Synopsis

Keywords: Contrast mechanisms: fMRI

There has been a longstanding demand for noninvasive neuroimaging with high spatiotemporal resolution. Recently, an approach has been proposed that enables Direct Imaging of Neuronal Activity (DIANA) with milliseconds temporal resolution, demonstrated by in vivo mouse brain imaging at 9.4T. DIANA showed high correlations with neuronal spikes, capturing neuronal-activity propagation along the thalamocortical pathway. The DIANA contrast mechanism may be attributed to changes in membrane potential-associated T2 relaxation time. Finally, DIANA may require different considerations in data acquisition and analysis than BOLD-fMRI, as DIANA is directly related to neuronal activity including spontaneous ongoing activity as well as responses to stimuli.

Advanced noninvasive neuroimaging methods provide valuable information on the brain function, but they have obvious pros and cons in terms of temporal and spatial resolution. Functional magnetic resonance imaging (fMRI) using blood-oxygenation-level-dependent (BOLD) effect provides good spatial resolution in the order of millimeters, but has a poor temporal resolution in the order of seconds due to slow hemodynamic responses to neuronal activation, providing indirect information on neuronal activity. In contrast, electroencephalography (EEG) and magnetoencephalography (MEG) provide excellent temporal resolution in the millisecond range, but spatial information is limited to centimeter scales. Therefore, there has been a longstanding demand for noninvasive brain imaging methods capable of detecting neuronal activity at both high temporal and spatial resolution. In this talk, I will introduce a novel approach that enables Direct Imaging of Neuronal Activity (DIANA) using MRI that can dynamically image neuronal spiking activity in milliseconds precision¹, achieved by data acquisition scheme of rapid 2D line scan synchronized with periodically applied functional stimuli. DIANA was demonstrated through in vivo mouse brain imaging on a 9.4T animal scanner during electrical whisker-pad stimulation. DIANA with milliseconds temporal resolution had high correlations with neuronal spike activities, which could also be applied in capturing the sequential propagation of neuronal activity along the thalamocortical pathway of brain networks. In terms of the contrast mechanism, DIANA was almost unaffected by hemodynamic responses, but was subject to changes in membrane potential-associated T2 relaxation time. Finally, it will be discussed that DIANA may require some different considerations in data acquisition and analysis than BOLD-fMRI, as DIANA is directly related to neuronal activity and it appears that spontaneous ongoing neuronal activity^2,3 as well as deterministic responses to stimuli contribute to DIANA signal. For example, averaging over more runs may not guarantee higher sensitivity of the DIANA response. Also, spontaneous ongoing neuronal activity seems to be significantly reduced averaging across subjects.

Acknowledgements

Brain Research Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT (Project ID: NRF-2019M3C7A1031993).