October 3rd, 2014 – Our work on Functional Ultrasound Imaging of intrinsic connectivity in the living rat Brain is published in Nature Communications
The results presented in this manuscript provide the first evidence of fUltrasound as a new neuroimaging modality for the study of small animal whole brain functional connectivity.
The brain dynamically integrates and coordinates the respose to internal and external stimuli across multiple spatiotemporal scales, through large-scale functional networks. Assessment of functional connectivity (FC) with functional magnetic resonance imaging (fMRI), particularly during resting-state/task-free periods, has greatly advanced our understanding of functional brain organization (Hutchison et al., NeuroImage 2013 80). Intrinsic connectivity networks, such as the default-mode network (Raichle et al. 2001, PNAS 98; Buckner et al. 2008, Ann N Y Acad Sci 1124), ventral and dorsal attention networks (Corbetta and Shulman 2002 Nat Rev Neurosci 3; Fox et al. 2006 PNAS 103), and salience network (Seeley et al. 2007 J Neurosci 27), have been intensely studied in basic and clinical cognitive neuroscience.
Most, if not all, physiological and psychiatric diseases disrupt large-scale functional and/or structural properties (Greicius, 2008 Curr Opin Neurol 21; Menon, 2011 Trends Cogn Sci 15). These include major pathologies such as schizophrenia, depression, and Alzheimer’s disease. Consequently, investigation of FC in well-controlled and accessible experimental conditions is of major scientific interest since it may lead, through a better understanding of the disorders, to better diagnostic or prognostic indicators and to more targeted drug treatment. Thus, development of translational rodent models elicits great interest and recent reports show the presence of eminent intrinsic networks, par example in the monkey (Vincent et al. 2008 Nature 447) and rat brain (Lu et al. PNAS 2012 vol. 109).
To date only fMRI is capable to image intrinsic brain networks with appropriate spatial resolution. However, MRI-based approaches face major challenges, such as necessity for very high magnetic fields and issues of motion artifacts, electromagnetic compatibility, cost and portability. These concerns seriously hinder the broad dissemination of FC research into translational pre-clinical settings.
Recently, our group demonstrated that very high frame rate ultrasound imaging enables the emergence of high resolution and high sensitivity Doppler imaging of blood flow deep into the rodent brain, leading to functional ultrasound (fUS) imaging of brain activity. However, this study used highly invasive craniotomy, which is not optimal for the imaging of resting-state functional networks. Indeed, frequent post-craniotomy complications in humans include moderate to severe pain, swellings and seizures. In consequence, up to date, we did not know whether fUS was capable of mapping intrinsic FC patterns and we did not know the relative performance of this method as compared to the standard fMRI-based approach.
Here we developed a large thinned-skull cranial window preparation and an ultrasonic sequence for ultrafast imaging of intrinsic FC of the living adult rat brain. This approach enabled us to identify highly-contrasted intrinsic functional connectivity patterns. Our method represents a simple, portable, cost and space effective, highly resolved and motion-artifact free method, capable to image functional connectivity deep into the rodent brain.
The manuscript demonstrates several major features:
– Both seed-based and singular value decomposition analysis of the spatial coherence in the low frequency (<0.1 Hz) spontaneous fUS signal fluctuations reproducibly found, at different coronal planes, high-contrast intrinsic functional connectivity patterns. These patterns corresponded to known major functional networks, such as the task-dependent lateral sensorimotor network, which was temporally anticorrelated with the prominent midline hubs of the default-mode network.
– The reproducibility of functional connectivity patterns between animals (N=6) is very high (r= 0.85+-0.03, p<0.001).
– Due to the unprecedented 100 µm x 100 µm in-plane spatial resolution, our technique shows an improved spatial delineation of the rodent brain functional networks, such as a differential involvement of primary and secondary motor cortices.
– Finally, whereas the fluctuations of resting-state in fMRI can be affected by cardio-respiratory motion, the millisecond temporal resolution of fUS allowed unambiguous cancellation of such low-frequency artifacts.
These results reveal fUS as a full-fledged novel neuroimaging method for functional connectivity mapping. Most importantly, major upcoming implications are envisioned as this approach can straightforwardly be extended to portable probes for functional connectivity imaging in awake and freely moving rodents both in normal and pathological conditions.
Functional Ultrasound Imaging of Intrinsic Connectivity in the Living Rat Brain. B.F. Osmanski, S. Pezet, A. Ricobaraza, Z. Lenkei, M. Tanter. Nature Communications, October 3rd, 2014
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