Neuronal Current Imaging with Ultra-Low-Field NMR Techniques

Neuronal current imaging (NCI) aims at detecting the influence of neuronal magnetic fields on an NMR signal which might be easier at ultra-low fields (~μT) than at high fields (~T). In the so-called ‘DC effect’, long-lived neuronal activity shifts the Larmor frequency of the surrounding protons and changes the NMR line-shape. An alternative strategy is to use fast neuronal activity as a tipping pulse. This so-called ‘AC effect’ requires the proton Larmor frequency to match the frequency of the neuronal activity. Phantom studies validating both principal working mechanisms are described assessing the feasibility of NCI at ultra-low fields. MRI on phantoms taken at Larmor frequencies of 100 and 731 Hz are also shown and discussed in an attempt to combine the AC effect and ULF MRI. These frequencies are examples of brain activity triggered by electrostimulation of the median nerve.