Echo planar imaging
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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.
- 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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