The second step in spatial encoding consists in applying a phase encoding gradient, which we will choose to apply in the vertical direction. The phase encoding gradient (GPE) intervenes for a limited time period. While it is applied, it modifies the spin resonance frequencies, inducing dephasing, which persists after the gradient is interrupted. This results in all the protons precessing in the same frequency but in different phases. The protons in the same row, perpendicular to the gradient direction, will all have the same phase. This phase difference lasts until the signal is recorded.
On receiving the signal, each row of protons will be slightly out of phase. This translates as their signals being more or less out of phase.
To obtain an image, it is necessary to multiply the different dephased acquisitions, which are regularly incremented. For a spin echo sequence with « n » rows, we make « n » acquisitions each with a different phase encoding gradient.
Duration of a 2D imaging sequence
Duration = TR ∙ NPy ∙ Nex
- TR = Repetition time
- NPy = Number of phase encoding steps
- Nex= Number of excitations