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 180o RF 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 as illustrated in the diagram to the right. 

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 msec, 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 gradient echo train causes greater T2* weighting
  • requires high-performance gradients
  • cardiac imaging
  • abdominal imaging, i.e. breath-hold sequences and 3D MR angiography
  • diffusion imaging
  • perfusion imaging
  • functional imaging
Physics and Imaging Technology: MRI

Factors affecting T1

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Article information

rID: 19116
Section: Physics
Tag: epi, mri, refs
Synonyms or Alternate Spellings:
  • Echo-planar imaging
  • Echo planar imaging (EPI)

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