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Hybrid Volumetric Optoacoustic-Ultrasound Tomography for Noninvasive Large-Scale Recording of Brain Activity with High Spatiotemporal Resolution

Periodic Reporting for period 3 - OPTOACOUSTOGENETICS (Hybrid Volumetric Optoacoustic-Ultrasound Tomography for Noninvasive Large-Scale Recording of Brain Activity with High Spatiotemporal Resolution)

Reporting period: 2019-01-01 to 2020-06-30

Efforts to scale neuroimaging towards the direct visualization of mammalian brain-wide neuronal activity have so far faced major challenges. Although high-resolution optical imaging of the whole brain in small animals has been achieved ex vivo, the realtime and direct monitoring of large-scale neuronal activity remains difficult, owing to the performance gap between localized, largely invasive, optical microscopy of rapid, cellular-resolved neuronal activity and whole-brain macroscopy of slow haemodynamics
and metabolism. In the OPTOACOUSTOGENETICS project we embarked on a new technological development aimed at revolutionizing the way brain signaling is studied. In particular, a new generation of volumetric hybrid optoacoustic-ultrasound imaging technology is developed that can perform rapid volumetric imaging of neural activation in whole brains of rodents with spatial resolution approaching dimension of single neurons.
In the course of the project we were able to demonstrate, for the first time, the fundamental ability of the newly developed functional optoacoustic neuro-tomography approach to detect calcium changes in isolated brains of adult zebrafish labeled with genetically encoded calcium indicator GCaMP5g. Most recently, we also examined fast optoacoustic signatures of calcium indicator GCaMP6f in the densely vascularized and light-scattering mammalian brain. We were able to show that changes in the fluorescence emission of the calcium indicator are directly related to its optoacoustic responses, both in vitro and in vivo. The latter results also demonstrate that the optoacoustic modality is sensitive enough to record sensory-evoked brain activity via calcium-related signal changes in the presence of strong background haemoglobin absorption. Thus, by providing direct volumetric neuroimaging at depths and spatiotemporal resolutions superior to optical fluorescence imaging, the functional optoacoustic neuroimaging bridges the gap between functional microscopy and whole-brain imaging modalities looking at slow hemodynamic changes.
The effective resolution achieved in our initial experiments was in the 150 micron range, still far from the single neuron dimensions. Furthermore, the existing GCaMP indicators as well as their red-shifted versions are all excited at wavelengths below 600nm, which so far restricted the effective in vivo imaging depth to the cortical areas of the mouse brain. In the second half of the project, we aim to achieve real-time volumetric optoacoustic imaging at cellular resolution while retaining large field of
view containing millions of neurons. Furthermore, we will develop and test optimal markers for deep brain optoacoustic monitoring of neural activity excitable in the far-red and near-infrared spectral regions. Finally, we will establish ultimate sensitivity limits of the functional optoacoustic neuro-tomography when monitoring responses due to various stimuli in healthy and diseased animal models.