Echo Planar Imaging
Rapid changes of frequency-encoding gradient polarity to refocus the MR echo many times after a single RF excitation, with a small change in phase encoding for each echo to read out different lines of k-space.
EPI is one of the fastest methods of acquiring MRI k-space data. If all data for an image is acquired after one RF excitation, it is single-shot EPI, in which each slice is acquired in one TR.
“The breakthrough came in 1977 with the introduction of echo-planar imaging (EPI). This snap-shot technique meant that in principle complete two-dimensional images could be achieved in extremely short times ranging from 20–50 ms. However, to achieve these acquisition times, yet another inventive step was required. This was the introduction of active magnetic screening.”Sir Peter Mansfield, Nobel lecture, 2003