Restraint system with integrated receive array for minimizing head motion during awake marmoset imaging
Kyle M Gilbert1, David J Schaeffer1, Stefan Everling1, and Ravi S Menon1

1Centre for Functional and Metabolic Mapping, The University of Western Ontario, London, ON, Canada


Marmoset monkeys are an increasingly popular animal model for functional MRI studies due to their close homology with humans. To negate the confounds of anesthesia on brain activation, marmosets can be imaged awake. A restraint chair with an integrated receive array is described for minimizing motion during awake imaging. Motion was limited to 129 μm and 0.41°, allowing for comparable temporal SNR with respect to anesthetized imaging.


The marmoset monkey is becoming an increasingly prevalent animal model in translational neuroscience studies1. By imaging animals awake, the confounding effects of anesthesia on the BOLD signal can be avoided2. Awake imaging, however, has technical challenges, as the animal must be sufficiently restrained to prevent head motion during scanning, while restraining the animal must be an efficient process to limit stress to the animal. At present, relatively few examples exist of RF-coil systems intended for awake marmoset imaging3,4. In this study, a restraint system with an integrated receive coil is described that is capable of minimizing motion during functional MRI at 9.4T, while achieving high temporal SNR. The first images acquired of an awake marmoset with this restraint system/coil are presented.


The marmoset was head-fixed by clamping an implanted chamber5 to a restraint system. The restraint system was comprised of a tube for containing the body, a neck restraint, two clamps that could pivot to clamp the marmoset’s chamber, and an adjustable feeding tube and camera (Figure 1). By securing the marmoset by a chamber adhered to the skull, the design allows for future studies combining electrophysiological recordings.

The marmoset was inserted at the rear of the tube; a neck plate and rear restraint plate were then attached to confine the animal. Two side clamps were subsequently moved into place, with a screw on one clamp tightened to securely clamp the chamber. The coil former was attached to the pivoting clamps, so the coil location would be reproducible without any intervention by the handler. The coil former was designed to minimize obstructions to the visual angle of the marmoset.

The receive array (Figure 2) was comprised of five loop elements integrated into the coil former. When tightening the screw to secure the chamber, two conducting posts on one clamp push into two conductive pads on the opposite clamp to electrically connect the element circumscribing the chamber; this provides whole-brain receive sensitivity despite the presence of a chamber. RF transmission was accomplished with a 12-cm-diameter quadrature birdcage coil. Imaging was performed on a Bruker AV3HD console interfaced to a custom 15-cm-diameter gradient coil in a Magnex 9.4-T 31-cm-diameter bore magnet.

Temporal SNR was evaluated on a marmoset, when anesthetized (four 600-volume, 15-minute EPI time series) and when awake (a single 600-volume time series). Head motion during these time series was assessed by registering each volume to the central time-point volume of the respective series using AFNI6. The maximum displacement and rotation of each run was determined—multiple time series were averaged in the case of the anesthetized marmoset.


The mean and maximum S12 between receive elements was -18 dB and -12 dB, respectively. Preamplifier decoupling added a further 10 – 13 dB of isolation, resulting in a mean noise correlation of 19% in vivo. The range in noise level between receive channels was 11%.

Although the chamber placed restrictions on the geometry of the receive coil, five elements could still be placed around the head for whole-brain coverage. An acceleration of two-fold was still possible in the anterior-posterior and left-right directions to reduce the echo train length (and therefore image distortion at 9.4 T). Figure 3 demonstrates the image quality in a single-volume echo-planar image accelerated two-fold in the anterior-posterior direction.

Head motion in the anesthetized marmoset was limited to an average maximum displacement of 221 μm and an average maximum rotation of 0.22°. When awake, the maximum displacement was 129 μm and the maximum rotation was 0.41° (Figure 4). Head motion had a higher variance volume-to-volume when the marmoset was awake, yet gross motion over the time series was similar to the anesthetized state—in both cases, motion was limited to less than a pixel. Minimal motion allowed the temporal SNR of the awake marmoset to be similar to that when anesthetized. Temporal SNR maps are shown in Figure 5.


The restraint system proved to be an efficient and practical solution for securing an awake marmoset and positioning a receive array. The marmoset could be safely restrained, with the receive array in place, within minutes, thereby limiting the stress to the animal. The rigidity of the head restraint minimized motion in awake imaging, resulting in the conservation of temporal SNR when compared to anesthetized imaging.


No acknowledgement found.


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Figure 1. (a) CAD drawing of the body-restraint system, coil positioning/head fixation, camera, and feeding tube. (b) A cross-sectional view showing the coil/neck-plate angle at the back of the head—this allows the marmoset’s arms to extend forward into the cavity below. (c) By raising the neck and end plates and flanging out the side clamps, the marmoset can enter the tube without obstruction. (d) The neck and end plates are then lowered, side clamps locked into place, and the screw tightened to clamp the chamber, as seen in (e).

Figure 2. A photograph of the marmoset coil when (a) open and (b) closed. Two screws are pressed into opposing conductive pads to electrically complete the loop encircling the chamber. The tightening screw (see Figure 1) ensures a solid connection to the pads, thereby preventing spiking artifacts in images. Circuits for matching, active detuning, and balancing are located on boards mounted to the top of the coil former. Low-input-impedance preamplifiers are located behind the animal, so as not to obstruct its view.

Figure 3. The first EPI volume of an awake marmoset acquired with the restraint system/coil. EPI parameters: resolution = 500-μm isotropic, FOV = 64 x 64 mm, slices = 42, TE/TR = 15/1500 ms, flip angle = 40°, bandwidth = 500 kHz, echo spacing = 256 μs.

Figure 4. (a) Rotation and (b) displacement of the marmoset brain during a 15-minute run when the marmoset was anesthetized and awake. The four-point fixation of the chamber, which is rigidly attached to the skull, greatly minimizes motion, even during awake imaging.

Figure 5. Temporal SNR maps of the marmoset when anesthetized and awake. By minimizing motion, the temporal SNR is consistent for anesthetized and awake imaging. In functional studies, increases in brain activation when in the awake state will further improve sensitivity to correlations between networks. EPI parameters: resolution = 500-μm isotropic, FOV = 64 x 64 mm, slices = 42, TE/TR = 15/1500 ms, flip angle = 40°, bandwidth = 500 kHz, echo spacing = 256 μs, volumes = 600, acquisition time = 15 minutes.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)