The Slice Selection means it determines the position of a slice
to be excited by an RF pulse.
And the slice was, let's consider what slice selection created to.
The gradient linearly changes,
the precession frequency of the magnetic momentums along a specific selection.
For instance, if that is applied along g direction, and that is combined,
with RF pulse so
is now combined with slice selection gradient they applied at the same time.
And then what happens both frequency,
preferential frequency, will be specially modulated as shown in the figure.
So, this figure will be typically used to represent what gradient does for
spatial frequency, and both goes direction, to present frequency.
And which is same as magnetic field or b field, so
because Omega equals comma B, they have a linear relationship.
So this Omega equals comma B, that relationship is going to be maintained
along the spatial direction in a linear way as shown in the figure.
So to each slice positions,
so this position projection represent spatial position.
And that have each of them would have different precession frequency
if that RF pulse is combined with this gradient applied at the same time.
And then, so without this slice selection gradient,
all the proton spins within the body will have the same precession frequency and
that is not actually true but you can assume like that.
That is true in some sense, so if we excite, if we apply for
RF pulse, Omega 2 in this case at this Larmor frequency
without this gradient then all the body part is going to be excited.
But if we want to excite at a specific position as shown in this figure here and
then we can change this modulation frequency of this RF pulse.
So this RF pulse represent the envelope shape of the RF pulse and
in fact, inside this envelope is modulated with Larmor frequency, okay?
And that Larmor frequency can be changed.
To change it, the excitation position, so as shown here, so
Omega 0 is going to be the same as Larmor frequency and then we can slightly
change the transmission RF pulse modulation frequency from Omega 2 to Omega
1 to change the location of excitation as shown in this figure, okay.
So without this gradient, if we applied for Omega 0,
then all the body part is going to be excited.
But if we apply for RF pulse, we need the frequency Omega 1 with no gradient,
then none of the spins is going to be excited.