# Ultra-high-resolution imaging of NHP head at 11.7T with a 32-channel Honeycomb modular receive array

Elias Djaballah, Paul-François Gapais, Michel Luong, Maxime Roustan, Thomas Lilin, Nicolas Boulant, Alexandre Vignaud, Qi Zhu, Alexis Amadon

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Elias Djaballah, Paul-François Gapais, Michel Luong, Maxime Roustan, Thomas Lilin, et al.. Ultra-high-resolution imaging of NHP head at 11.7T with a 32-channel Honeycomb modular receive array. ISMRM 2025, May 2025, Honolulu, United States. hal-05090048

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# Ultra-high-resolution imaging of NHP head at 11.7T with a 32-channel Honeycomb modular receive array

Elias Djaballah¹,³* & Paul-François Gapais¹*, Michel Luong², Maxime Roustan³, Thomas Lilin³, Nicolas Boulant¹, Alexandre Vignaud¹, Qi Zhu³ and Alexis Amadon¹

*Joint-first authors, ¹BAOBAB, Université Paris-Saclay, CEA/Joliot/NeuroSpin, GIF-SUR-YVETTE, France, ²Université Paris-Saclay, CEA/DRF/IRFU/DACM, GIF-SUR-YVETTE, France, ³Cognitive Neuroimaging Unit, INSERM, CEA, Université Paris-Saclay, NeuroSpin Center, GIF-SUR-YVETTE, France

## Synopsis:

**Motivation:** Our operational 11.7T recently delivered the first in-vivo human brain images. Yet, non-human-primate (NHP) brain is also of interest to the neuroscientists because of its similarity to human.

**Goal(s):** To establish ultra-high-resolution whole-brain imaging for NHP at 11.7T using a 32-Rx coil.

**Approach:** A 32-channel receive array, initially designed for human brain temporal lobes, was adapted to the NHP thanks to repositionable high-impedance loops and an adaptative former.

**Results:** In-plane resolutions below 100 µm were achieved.

**Impact (<40):** The modularity of our receivers allows to image both human and non-human primate brains without the need for coil retuning. Ultra-high-resolution MRI of whole-head anesthetized NHP is demonstrated at 11.7T.

## Introduction:

Ultra-high-field (UHF) MRI offers significant gains in SNR¹ and image resolution². Recently, the first human brain images at 11.7T were released to the community³ and revealed the full potential of this unique machine. Ultra-high-field MRI of non-human-primate (NHP) brain is also of interest to neuroscientists, as they share similarities to humans in terms of structure, function, and cognitive abilities. Since the monkey's brain is about 12 times smaller (in volume) than the human brain⁴, dedicated receive arrays are critical to achieve submillimeter resolution. Several receive arrays have been built at UHF for NHP studies, demonstrating very high cortical SNR and acceleration capabilities⁵,⁶, but with limited coverage for whole-brain imaging.

A modular 32-channel array, referred to as *Honeycomb*, dedicated to the human temporal lobes has been recently proposed and demonstrated an improved SNR compared to a whole-brain receive array⁷. In this abstract, the modularity of the *Honeycomb* receive array is exploited and adapted for NHP brain imaging. High-resolution anatomical images of anesthetized NHP are pushed to the limit of acceptable SNR.

## Methods:

**Setup:** The *Honeycomb* coil, originally developed for human temporal lobes, consisting of 32 hexagonal high-impedance loops (HICs) with a 44-mm circumdiameter, was adapted for NHP studies. The loops were easily positioned using Velcro straps on a thermoformed plastic molded from a NHP's head phantom (Figure 1A). After repositioning, retuning was unnecessary for either the loops or the homemade preamplifiers. The loops are not overlapped and "reverse" preamplifier decoupling⁸ is applied. The loops cover a much larger area than just the brain, extending down to the neck. An 8-Tx-only array, composed of large overlapping loops⁹ was used in circularly polarized (CP) mode.

**Animal:** One female adult monkey (Macaca Mulatta, 7.8kg) was examined at 11.7T in the supine position. All experiments were conducted in strict accordance with European (EC/2010/63) and French regulations (French Act Rural Code R214-87 to 126). The experimental protocol was approved by the Ethics committee and the French Ministry of Research and Education (Authorization n°APAFIS#32632-2021080216315953v1). The monkey was initially sedated with a mixture of Ketamine (3mg/kg) and Dexdomitor (0.015mg/kg) and maintained using Isoflurane at 1.5% in an O₂/Air mixture. Heart rate, respiratory rates, blood pressure, blood oxygen saturation and body temperature were monitored throughout the experiment. A flexible heating pad maintained the animal's temperature at 38°C.

**MR imaging:** The transmit coil was used in its default Tx-combined CP mode for all acquisitions. The SNR was evaluated using a 3D-GRE sequence (1.5-mm³ resolution, FOV69x69x74, BW 780Hz/Px, FA7, TR/TE=15/2ms). It was corrected for flip angle and TR, and reconstructed using a noise-covariance-weighted root-sum-of-square using the same GRE sequence but without applied voltage¹⁰. The imaging protocol consisted of three T₂*-weighted 2D gradient echo sequences acquired with different resolutions and parameters (Table 1) and a Maximum Intensity Projection (MIP) image from a T₁-weighted sequence.
## Results and discussions:

The in vivo noise correlation matrix and SNR maps are shown in Figure 1B and 1C, respectively. The non-overlapping design and inherent low coupling of HICs (paired with efficient preamplifier decoupling) provide a low mean correlation. The SNR distribution shows significant coverage of the monkey's head. However, since the animal is placed in the supine position, the loops at the top of the coil contribute little to the overall SNR as they are almost orthogonal to the B₀ field. The right-left asymmetry in the SNR distribution may be due to a slight tilt of the monkey's head inside the coil. Since the monkey's head is much smaller compared to the diameter of this transmit array (28.5cm), the CP mode provides a sufficiently homogeneous B₁⁺ excitation profile (Figure 1D).

The ultra-high-resolution T₂\*-weighted anatomical images (Figure 2) demonstrate the capability of 11.7T MRI combined with a dense, close-to-subject receive array. The 60µm in-plane resolution shows a lower SNR, but better contrast and finer-grained details of vascular structures, particularly microveins, as well as cortical and subcortical structures. The broad coverage of the coil allows clear imaging of the monkey's orbital nerve (Figure 3) and veins extending down to the neck (Figure 4).

## Conclusion and perspectives:

The modularity and versatility of our Honeycomb receive array were demonstrated, achieving ultra-high-resolution whole-head NHP imaging at 11.7T with minimal setup adjustments. This non-invasive imaging approach paves the way towards more comprehensive studies on fine-grained brain structure and function in NHP.

At 11.7T, strong B₀ artifacts are expected, challenging the acquisition of EPI images for fMRI studies. Therefore, a dedicated B₀ shimming system is paramount to mitigate the distortions¹¹,¹², to be paired with a tighter transmit coil. In this context tailored pulses shall be used¹³ to provide a homogenous excitation.

## Acknowledgements:

We acknowledge the financial support of the French national research agency (ANR) under the reference ANR-20-CE37-0005 and the exploratory program of CEA, the French Alternative Energies and Atomic Energy Commission. This work benefited from government funding (ESR/EquipEx PRESENCE, ANR-21-ESRE-0006) managed by the French National Research Agency under the France 2030 Future Investment Program.

## References:

1. Le Ster C, Grant A, Van de Moortele P-F, et al. Magnetic field strength dependent SNR gain at the center of a spherical phantom and up to 11.7T. Magn Reson Med. 2022.

2. Trattnig S, Springer E, Bogner W, et al. Key clinical benefits of neuroimaging at 7T. Neuroimage 2018.

3. Boulant, N., Mauconduit, F., Gras, V. et al. In vivo imaging of the human brain with the Iseult 11.7-T MRI scanner. Nat Methods 2024.

4. Paula L Croxson, Stephanie J Forkel, Leonardo Cerliani, Michel Thiebaut de Schotten, Structural Variability Across the Primate Brain: A Cross-Species Comparison, *Cerebral Cortex*, Volume 28, Issue 11, November 2018, Pages 3829–3841

5. Lagore, R. L., Moeller, S., Zimmermann, J., et al. An 8-dipole transceive and 24-loop receive array for nonhuman primate head imaging at 10.5 T. NMR in Biomedicine 2020.

6. Zhang, X., Zhang, J., Gao, Y., et al. A 16-Channel Dense Array for In Vivo Animal Cortical MRI/fMRI on 7T Human Scanners. IEEE Transactions on biomedical engineering 2021.

7. Gapais, P.-F., Luong, M., Giacomini, E., et al. A 32-channel high-impedance honeycomb-shaped receive array for temporal lobes exploration at 11.7T. Magn Reson Med. 2024.

8. Zhang, B., Sodickson, D. K., Cloos, M. A., A high-impedance detector-array glove for magnetic resonance imaging of the hand. Nat Biomed Eng. 2018.

9. Chu S, Gras V, Mauconduit F, Massire A, Boulant N, Gunamony S. Electromagnetic and RF pulse design simulation based optimization of an eight-channel loop array for 11.7T brain imaging. Magn Reson Med. 2023.

10. Kellman, P., McVeigh, E. R., Image Reconstruction in SNR Units: A General Method for SNR Measurement. Magn Reson Med. 2005.

11. Meneses, B. P., Stockmann, J. P., Arango N., et al., Shim coils tailored for correcting B0 inhomogeneity in the human brain (SCOTCH): Design methodology and 48-channel prototype assessment in 7-Tesla MRI, Neuroimage 2022.

12. Djaballah, E., Uthayakumar, A., Bouchet, M., Kalouguine, A., Amadon, A., ZHU, Q., A 48-channel multi-coil shim array for B0 inhomogeneities correction in the NHP brain at 7T. Proc. Intl. Soc. Mag. Reson. Med. 32 (2024).

13. Cloos, M. A., Boulant, N., Luong, M. et al. kT-Points: Short Three-Dimensional Tailored RF Pulses for Flip-Angle Homogenization Over an Extended Volume. Magn Reson Med. 2012.
<figure><img src="image_2.png" /><figcaption>Figure 1: Honeycomb receive array setup and in-vivo performance metrics (A): Honeycomb receive array made of 32 HICs. The loops are placed on a former mimicking the NHP's head. (B) Noise correlation matrix with a mean correlation of 0.106 (note the scale from 0 to 0.8). (C) In-vivo SNR, reconstructed with a noise-covariance weighted root-sum-of-square and corrected for flip angle and TR. (D) B1+ maps in T/sqrt(W) with an 8-loop transmit coil in its circularly polarized mode.</figcaption></figure>

<table><caption>Table 1: Sequence parameters of the images shown in the subsequent figures.</caption><thead><tr><th>Acquisition</th><th>#1</th><th>#2</th><th>#3</th><th>#4</th></tr></thead><tbody><tr><th>Sequence</th><td>T<sub>2</sub>* 2D-GRE</td><td>T<sub>2</sub>* 2D-GRE</td><td>T<sub>2</sub>* 2D-GRE</td><td>T<sub>1</sub> 3D-MPRAGE</td></tr><tr><th>Resolution (mm<sup>3</sup>)</th><td>0.06x0.06x1</td><td>0.10x0.10x1</td><td>0.15x0.15x1</td><td>0.25x0.25x0.25</td></tr><tr><th>TR (ms)</th><td>2070</td><td>2000</td><td>2000</td><td>3950</td></tr><tr><th>TE (ms)</th><td>23.7</td><td>15</td><td>15</td><td>3.93</td></tr><tr><th>Flip angle (*)</th><td>60</td><td>45</td><td>45</td><td>8</td></tr><tr><th>Matrix size</th><td>2048x2048</td><td>1200x1200</td><td>800x838</td><td>560x560</td></tr><tr><th>FOV</th><td>120x120</td><td>120x120</td><td>120x126</td><td>140x140</td></tr><tr><th>Bandwidth (Hz/Px)</th><td>30</td><td>50</td><td>50</td><td>230</td></tr><tr><th>GRAPPA</th><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><th>Number of acquisitions</th><td>2</td><td>1</td><td>1</td><td>1</td></tr><tr><th>Scan time (s)</th><td>4272</td><td>1208</td><td>848</td><td>1197</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>Figure 2: T2*-weighted images with in-plane resolutions of 60, 100, and 150 µm. The sequence parameters are not equivalent between these three acquisitions (cf. Table 1).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3: Zoom on the orbital nerves from the 2D-GRE T₂*-weighted image at 150 µm isotropic resolution.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4: Maximum Intensity Projection (MIP) reconstructed using the isotropic 0.25 mm T1-weighted MPRAGE sequence (cf. Table 1).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(Preview figure)</figcaption></figure>