Temporal resolution of fMRI is limited by: (1) the feedback mechanism that raises the blood flow operating slowly; (2) having to wait till net magnetization recovers before sampling a slice again; and (3) having to acquire multiple slices to cover the whole brain or region of interest.
Typical gradient-echo EPI uses two gradient coils within a slice, and turns on first one coil and then the other, tracing a set of lines in k-space.