Introduction

The macaque monkey is an important animal model for neuroscience. MR imaging techniques such as functional MRI (fMRI) and diffusion MRI (dMRI) have provided valuable insights for understanding brain organization^1,2,3, as well as cross-species anatomy^4,5. Recent advances in high-temporal resolution fMRI^6,7 have facilitated artifact removal⁸ and sensitive detection of complex functional networks⁹. We previously developed a 24-channnel multi-array receive coil for anesthetized macaque imaging in a 3T MRI scanner, and confirmed its usefulness for fast imaging to explore and validate non-invasive connectivity measures¹⁰. However, there is a compelling need to address brain organization in conscious behaving animals. Here, we developed a separate 24-channel multi-array coil optimized for headpost-implanted monkeys and evaluated measures of coil quality and efficiency in phantoms and animal studies.

Methods

Animal experiments were approved by the Animal Care and Use Committee of the Kobe Institute of RIKEN. The subject was a male rhesus monkey (Macaca mulatta; 6.5 kg). Experiments were performed in a 3T MRI scanner with a powerful gradient (80mT/m) (MAGNETOM Prisma, Siemens, Erlangen, Germany). The coil frame geometry was designed with a 3D digital design software (Rhinoceros5, McNeel, Seattle, USA) to closely fit our largest monkey’s head. The coil elements covered the whole monkey’s head and were placed with some overlap with each other to avoid coupling. To easily equip the coil over the headpost-implanted monkey, the coil was separated into anterior and posterior halves (Figure 1A), and the coil elements between two halves were overlapped by bending a part of elements outward and facing each other (Figure 1B). Element coupling was estimated with noise correlation measurements. B1 transmission (B1+) was assessed with a flip-angle (FA) sequence. B1 receive field (B1-) was estimated by calculating signal ratio between 24-channel and body receive coils with gradient-echo sequence. To evaluate the quality of functional MR images without the additional confounds of awake imaging such as motion, we scanned resting-state fMRI (rfMRI) using imaging parameters in one anesthetized animal (FOV 95x95 mm, matrix size 76x76, slice thickness 1.25 mm, multiband factor=5, TR=755ms, TE 30ms, number of slices 50, flip-angle 55°, bandwidth 1370 Hz/pixel, echo spacing 0.95ms, the number of total volumes 4096, and scan time 51 min). Data was analyzed with non-human primate version of Human Connectome Project (HCP) pipelines¹¹. The fMRI data was analyzed with a single-session independent component analysis (ICA) and manually surveyed for structured noise and neural components. T1w and T2w images were acquired as described in our other abstract¹⁰.

Results

The element decoupling was verified by measuring the noise correlation coefficient matrix for the 24-channel coil (Figure 1C). The average noise correlation coefficient was 8.5±5.3%. B0 field-maps illustrated inhomogeneities near nasal and mastoid cavities (Figure 2A). Flip-angle (B1+) maps showed that the transmission was slightly higher at subcortical regions but fairly uniform over the cortical surface (89°±4.5°) (Figure 2B), and B1 receive map (B1-) also showed signal sensitivity is higher in the peripheral part of the brain, particularly at the gyrus crown located close to the coil elements (Figure 2C). The B1 bias correction using T1w and T2w worked fine with the HCP pipeline, and myelin map was created as in Figure 2D. ICA of a single resting state fMRI scan showed 231 structured noise components and 19 neural signal components. There was no structured noise related to slice overlapping. Figure 3 shows representative resting-state network (RSN) components (default mode, frontoparietal, ventral sensorimotor, and auditory networks).

Discussion

Our new system with a 24-ch multi-array coil for awake macaques in a 3T scanner showed promising coil performance. Although the coil was separated in two units of for usability, the coupling of elements was minimized and average value was almost same as in our previous single-unit 24-ch coil for anesthetized macaques¹⁰. The B1 transmission field was fairly homogenous over the cortical surface. B1 receive field was sensitive to the superficial part of the brains such as gyral crowns, however, the biasfield was well corrected within the HCP pipeline using T1w and T2w, and resulted in myelin maps consistent with previous reports in macaque¹². The coil detected a number of RSNs, some of which were consistent with our other report¹⁰. Overall findings suggest the potential of our system, although we need to test it in awake animals with a focus on head motion and the tolerance of the animals.

Conclusion

We developed a system with 24-ch multi-array coil for awake macaques in a 3T MRI scanner. Our preliminary data confirmed that coil performance was high enough for multi-modal imaging in awake monkeys.

Acknowledgements

This research is partially supported by the program for Brain Mapping by integrated Neurotechnologies for Disease Studies (Brain/MINDS) from Japan Agency for Medical Research and development, AMED, by MEXT KAKENHI Grant (17K01981, 16H03300, 16H01626), and by a grant from Sumitomo Dainippon Pharma Co., Ltd.

References

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