Selecting the slice plane: selective excitation
The first step of spatial encoding consists in selecting the slice plane. To do this, a magnetic field gradient, the Slice Selection Gradient (GSS), is applied perpendicular to the desired slice plane. This is added to B0, and the protons present a resonance frequency variation proportionate to GSS (Larmor equation). An RF wave is simultaneously applied, with the same frequency as that of the protons in the desired slice plane. This causes a shift in the magnetization of only the protons on this plane. As none of the hydrogen nuclei located outside the slice plane are excited, they will not emit a signal. The RF wave associated with the slice selection gradient and the adapted resonance frequency, is called the selective pulse.
These protons located in the slice plane will again be stimulated by the magnetic field gradients to encode their position in horizontal and vertical directions.
Thickness, slice profile and selective RF pulse
An RF pulse does not have one frequency only (for this, it would need to be of infinite duration). It covers a certain bandwith, which depends on the shape of the pulse and its duration.
The thickness of the slice can be varied by adjusting the bandwidth of the selective pulse and the amplitude of the slice selection gradient:
- For a fixed amplitude gradient, the wider the bandwidth, the greater the number of protons excited and the thicker the slice
- For a fixed bandwidth, the stronger the gradient, the greater the variation of precession frequency in space and the thinner the slice
Moreover, the shape of the RF pulse in time will also determine the bandwidth profile in frequency, and thus the slice profile.
Additional slice selection gradient lobe(s)
During selective pulse delivery, the magnetization shift gives rise to transversal magnetization, which will be subjected to the slice selection gradient.
In the case of an excitation pulse at an angle below 180°, the slice selection gradient will have a spin dephasing effect due to the dispersion in the resonance frequency produced. To neutralize this effect, after applying the selective RF pulse (concomitant with the gradient) another gradient lobe is applied, along the same axis but in the opposite direction and with a surface (amplitude x time) equal to half the initial gradient lobe.
In the case of a 180° pulse, the dephasing effects neutralize symmetrically in relation to the centre of the RF pulse, so no rephasing lobe needs to be applied.
On the other hand, as the slice profile is imperfect, a 180° rephasing pulse, which will simply invert the magnetization of the excited spins, will also cause a shift in the undesired spins on the edge of the slice. To avoid this phenomenon, two identical gradient lobes can be added on each side of the 180° pulse:
- These lobes will have a symmetrical effect on the existing magnetization, before and after the 180° pulse, and will neutralize each other
- Magnetization on the edge of the slice created by the 180° pulse will be offset by the effect of the second added gradient lobe, which will cancel it out.