Magnetic resonance imaging (MRI) requires the use of a non-uniform magnetic field superimposed on a large uniform magnetic field.

Question 5

Magnetic resonance imaging (MRI) requires the use of a non-uniform magnetic field

superimposed on a large uniform magnetic field.

State and explain the purpose of

(a) the large uniform magnetic field, [3]

(b) the non-uniform magnetic field. [3]

Reference: Past Exam Paper – November 2013 Paper 43 Q10

Solution:

(a) The (hydrogen) nuclei possess a ‘spin’ and they precess about the direction of the magnetic field. The frequency of precession depends on the magnetic field strength. Applying a large magnetic field (in excess of 1 tesla) gives rise to a frequency of precession that is in the radio-frequency region of the electromagnetic spectrum.

(b) The non-uniform field gives a unique value of magnetic field strength at each point in a person. Therefore the frequency of precession of hydrogen nuclei is different at different points (regions of the subject).

This enables the location of the precessing nuclei to be determined.

Furthermore, as the non-uniform field is changed it will enable the nuclei in different regions of the person to be detected.