Object ID: 00000018WIA30403970GYZ
Topic ID: id_40025923 Version: 1.4
Date: Mar 2, 2022 1:10:36 PM

EPI

EPI uses multiple oscillating gradient pulses within a TR period to create the echo. EPI is similar to an FSE sequence, except it uses gradients (which take less time to turn on and off) instead of RF pulses to produce echoes. The efficiency of a gradient versus an RF pulse to produce the echo results in reduced SAR and increased number of slices in comparison to an FSE acquisition. Like FSE, EPI uses an ET to produce numerous echoes within a TR period filling k-space rapidly and thus dramatically reducing scan time.

Considerations

When fat signal is not properly suppressed, it shows up with a large spatial shift in phase encoding direction due to the low bandwidth in phase encoding direction. The fat signal is suppressed by default using spatial spectral RF pulses.
Areas near the boundary of two regions with different magnetic susceptibility, e.g. water and air, cause shifts in the B0 field which often results in geometric distortion and signal loss in the EPI images.
All EPI pulse sequences are sensitive to field inhomogeneities (opposed to an FSE which virtually eliminates those effects). Therefore, pathologies that cause disruptions in the local magnetic field have a higher potential for contrast visualization in an EPI image.