Echo planar imaging

Last revised by Zemar Vajuhudeen on 1 Mar 2022

Echo planar imaging (EPI) is performed using a pulse sequence in which multiple echoes of different phase steps are acquired using rephasing gradients instead of repeated 180-degree radiofrequency pulses following the 90°/180° in a spin echo sequence. This is accomplished by rapidly reversing the readout or frequency-encoding gradient. This switching or reversal may also be done in a sinusoidal fashion. Echo planar sequences may use entirely gradient echoes or may combine a spin-echo with the train of gradient echoes. 

In a single-shot echo-planar sequence, the entire range of phase encoding steps, usually up to 128, are acquired in one TR. In multi-shot echo-planar imaging, the range of phase steps is equally divided into several "shots" or TR periods. For example, an image with 256 phase steps could be divided into 4 shots of 64 steps each. 

Of note, although echo-planar imaging is considered a gradient echo based technique, some fast spin-echo acquisitions such as HASTE and SS-FSE can be categorized as multi-shot echo planar imaging.

As a result, an image can be acquired in 20-100 ms allowing excellent temporal resolution such as that required in cardiac imaging. Each subsequent echo results in a progressively T2-weighted signal. 

  • reduced imaging time
  • decreased motion artifact
  • ability to image rapid physiologic processes of the human body
  • sensitive to susceptibility effects
  • sensitive to main magnetic field inhomogeneity
  • long echo train length causes greater T2* weighting
  • requires high-performance gradients

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