Slice selection


 

Slice selection

 

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.


Slice selection

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.

Selecting slice with gradient

 

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.

Selecting slice 1

 

3. Reset

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.

Resetting the gradient

 

Summary

  1. A magnetic field gradient is applied in the z-axis
  2. The Larmor frequencies of the nuclei varies along the z-axis
  3. An RF pulse with a frequency matching the Larmor frequency of the nuclei we want to select is applied
  4. In this way a slice along the z-axis is selected (correlates with an axial slice of the patient)
  5. The phases of the nuclei are reset by reversing the gradients

 

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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

Slice selection - Changing RF bandwidth 

 

2. RF pulse frequency

Changing the RF pulse frequency moves the slice selected up and down the z-axis 

Slice selection - changing pulse frequency

 

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

Slice selection - gradient strength

 

Next page: Frequency encoding


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