In a nutshell: Electrical impedance tomography could be used to measure brain activity in previously difficult-to-access areas.

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Information in the brain is transmitted via electrical signals. To send or receive these signals, brain cells change their conductivity. By mapping changes in the levels of conductivity in the brain, scientists can learn how different parts of the brain communicate with one another and interact.

Electrical impedance tomography (EIT) is a new medical imaging technique that measures conductivity using arrays of electrodes. It has been used successfully in shallow areas of the brain, close to the skull, such as the cortex – the thick layer of cells that covers the brain’s surface.

However, EIT has not yet been shown to work as effectively in deeper brain structures, such as the thalamus – which sends motor and sensory signals to the cortex. Measuring activity further inside the brain could be useful for patients with deep brain implants, for use in conditions such as Parkinson’s disease.

To test whether EIT might be able to measure brain conductivity in the thalamus using electrodes implanted in the brain, Brain Function CoE researcher Calvin Eiber and his colleagues at the University of Sydney built an anatomically accurate brain model. Using this model, the researchers simulated how EIT would work to detect changes in groups of brain cells.

They looked at how large an area could be mapped using EIT, and whether or not conductivity changes in multiple areas of the brain could be measured simultaneously.

They found that EIT could reliably detect conductivity changes in brain cells up to around 300 micrometres away from the electrodes. This may not seem like much – it’s about three times the thickness of a human hair – but this sensitivity is similar or better than existing techniques, and could be used to measure activity in the thalamus. The researchers also showed that EIT could detect activity in multiple areas of the brain at the same time.

These results help to bring EIT into the clinic as a promising new technique for visualizing activity in deep brain structures, which are otherwise difficult to access.

Next steps:
The team is planning to set up a real EIT system to collect measurements of brain activity. They are first measuring a relatively well understood part of the brain using existing methods, so they can compare these values against those obtained using an implanted electrode array.

Zhu, D., McEwan, A., & Eiber, C. (2019). Microelectrode array electrical impedance tomography for fast functional imaging in the thalamus. NeuroImage, 198, 44–52. doi: 10.1016/j.neuroimage.2019.05.023

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