Toward direct mapping of neuronal activity: MRI detection of ultraweak, transient magnetic fields changes

A novel method based on selective detection of rapidly changing DeltaB(o), magnetic fields and suppression of slowly changing DeltaB(o), fields is presented. The ultimate goal of this work is to present a method that may allow detection of transient and subtle changes in B-o in cortical tissue associated with electrical currents produced by neuronal activity. The method involves the detection of NMR phase changes that occur during a single-shot spin-echo (SE) echo-planar sequence (EPI) echo time. SE EPI effectively rephases all changes in B-o that occur on a time scale longer than the echo time (TE) and amplifies all DeltaB(o) changes that occur during TE/2. The method was tested on a phantom that contains wires in which current can be modulated. The sensitivity and flexibility of the technique was demonstrated by modulation of the temporal position and duration of the stimuli-evoked transient magnetic field relative to the 180 RF pulse in the imaging sequence-requiring precise stimulus timing. Currently, with this method magnetic field changes as small as 2 x 10(-10) T (200 pT) and lasting for 40 msec can be detected. Implications for direct mapping of brain neuronal activity with MRI are discussed.