"Monitoring electromagnetic signals in the brain with MR

Miniature antennas/coils like the ones used for MRI, implantable, detect electromagnetic signals

"MRI works by detecting radio waves emitted by the nuclei of hydrogen atoms in water".

"The sensor is initially tuned to the same frequency as the radio waves emitted by the hydrogen atoms. When the sensor picks up an electromagnetic signal from the tissue, its tuning changes and the sensor no longer matches the frequency of the hydrogen atoms. When this happens, a weaker image arises when the sensor is scanned by an external MRI machine.

The researchers demonstrated that the sensors can pick up electrical signals similar to those produced by action potentials (the electrical impulses fired by single neurons), or local field potentials (the sum of electrical currents produced by a group of neurons)".

Paper: https://www.nature.com/articles/s41551-018-0309-8