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echo-planar_imaging [2019/08/18 21:08]
administrador created
echo-planar_imaging [2019/08/18 21:08] (current)
administrador
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 +====== 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 echos or may combine a spin echo with the train of gradient echos 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. ​
 +
 +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, i.e. breath-hold sequences and 3D MR angiography
 +diffusion imaging
 +perfusion imaging
 +functional imaging
echo-planar_imaging.txt · Last modified: 2019/08/18 21:08 by administrador