Echo Planar Imaging

Characterized by a series of rapid gradient reversals by the readout gradient

Echoplanar imaging is a gradient echo technique related to fast gradient echo imaging. Instead of one to 16 phase steps per acquisition TR, the entire set of 64 or 128 phase steps is acquired during one acquisition TR. This is accomplished by rapidly reversing the readout or frequency- encoding gradient. This switching or reversal may also be done in a sinusoidal fashion. Echoplanar sequences may use entirely gradient echos or may combine a spin echo with the train of gradient echos as illustrated in the diagram below. Echoplanar images may be acquired in less that 1/10th of a second and therefore may be useful in cardiac imaging and other rapidly changing processes.

The T2 of many spins of interest ranges from 50 to 100 msec. In other words, after an excitation RF pulse, there is measurable (but decaying) signal for about 100 msec. In contrast, one set of frequency-encoded measurements (at one fixed phase encoding) can be collected in a few msec or less. If short imaging time is paramount, it is inefficient to measure the excited signal for a small fraction of the time that it is available. Rather than leaving the phase-encoding twist fixed during a readout while collecting only one complete set of frequency-encoding twists, the magnetic field gradient in the frequency-encoding gradient can be oscillated (producing the same complete set of twists multiple times). Concomitantly, the phase-encoding twist is altered slightly between each frequency-encoding gradient oscillation by a very small application of the phase-encoding field gradient. The entire set of waves (single-shot echo-planar imaging, EPI) or some subset of them (multi-shot EPI) can thereby be measured after a single excitation (in a single TR).[5] The advantage is that all of the available signal is used for image data collection. All the data for an EPI image (Fig. 11A) can be collected in less than 100 msec. A significant disadvantage of EPI is that the long duration of data collection can cause blurring (from T2-related amplitude decay) and geometrical warping (from spin phase accrual related to inhomogeneous magnetic fields).