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Slice selection : Selective excitation

The first step of spatial localization is called slice selection. A magnetic field gradient is applied perpendicular to the desired slice plane (Slice-selection gradient). As the magnetic field varies in the direction of the gradient, all the planes perpendicular to the direction of the gradient have different precessional frequencies.
Then, an RF pulse is applied at the resonance frequency of the spins on the desired slice plane. The protons on either side of the slice will be at different precessional frequencies so that only the protons of the desired slice will be excited.

Once a slice has been selectively excited, the protons of this slice must be localized on this plane.
We are going to explain how magnetic field gradients applied in the direction of rows (Ox) and columns (Oy) allow encoding of spatial information on the slice plane.