A geometrically adjustable receive array for imaging marmoset cohorts

Design and application of a multimodality-compatible 1Tx/6Rx RF coil for monkey brain MRI at 7T

OBJECTIVE

Blood-oxygen-level-dependent functional MRI allows to investigte neural activities and connectivity. While the non-human primate plays an essential role in neuroscience research, multimodal methods combining functional MRI with other neuroimaging and neuromodulation enable us to understand the brain network at multiple scales.

APPROACH

In this study, a tight-fitting helmet-shape receive array with a single transmit loop for anesthetized macaque brain MRI at 7T was fabricated with four openings constructed in the coil housing to accommodate multimodal devices, and the coil performance was quantitatively evaluated and compared to a commercial knee coil. In addition, experiments over three macaques with infrared neural stimulation (INS), focused ultrasound stimulation (FUS), and transcranial direct current stimulation (tDCS) were conducted.

MAIN RESULTS

The RF coil showed higher transmit efficiency, comparable homogeneity, improved SNR and enlarged signal coverage over the macaque brain. Infrared neural stimulation was applied to the amygdala in deep brain region, and activations in stimulation sites and connected sites were detected, with the connectivity consistent with anatomical information. Focused ultrasound stimulation was applied to the left visual cortex, and activations were acquired along the ultrasound traveling path, with all time course curves consistent with pre-designed paradigms. The existence of transcranial direct current stimulation electrodes brought no interference to the RF system, as evidenced through high-resolution MPRAGE structure images.

SIGNIFICANCE

This pilot study reveals the feasibility for brain investigation at multiple spatiotemporal scales, which may advance our understanding in dynamic brain networks.

A radiofrequency coil to facilitate task-based fMRI of awake marmosets

BACKGROUND

The small common marmoset (Callithrix jacchus) is an ideal nonhuman primate for awake fMRI in ultra-high field small animal MRI scanners. However, it can often be challenging in task-based fMRI experiments to provide a robust stimulus within the MRI environment while using hardware (an RF coil and restraint system) that is compatible with awake imaging.

NEW METHOD

Here we present an RF coil and restraint system that permits unimpeded access to an awake marmoset's head subsequent to immobilization, thereby permitting the setup of peripheral devices and stimuli proximal to the head.

RESULTS

As an example application, an fMRI experiment probing whole-brain activation in response to marmoset vocalizations was conducted-this paradigm showed significant bilateral activation in the inferior colliculus, medial lateral geniculate nucleus, and auditory cortex.

COMPARISON WITH EXISTING METHOD(S)

The coil performance was evaluated and compared to a previously published restraint system with integrated RF coil. The image and temporal SNR were improved by up to 58 % and 27 %, respectively, in the peripheral cortex and by 30 % and 3 % in the centre of the brain. The restraint-system topology limited head motion to less than 100 µm of translation and 0.30° of rotation when measured over a 15-minute acquisition.

CONCLUSIONS

The proposed hardware solution provides a versatile approach to awake-marmoset imaging and, as demonstrated, can facilitate task-based fMRI.

Optimization of a quadrature birdcage coil for functional imaging of squirrel monkey brain at 9.4T

A quadrature transmit/receive birdcage coil was optimized for squirrel monkey functional imaging at the high field of 9.4 T. The coil length was chosen to gain maximum coil efficiency/signal-to-noise ratio (SNR) and meanwhile provide enough homogenous RF field in the whole brain area. Based on the numerical simulation results, a 16-rung high-pass birdcage coil with the optimal length of 9 cm was constructed and evaluated on phantom and in vivo experiments. Compared to a general-purpose non-optimized coil, it exhibits approximately 25% in vivo SNR improvement. In addition to the volume coil, details about how to design and construct the associated animal preparation system were provided.

Magnetic Resonance Imaging of Marmoset Monkeys

The use of the common marmoset monkey (Callithrix jacchus) for neuroscientific research has grown markedly in the last decade. Magnetic resonance imaging (MRI) has played a significant role in establishing the extent of comparability of marmoset brain architecture with the human brain and brains of other preclinical species (eg, macaques and rodents). As a non-invasive technique, MRI allows for the flexible acquisition of the same sequences across different species in vivo, including imaging of whole-brain functional topologies not possible with more invasive techniques. Being one of the smallest New World primates, the marmoset may be an ideal nonhuman primate species to study with MRI. As primates, marmosets have an elaborated frontal cortex with features analogous to the human brain, while also having a small enough body size to fit into powerful small-bore MRI systems typically employed for rodent imaging; these systems offer superior signal strength and resolution. Further, marmosets have a rich behavioral repertoire uniquely paired with a lissencephalic cortex (like rodents). This smooth cortical surface lends itself well to MRI and also other invasive methodologies. With the advent of transgenic modification techniques, marmosets have gained significant traction as a powerful complement to canonical mammalian modelling species. Marmosets are poised to make major contributions to preclinical investigations of the pathophysiology of human brain disorders as well as more basic mechanistic explorations of the brain. The goal of this article is to provide an overview of the practical aspects of implementing MRI and fMRI in marmosets (both under anesthesia and fully awake) and discuss the development of resources recently made available for marmoset imaging.

A 16-Channel Dense Array for In Vivo Animal Cortical MRI/fMRI on 7T Human Scanners

OBJECTIVE

The purpose of the present study was to fabricate a novel RF coil exclusively for visualizing submillimeter tissue structure and probing neuronal activity in cerebral cortex over anesthetized and awake animals on 7T human scanners.

METHODS

A novel RF coil design has been proposed for visualizing submillimeter tissue structure and probing neuronal activity in cerebral cortex over anesthetized and awake animals on 7T human scanners: a local transmit coil was utilized to save space for auxiliary device installation; 16 receive-only loops were densely arranged over a 5 cm-diameter circular area, with a diameter of 1.3 cm for each loop.

RESULTS

In anesthetized macaque experiments, 60 μm T₂^*-weighted images were successfully obtained with cortical gyri and sulci exquisitely visualized; over awake macaques, bilateral activations of visual areas including V1, V2, V4, and MST were distinctly detected at 1 mm; over the cat, robust activations were recorded in areas 17 and 18 (V1 and V2) as well as in their connected area of lateral geniculate nucleus (LGN) at 0.3 mm resolution.

CONCLUSION

The promising brain imaging results along with flexibility in various size use of the presented design can be an effective and maneuverable solution to take one step close towards mesoscale cortical-related imaging.

SIGNIFICANCE

High-spatial-resolution brain imaging over large animals by using ultra-high-field (UHF) MRI will be helpful to understand and reveal functional brain organizations and the underlying mechanism in diseases.

Design and construction of a gradient coil for high resolution marmoset imaging

A gradient coil with integrated second and third order shims has been designed and constructed for use inside an actively shielded 310 mm horizontal bore 9.4 T small animal MRI. An extension of the boundary element method, to minimise the power deposited in conducting surfaces, was used to design the gradients, and a boundary element method with a constraint on mutual inductance was used to design the shims. The gradient coil allows for improved imaging performance and was optimized for an imaging region appropriate for marmoset imaging studies. Efficiencies of 1.5 mT m^-1 A^-1 were achieved in a 15 cm wide bore while maintaining gradient uniformity ≤5% over the 8 cm region of interest. Two new cooling methods were implemented which allowed the gradient coil to operate at 100 A RMS, 25 % of max current with a temperature rise below 30 C.

Reproducibility of Neurite Orientation Dispersion and Density Imaging (NODDI) in rats at 9.4 Tesla

PURPOSE

Neurite Orientation Dispersion and Density Imaging (NODDI) is a diffusion MRI (dMRI) technique used to characterize tissue microstructure by compartmental modelling of neural water fractions. Intra-neurite, extra-neurite, and cerebral spinal fluid volume fractions are measured. The purpose of this study was to determine the reproducibility of NODDI in the rat brain at 9.4 Tesla.

METHODS

Eight data sets were successfully acquired on adult male Sprague Dawley rats. Each rat was scanned twice on a 9.4T Agilent MRI with a 7 ± 1 day separation between scans. A multi-shell diffusion protocol was implemented consisting of 108 total directions varied over two shells (b-values of 1000 s/mm2 and 2000 s/mm2). Three techniques were used to analyze the NODDI scalar maps: mean region of interest (ROI) analysis, whole brain voxel-wise analysis, and targeted ROI analyses (voxel-wise within a given ROI). The coefficient of variation (CV) was used to assess the reproducibility of NODDI and provide insight into necessary sample sizes and minimum detectable effect size.

RESULTS

CV maps for orientation dispersion index (ODI) and neurite density index (NDI) showed high reproducibility both between and within subjects. Furthermore, it was found that small biological changes (<5%) may be detected with feasible sample sizes (n < 6-10). In contrast, isotropic volume fraction (IsoVF) was found to have low reproducibility, requiring very large sample sizes (n > 50) for biological changes to be detected.

CONCLUSIONS

The ODI and NDI measured by NODDI in the rat brain at 9.4T are highly reproducible and may be sensitive to subtle changes in tissue microstructure.

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

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.

A surface loop array for in vivo small animal MRI/fMRI on 7T human scanners

Small animals such as non-human primate (NHP) and rodent are valuable models in frontier neuroscience researches, and comparative research between the animal model and human is helpful to understand and reveal the functional brain circuits in cognition and underlying mechanism in psychological disease. Small animals can be trained or anesthetized to endure long-term and multiple imaging scans; however, the concomitant needs in subcortical structure and function investigations pose major challenges in, e.g. spatial resolution, scan time efficiency, spatial/temporal signal-to-noise-ratio, as well as apparatus mechanical fixation. In addition, comparative investigations across species are also expected to be conducted under similar physical environment (such as the main magnetic field strength, RF pulse shape, sequence protocols, gradient waveform, system stability, etc in MRI), in order to avoid possible deviation in signal detection under different platforms, as well as to reduce experiment complexity. We have proposed a novel 5-channel surface coil that is equipped on 7T human MRI scanners and designed for small animal structural and functional MRI. Through a series of in vivo experiments over an anesthetized rat and macaque, the presented coil shows its main characteristics in, i.e. flexible coil mounting, reduced FOV, high temporal SNR, and parallel imaging capability. Such design is able to compensate the relatively lower gradient slew rate of human scanners versus those with smaller bores, and thus effectively facilitates in vivo microscopic structural MR images being obtained within a shortened and safe period of anesthesia; besides, it also enables high-resolution functional MRI (i.e. spin-echo based) being achieved with reasonable temporal resolution, distortion level and functional contrast.

Investigating the Influence of Spatial Constraints on Ultimate Receive Coil Performance for Monkey Brain MRI at 7 T

The RF receive coil array has become increasingly vital in current MR imaging practice due to its extended spatial coverage, maintained high SNR, and improved capability of accelerating data acquisition. The performance of a coil array is intrinsically determined by the current patterns generated in coil elements as well as by the induced electromagnetic fields inside the object. Investigations of the ultimate performance constrained by a specific coil space, which defines all possible current patterns flowing within, offer the opportunity to evaluate coil-space parameters (i.e., coverage, coil-to-object distance, layer thickness, and coil element type) without the necessity of considering the realistic coil element geometry, coil elements layout, and number of receive channels in modeling. In this paper, to mimic 7-T monkey RF head coil design, seven hypothetical ultimate coil arrays with different coil-space configurations were mounted over a numerical macaque head model; by using Huygens's surface approximation method, the influences of coil-space design parameters were systematically investigated through evaluating the spatial constrained ultimate intrinsic SNR and ultimate g-factor. Moreover, simulations were also conducted by using four coil arrays with limited number of loop-only elements, in order to explore to what extent the ultimate coil performance can be achieved by using practical coil designs, and hence several guidelines in RF coil design for monkey brain imaging at 7 T have been tentatively concluded. It is believed that the present analysis will offer important implications in novel receive array design for monkey brain MR imaging at ultra-high field.

Concentric radiofrequency arrays to increase the statistical power of resting-state maps in monkeys

The close homology of monkeys and humans has increased the prevalence of non-human-primate models in functional MRI studies of brain connectivity. To improve upon the attainable resolution in functional MRI studies, a commensurate increase in the sensitivity of the radiofrequency receiver coil is required to avoid a reduction in the statistical power of the analysis. Most receive coils are comprised of multiple loops distributed equidistantly over a surface to produce spatially independent sensitivity profiles. A larger number of smaller elements will in turn provide a higher signal-to-noise ratio (SNR) over the same field of view. As the loops become physically smaller, noise originating from the sample is reduced relative to noise originating from the coil. In this coil-noise-dominated regime, coil elements can have overlapping sensitivity profiles, yet still possess only mildly correlated noise. In this manuscript, we demonstrate that inductively decoupled, concentric coil arrays can improve temporal SNR when operating in the coil-noise-dominated regime-in contrast to what is expected for the more ubiquitous sample-noise-dominated array. A small, thin, 7-channel flexible coil is developed and operated in conjunction with an existing whole-head monkey coil. The mean and maximum noise correlation between the two arrays was 5% and 23%, respectively. When the flex coil was placed over the sensorimotor cortex, the temporal SNR improved by up to 2.3-fold in the peripheral cortex and up to 1.3-fold at a 2- to 3-cm depth within the brain. When the flex coil was placed over the frontal eye fields, resting-state maps showed substantially elevated sensitivity to correlations in the prefrontal cortex (54%), supplementary eye fields (39%), and anterior cingulate cortex (41%). The concentric-coil topology provided a pragmatic and robust means to significantly improve local temporal SNR and the statistical power of functional connectivity maps.