evaluated the coil performance through in vivo monkey brain imaging at 7T.Method
Fig.1a shows the customized 7T monkey coil with 16ch receive array, inserted into a quadrature 16-rung birdcage transmit-only coil with 29cm inner diameter and 24cm length. The receive helmet sits on a stereotaxic frame which used two ear-bars and one mouth-bar for monkey head fixation. 16 AWG wire loops (5cm diameter) were laid out on a 3D printed polycarbonate former. Four openings (2~3cm diameter) were placed in the housing to allow neural stimulation methods (e.g., optogenetics or infrared light [8]) accessing brain regions of temporal, parietal and occipital lobes. Toroid inductors with proper resonance frequency were used as RF chokes bypassing distributed capacitors on each RF loop. Open-source, low-cost DC current amplifiers were used to generate multi-channel B0-shim current [9].
Images were acquired on a 7T research scanner (Magnetom, Siemens Healthcare, Erlangen, Germany) with a whole-body gradient set (70mT/m and 200T/m/s). A commercial 28-channel QED knee coil (Mayfield Village, OH, USA) with 15cm inner diameter was selected as a reference for comparison.
Two healthy female macaques (2-3 years, 3.5kg and 4.5kg, respectively) were used in the study. All procedures were in accordance with NIH standards and approved by the local Institutional Animal Care Committee. Anesthesia was maintained by 1.5~2% isoflurane and heart rate, expired CO2, oxygen saturation, respiration, and temperature monitored. Monkeys were placed in an MR-compatible stereotaxic with the head centered within the birdcage and knee coils. Monkey A was used for coil Q&A measurements and anatomical image acquisition, while Monkey B, which had undergone skull removal and an optical chamber implantation over occipital cortex, was used to evaluate B0 shimming performance in local B0 inhomogeneity correction.
System 2nd-order B0 shim was applied prior to all scans. SNR and g-factor maps were obtained from PD-weighted FLASH images (TR/TE/α: 30ms/6ms/10°, 1×1×3mm3). AFI-B1 maps [TR1/TR2/TE/α: 20ms/50ms/2.93ms/60°] were acquired to evaluate the transmission efficiency and homogeneity of the birdcage coil. High-resolution MPRAGE and SWI were both acquired with an GRAPPA acceleration rate of 3 along R/L phase encoding direction (MPRAGE: TR/TE/α: 2,500ms/3.21ms/7°, matrix size 192×192, FOV 96×96mm2, slice thickness 0.5mm, 5 averages, scan time 19'49''; SWI: TR/TE/α: 66ms/25ms/16°, matrix size 640×300, FOV 192×90mm2, slice thickness 0.3mm, 1 average, scan time 20'47''). To evaluate the efficacy of B0 shimming, single shot EPI [TR/TE/α: 2,000ms/20ms/80°; matrix size 138×104, FOV 138×104mm2, slice thickness 2mm, bandwidth 1,449Hz/px] with shim currents OFF/ON were acquired and compared.
Results
The QU/QL-ratio of the 50 mm loop was measured to be 4. As in Fig.2, the array’s noise correlation ranged from 1.8% to 47.7% (avg. 15.7%), with a 1.5-fold SNR improvement compared to the 28-channel knee coil at the cortical region and 2-fold improvement in deep brain regions, and with improved B1+ homogeneity but slightly lower transmit efficiency by using the birdcage transmit coil.
Fig.3 indicates that, with the acceleration rate of 3, 1/g-factor higher than 0.8 can be achieved over most brain region for all three phase encoding directions when using the 16ch array.
Fig.4 shows 0.5 mm isotropic MPRAGE (monkey B) and 0.3 mm isotropic SWI (monkey A) images can be obtained with ~20 minutes.
Fig.5 demonstrates that the present multi-coil-enabled higher order global shim is capable of improving the recovery of signal drop out and image distortion due to B0 inhomogeneity.
Discussion and Conclusion
The feasibility of constructing high-density receive array integrated with B0-Shim has been demonstrated. With DC-enabled higher-order local B0 shimming, obvious improvement in recovering local signal drop out and image distortion around brain surgery region, implying its promising application in high-resolution multi-modal monkey neuroimaging.Acknowledgements
National Natural Science Foundation 81701774 and 61771423. We thank Joshua Park, Sean Bradley, Sean Down, Simon Sigalovsky, Meizhen Qian, Jialu Zhang and Dengfeng Zhou for helpful technical support.References
[1] Gao, Yang, Weidao Chen, and Xiaotong Zhang. "Investigating the Influence of Spatial Constraints on Ultimate Receive Coil Performance for Monkey Brain MRI at 7T." IEEE Transactions on Medical Imaging (2018).
[2] Chernov, Mykyta, and Anna Wang Roe. "Infrared neural stimulation: a new stimulation tool for central nervous system applications." Neurophotonics 1.1 (2014): 011011.
[3] Roe, Anna Wang, et al. "In vivo mapping of cortical columnar networks in the monkey with focal electrical and optical stimulation." Frontiers in neuroanatomy 9 (2015): 135.
[4] Juchem, Christoph, et al. "Dynamic multi-coil technique (DYNAMITE) shimming for echo-planar imaging of the human brain at 7 Tesla." Neuroimage 105 (2015): 462-472.
[5]Truong T K, Darnell D, Song A W. Integrated RF/shim coil array for parallel reception and localized B0 shimming in the human brain[J]. NeuroImage, 2014, 103: 235-240.
[6] Stockmann J P, Witzel T, Keil B, et al. A 32-channel combined RF and B0 shim array for 3T brain imaging. Magn Reson Med, 2015, 75(1): 441-451.
[7] Stockmann, Jason P., and Lawrence L. Wald. "In vivo B0 field shimming methods for MRI at 7 T." NeuroImage 168 (2018): 71-87.
[8] Xu G., et al., “A novel method for mesoscale connectome mapping: focal infrared neural stimulation in high-field functional MRI” ISMRM 2018: 1011
[9] Arango, N., et al. "Open-source, low-cost, flexible, current feedback-controlled driver circuit for local B0 shim coils and other applications." Int Soc Magn Res Med. Vol. 1157. 2016.
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