Having trouble deciding which part of your mouse brain you need to image? Why not monitor activity across the entire brain? The new paper by Prof. Daniel Razansky and colleagues shows exciting optoacoustic methods to image neural activity at depths previously not attainable with the existing optical microscopy techniques.
Optoacoustic (photoacoustic) imaging is a powerful investigation method that can overcome the major drawbacks of conventional optical microscopy techniques that commonly need invasive procedures, have restricted field of view, slow imaging speed and shallow penetration depth. The method uses the advantages of both light and sound. Short-pulsed laser radiation is launched into biological tissue and tiny ultrasound vibrations induced by light absorption are subsequently detected. Because sound scatters through tissue much less than photons, optoacoustic imaging achieves high spatial resolution at centimeter-scale depths, far from reach of the optical imaging. See also ZNZ News Super vision of the brain.
Imaging through skin and skull
In their paper published in Nature Biomedical Engineering, Razansky and co-authors first demonstrated brain-wide 3D optoacoustic imaging of drug-induced calcium waves in an isolated mouse brain preparation expressing the fluorescent calcium sensor GCaMP6f, commonly employed in optical imaging. The optoacoustic results correlated well with fluorescence imaging measurements taken from the superficial brain layers.
The authors then used optoacoustic imaging in living animals to non-invasively detect calcium evoked activity induced by electric stimulation of hind paws of GCaMP6f-expressing mice. They were able to detect rapid neural activity across the entire cortex volumetrically at up to 2 mm depth through the intact skin and skull. The overall brain dynamics was validated with planar fluorescence imaging.
Increasing the depth of penetration
Fluorescent sensors used in optical imaging typically have peak absorption wavelengths in the visible range (~450–600 nm). The main drawback of this spectral range is the strong absorption by blood haemoglobin that limits the depth of penetration of optoacoustic imaging for these sensors and creates undesired background signals.
A possible solution to this problem was presented in a second collaborative paper by the Razansky group where development and application of the first near-infrared-shifted calcium ion indicator (NIR-GECO) was reported. It has excitation and emission maxima above 670 nm, a wavelength range for which absorption by haemoglobin is minimized allowing for a better tissue penetration. In this first proof-of-concept paper, only fluorescence traces were reliably detected while development of an improved variant for optoacoustic imaging is underway.
With these experiments the authors demonstrate that the optoacoustic method is sufficiently sensitive for detecting rapid and distributed sensory responses and set the stage for future advances.
Rapid volumetric optoacoustic imaging of neural dynamics across the mouse brain. Gottschalk S, Degtyaruk O, Mc Larney B, Rebling J, Hutter MA, Deán-Ben XL, Shoham S, Razansky D. Nature Biomed Eng. 2019 May;3(5):392-401. PubMed Abstract
A genetically encoded near-infrared fluorescent calcium ion indicator. Qian Y, Piatkevich KD, McLarney B, Abdelfattah AS, Mehta S, Murdock MH, Gottschalk S, Molina RS, Zhang W, Chen Y, Wu J, Drobizhev M, Hughes TE, Zhang J, Schreiter ER, Shoham S, Razansky D, Boyden ES, Campbell RE. Nature Meth. 2019 Feb;16(2):171-174. PubMed Abstract
Image: Cover of the May issue of Nature Biomedical Engineering: Optoacoustic imaging of neuronal calcium activity across a mouse brain expressing a genetically encoded calcium indicator.