Observation of the fast response of a magnetic resonance signal to neuronal activity: a snail ganglia study.

We observed the response of a magnetic resonance signal to neuronal activity in dissected snail ganglia to investigate faster responding components than the blood oxygenation level dependent (BOLD) component. To eliminate the BOLD component completely from the magnetic resonance signal, we used dissected snail ganglia which have non-magnetic hemocyanin, other than hemoglobin, as an oxygen carrying protein. To activate the snail ganglia in synchronization with the magnetic resonance signal measurement, we applied 30 Hz electrical stimulation with a pulse width of 2 s to the ganglia just before the 90 degrees RF pulse. The increase of neuronal activity in the snail ganglia after the electrical stimulation was confirmed with extracellular potential recording. The magnetic resonance signal intensity, measured with the volume selection pulse sequence in a 3.0 T MRI, decreased by about 2.97 +/- 1.10% when the ganglia were activated by the electrical stimulation. We think that the MRI signal decrease is partly due to the magnetic field produced by the neuronal current and partly due to the Lorentz force exerted on the neurons when they were firing.