Integrated radiofrequency array and animal holder design for minimizing head motion during awake marmoset functional magnetic resonance imaging

Highlights

• The marmoset is a powerful preclinical model for studying human brain diseases.

• fMRI holds tremendous potential for functional brain mapping in marmosets.

• Awake marmoset fMRI (task-based) is not trivial with little available MRI

• hardware.

• Here, we provide openly available designs allowing for fully awake marmoset fMRI.

Abstract

Marmosets are small New World primates that are posited to become an important preclinical animal model for studying intractable human brain diseases. A critical step in the development of marmosets as a viable model for human brain dysfunction is to characterize brain networks that are homologous with human network topologies. In this regard, the use of functional magnetic resonance imaging (fMRI) holds tremendous potential for functional brain mapping in marmosets. Although possible, implementation of hardware for fMRI in awake marmosets (free of the confounding effects of anesthesia) is not trivial due to the technical challenges associated with developing specialized imaging hardware. Here, we describe the design and implementation of a marmoset holder and head-fixation system with an integrated receive coil for awake marmoset fMRI. This design minimized head motion, with less than 100  μm of translation and 0.5 degrees of rotation over 15 consecutive resting state fMRI runs (at 15 min each) across 3 different marmosets. The fMRI data was of sufficient quality to reliably extract 8 resting state networks from each animal with only 60–90 min of resting state fMRI acquisition per animal. The restraint system proved to be an efficient and practical solution for securing an awake marmoset and positioning a receive array within minutes, limiting stress to the animal. This design is also amenable for multimodal imaging, allowing for electrode or lens placement above the skull via the open chamber design. All computer-aided-design (CAD) files and engineering drawings are provided as an open resource, with the majority of the parts designed to be 3D printed.

Introduction

Marmosets are small New World primates with a mostly smooth (lissencephalic) cortex, offering unique possibilities for studying mechanistic neuron-type and layer-specific circuits supporting primate-specific higher cognitive and sensori-motoric functions not possible in other primate species (Walker et al., 2017). Marmosets are also posited to become an important preclinical animal model for studying intractable human brain diseases, offering advantages over both rodents and Old World non-human primate models. Marmosets, unlike rodents, have a granular dorsolateral prefrontal cortex (Preuss, 1995), a brain area frequently linked to neuropsychiatric disorders (Goldman-Rakic, 1999). As such, while not much larger than a rat (at ∼350 g), the marmoset possesses a brain that parallels the human brain more closely than does the rodent brain. Their small size is advantageous over other larger non-human primate models (e.g., macaques), as it is possible to image the marmoset brain using ultra-high field small-bore magnetic resonance imaging (MRI) scanners typically employed for rodent studies (often allowing for superior signal-to-noise ratio and resolution). Although possible, the implementation of small-bore marmoset MRI is not trivial due to the technical challenges associated with developing specialized imaging hardware (Papoti et al., 2013, 2014, 2017; Gilbert et al., 2017, 2019) – commercially available radiofrequency coils designed for rodents are often not optimized for the significantly larger marmoset head and brain, especially for accelerated echo-planar imaging sequences requiring multi-channel receive arrays (e.g., functional MRI (fMRI), diffusion tensor imaging, arterial spin labelling). Here, we offer open-source hardware designs for an integrated radiofrequency coil and animal holder that allows for stable head-fixation (via a surgically implanted chamber) and high-quality functional imaging (e.g., high temporal signal-to-noise (SNR) ratio, reliable resting state network maps) of awake marmosets at ultra-high field.

A critical step in the development of marmosets as a viable model for human brain dysfunction is to characterize brain networks that are homologous with human network topologies. Although tracing studies have started to provide detailed knowledge of marmoset brain connectivity (see Majka et al., 2016 for atlas) these studies do not provide information about functional interactions amongst regions across the brain. In this regard, the use of fMRI holds tremendous potential for functional brain mapping in marmosets. Indeed, our lab has employed anesthetized resting-state fMRI to demonstrate homologies in functional network topologies across marmosets, macaques, and humans (Ghahremani et al., 2017; Schaeffer et al., 2019a, 2019b). There are some caveats to imaging under anesthesia, however, with mounting evidence to suggest that anesthesia obfuscates the true connectivity profiles of the resting brain (Liu et al., 2013; Hutchison et al., 2014). Mapping these circuitries in fully awake marmosets, therefore, is invaluable for accurately mapping homologous brain circuitries in healthy and altered states (e.g., by way of optogenetic or pharmacological manipulation). Perhaps more importantly, awake marmoset imaging allows for the use of behavioral paradigms during functional imaging (i.e., task-based fMRI), allowing for identification of the circuities underlying marmoset behavior (and the manipulation thereof).

In recent years, clever hardware designs (Meyer et al., 2006; Papoti et al., 2013, 2014, 2017) have allowed for awake marmoset fMRI, and major in-roads have been made into mapping awake marmoset circuitry (Belcher et al., 2013, 2016; Liu et al., 2013; Hung et al., 2015b, 2015a; Silva, 2017; Toarmino et al., 2017; Hirano et al., 2018; Yen et al., 2018). The majority of these published studies, however, utilize custom-designed helmet coils (as described in Papoti et al., 2013; Silva et al., 2011) – this system is well conceived and ideal for truly non-invasive imaging, but requires a custom helmet to be built for each animal. Here, we present a more universal system, making use of a surgically implanted head chamber that allows for awake fMRI in stereotactic position while also being compatible (in principle) with simultaneous electrophysiological recoding in the MRI scanner. This same implanted head chamber is also compatible with existing published designs for upright behavioral training and electrophysiological recordings in a stereotactic frame (Johnston et al., 2018, 2019). To demonstrate the utility of the design, we present quality assessments from the coil (coil coupling, noise correlations, and SNR), estimates of head motion, and resting state data (independent component analysis (ICA) extracted resting state networks) from three marmosets. We have made all of the computer-aided-design files publicly available (https://web.gin.g-node.org/everling_lab_marmosets). The majority of the components (aside from the electronics and nylon hardware) can be three-dimensionally (3D) printed using MRI-compatible material at very low cost.