Echo planar imaging

Echo planar imaging (EPI) is an MRI acquisition methodology with an excellent temporal resolution that is required in specific clinical settings e.g. cardiac imaging.

There are single-shot and multi-shot echo-planar sequences.

Clinical applications

Echo planar imaging is used under the following circumstances:

• cardiac imaging

• abdominal imaging

  • breath-hold sequences

  • 3D MR angiography

• diffusion imaging

• perfusion imaging

• functional imaging

Benefits

• reduced imaging time

• decreased motion artifact

• ability to image rapid physiologic processes of the human body

Drawbacks

• sensitive to susceptibility effects

• sensitive to main magnetic field inhomogeneity

• long echo train length causes greater T2* weighting

• requires high-performance gradients

Technique

Echo planar imaging 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. 

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 e.g. required in cardiac imaging. Each subsequent echo results in a progressively T2-weighted signal.