In the absence of a linear gradient, all the hydrogen protons located within the magnetic field (Bo) will precess at the same frequency. If this frequency is the same as that of the electromagnetic wave (RF pulse), all the protons will be capable of absorbing energy.
To select a slice along a plane direction, it is necessary to apply a linear magnetic field gradient so that only the protons with the same precession frequency as the RF pulse will be able to absorb the energy. Modifying the magnetic gradient will allow the selection of different slices.
The slice thickness is usually between 3 and 5mm. In thick slices, more protons will be in resonance, resulting in a stronger signal, but the image will have less resolution and the S/N ratio will increase. By contrast, a thinner slice contains fewer protons in resonance, so the signal is weaker but the S/N ratio will decrease and the image will have a higher resolution. It is necessary to leave a space between slices, that is about 10% of the slice thickness.