Echo planar imaging

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 radio frequency 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. 

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. 

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 gradient echo train causes greater T2* weighting
• requires high-performance gradients

Applications

• cardiac imaging
• abdominal imaging
  • breath-hold sequences
  • 3D MR angiography
• diffusion imaging
• perfusion imaging
• functional imaging