1. Functional activation mapping
Mapping a stimulus
Functional ultrasound neuro imaging has been used to map the brain response to external stimuli. We realized very early in our seminal study on anesthetized rats (Mace et al, Nature Methods, 2011) , where we used functional ultrasound to map the response of the brain to whisker stimulation (including single whisker stimulation) that functional ultrasound imaging is a very sensitive neuroimaging modality with excellent spatial resolution.
Functional ultrasound (fUS) imaging evoked in the left (e) or right (f) somatosensory cortex, hindlimb part (S1HL) using electrical stimulation (5 Hz, 0.2 mA, 100 μs width for 10 s, separated by 20 s) of the right (e) or left (f) sciatic nerve applied either on the right (e) or left (f) side, respectively. (g) Time course changes in the evoked fUS response in the left S1HL following stimulation of the contralateral sciatic nerve. From: Osmanski et al, Nature Communications, 2014 - Open Access - Courtesy B. Osmanski
Functional activations can be used as a first control to validate probe positioning and animal response before more complex functional experiments. For instance, researchers using Iconeus technology are now quantifying functional response to standard stimuli to study genetically- or pharmacologically-induced alterations in the neurovascular coupling and pericyte functions (unpublished data).
Retinotopic and tonotopic maps
The high spatial resolution of the fUS technique has allowed to create detailed retinotopic maps of the rat, mouse and even awake primates performing complex visual task.
In the recent awake mice study of the Roska lab, Emilie Macé, alumnus of the Tanter lab, has shown that functional ultrasound could reveal 87 brain structures involved in the optokinetic reflex alone (Mace et al., Neuron, 2018).
Researchers have also began to study audition in awake ferrets, an eminent model for audition research. By mapping the response to auditory stimuli with different frequency, they have been able to construct highly-detailed sonotopic maps and to demonstrate that neighboring pixels at 100 micrometers distance show independant auditory response curves, further demonstrating the high resolution of functional ultrasound (Bimbard et al, eLife, 2018).
fUS imaging reveals the tonotopic organization of cortical, sub-cortical, and intracortical auditory structures in the awake ferret. (a) Left: UFD-T of the left and right craniotomies, superimposed on an MRI scan of a ferret brain. Right: magnification of the blue bounding box (left). Auditory structures: auditory cortices (AC), medial geniculate body (MGB), inferior colliculus (IC). Other structures: hippocampus (Hip), visual cortex (VC). (b) Structural view of a tilted parasagittal slice (~30° from D-V axis) of the visual and auditory cortices (represented as a blue plane on the 3D brain). Lining delineates the cortex. (c) Upper left: Tonotopic organization of the slice described in (b). Lower left: tuning curve (mean ± sem) and average responses in %CBV (see Materials and methods) for the voxel located in the upper panel (black cross). Upper right: combination of 16 similar slices over the surface of the AC, arrow depicts slice of (b). AEG/MEG/PEG: anterior/middle/posterior ectosylvian gyrus. Lower right: 3D reconstruction of the whole AC’s functional organization. Bimbard et al, eLife, 2018, CC-BY 4.0
Mapping activity in a behaving primate
In behaving non-human primates, researchers have shown how functional ultrasound can be used to map the regions involved in complex task and rule handling in the Supplementary Eye Field (SEF), by following the regions response trial-by-trial with high sensitivity. fUS is able to assess local changes in cerebral blood volume during cognitive tasks, with sufficient temporal resolution to measure the directional propagation of signals. In two macaques, they observed an abrupt transient change in supplementary eye field (SEF) activity when animals were required to modify their behaviour associated with a change of saccade tasks. Notably, SEF activation could be observed in a single trial, without averaging (Dizeux et al., Nature Communications, 2019).
Functional ultrasound neuro imaging is now also being investigated in non-human primates for its high potential for Brain Computer Interfaces (BCI).
Mapping odour responses with much higher sensitivity and resolution than BOLD fMRI
Finally, the Charpak lab provided recently an extensive comparison between calcium imaging, bold-fMRI at very high field (17.2 Tesla) and functional ultrasound neuro imaging in the same animal. By imaging functional responses in the mice olfactory bulb during odor presentation, researchers conclude that functional ultrasound “is a very efficient technique for measuring mesoscopic vascular responses. Its high SNR and temporal resolution allows the generation of voxel-based correlation maps even at low-odour concentration.” whereas the very same experiment “ was not ideal for BOLD-fMRI mapping, due to the lower SNR and temporal resolution of our BOLD-fMRI protocol.” (Boido et al, Nature Communication, 2019).