The first part of localising the signal is to localise it the location of the axial slice within the object being imaged. This is known as “slice selection“. The way this is done is by using the RF pulse to select which slice to activate i.e. which slice will have the magnetic vector of its nuclei flipped to the transverse plane in order to return a signal.
1. Apply gradient
A magnetic field gradient is applied in the Z-axis superimposed on the background magnetic field. Going back to the Larmor equation the frequency of precession depends on the magnetic field. This means that nuclei will have different frequencies throughout the z-axis.
2. Select slice
An RF pulse is applied to flip the magnetisation of the nuclei into the transverse plane and, therefore, give a signal. Remember, to flip the precession of the nuclei the RF pulse frequency should be the same as the Larmor frequency of the nuclei. As the Larmor frequency of nuclei is different along the z-axis we can select a slice to activate by altering the frequency of the RF pulse.
As the frequencies are different along the gradient, the nuclei begin to precess out of phase. Before selecting the next slice we need to reset the nuclei. This is done by temporarily reversing the gradient to reverse the precessional frequencies. The nuclei then rephase.
- A magnetic field gradient is applied in the z-axis
- The Larmor frequencies of the nuclei vary along the z-axis
- An RF pulse with a frequency matching the Larmor frequency of the nuclei we want to select is applied
- In this way, a slice along the z-axis is selected (correlates with an axial slice of the patient)
- The phases of the nuclei are reset by reversing the gradients
Factors affecting slice properties
1. RF pulse bandwidth
The RF pulse bandwidth is the range of frequencies within the pulse
Large bandwidth = large range of frequencies = larger slice
2. RF pulse frequency
Changing the RF pulse frequency moves the slice selected up and down the z-axis
3. Gradient strength
Altering the gradient strength alters the steepness of the gradient. The same RF pulse will then activate (select) a different size of slice
Larger gradient = smaller image slice
Smaller gradient = larger image slice