Question

Regarding echo-planar imaging (EPI): 

a. It allows the acquisition of high-resolution images 

b. Image formation is achieved by a series of alternating polarity gradients in the frequency-encoding direction 

c. Phase encoding is achieved by using gradients of varying strength that are rewound after each echo 

d. Different weightings can be achieved 

e. It is prone to artefacts

Answer 1

a. False. An acquisition matrix resolution is usually limited to 64 X 64 or 128 X 128. 

b. True. The polarity of the frequency-encoding gradient is repeatedly changed so that the k-space is traversed in a backwards–forwards (raster) fashion. 

c. False. A small but constant phase-encoding gradient is applied after each echo formation to move the k-space trajectory up one step. This is often referred to as a ‘blip’. 

d. True. EPI can be weighted for T₂, T₂ , T₁ or diffusion. 

e. True. It is particularly prone to chemical shift artefacts that need to be prevented by fat suppression and to artefacts resulting from non-perfect alignment of the forward and reverse echoes (‘N/2 ghosting’).