Principles of Magnetic Resonance Imaging

1. Principles of MRI

2. Magnetic Moment: All materials contain nuclei that are either protons or neutrons or a combination
of both (Show, 1971). Nuclei containing an
odd number of protons or neutrons or both in
combination, possess a nuclear ‘spin’ and a
magnetic moment which has both magnitude and
direction. In body tissue or any other specimen,
the magnetic moments of the nuclei making up
the tissue are randomly aligned (Fig. 22.1) and
have zero net magnetization (M = 0).
When a material is placed in a magnetic field
B0, some of the randomly oriented nuclei experience
an external magnetic torque which tends to

3. align the individual parallel or anti-parallel
magnetic moments to the direction of an
applied magnetic field. There is a slight excess
of nuclei aligned parallel with the magnetic
field and this gives the tissue a net magnetic
momentM0. It is this differential in a magnetic
moment that accounts for the nuclear magnetic
resonance signal on which the imaging is
based. With the magnetic moments being
randomly oriented with respect to one another,
the components in the X-Y plane cancel one
another out while the Z components along the
direction of the applied magnetic field add up to
produce this magnetic moment M0