When Is Phase Encoding?

The second step of spatial localization is called phase encoding. A magnetic gradient field is applied briefly in one direction. As the change in frequency is very brief, when the gradient is switched off, it causes a change in phase that is proportional to the distance.

When is the phase encoding gradient applied?

In the imaging sequence a phase encoding gradient is applied after the slice selection gradient. Assuming this is applied along the X axis, the spins at different locations along the X axis begin to precess at different Larmor frequencies.

What is the difference between a frequency encoding and a phase encoding?

The frequency-encoding direction is along the x-axis in K-space (may or may not be that axis in the image, if it is rotated); this represents the time samples of the signal. The y-axis is the phase-encoding direction: each phase-encoding step yields a separate horizontal line.