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.
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