Exploring human brain neuronal currents with phase MRI

Purpose: Brain activity‐associated neuronal currents produce weak transient magnetic fields that would affect both magnitude and phase of the local MRI signal, but these very small signal changes are not reliably detectable with conventional fMRI methodologies. A recent simulation study, using a realistic model specifically for human cerebral cortex, indicates that the phase signal change induced by spontaneous activity may reach a detectable level (up to 0.2°) in favorable conditions. This study aimed to investigate neuronal current‐induced signal changes in human visual cortex with phase MRI. Materials and Methods: Six healthy subjects participated in a phase fMRI study using a temporally well‐controlled visual stimulation paradigm with a known neuronal firing pattern in visual cortex. The precise timing of the paradigm provides a means of detecting and testing the neuronal current‐induced phase signal changes, and placing a series of acquisition windows to fully cover the entire response duration enables a thorough detection of any detectable phase signal changes induced by the stimulus‐evoked neuronal currents. Results: The presented phase MRI method demonstrated to be reliable, and the improved phase measure has achieved a sensitivity level of 0.2° for detecting any significant phase signal changes under a practical length of fMRI session. The test found no sign of any significant neuronal current‐induced phase signal changes in any subject and study. Conclusions: Under the experimental condition, the upper limit of the neuronal current‐induced phase signal changes was found to be less than 0.2° in the human visual cortex, consistent with the model prediction.