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PPENDIX
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General Principles of Magnetic
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Resonance Imaging
The nuclear spin system
Many atomic nuclei show the property of spin (i.e. spinning about an axis)
and, as a consequence, possess a magnetic moment aligned along the
axis of the spin. The spin may either be a multiple of 1 or 1/2 and it is
usually indicated as I . The inherent angular moment is therefore
where h is Plank's constant.
The magnetic moment, µ, is related to the angular momentum by
where J is known as the gyromagnetic ratio and is a constant for a
particular nucleus.
The nuclei most commonly used in Nuclear Magnetic Resonance (NMR)
are those with I = 1/2 and, among them, hydrogen (proton) is the most
studied because of its inherent properties and its abundant presence in
the human body, which comprises mainly water (around 80%).
When a nuclear spin is placed in a magnetic field, it will attempt to line up
with the field. For a nucleus with I = 1/2 there are two stable states,
namely parallel and anti-parallel to the field. These types of alignment are
on different energy levels. and the large number of spins present in a
sample will randomly distribute themselves between them, with the
majority of spin in the parallel direction, which requires less energy. This
is the quantity observed in NMR, It is called macroscopic magnetization
normally indicated by M
0
. M
0
is proportional to the applied field B
0
.
Although NMR phenomena should be described by quantum mechanics, in
the case of spins with I = 1/2, the principles of the classic model concur
exactly with those expected for the quantum mechanical model.
Therefore spin behavior in a magnetic field can be considered as similar to
that of a gyroscope or a spinning top in a gravitational field.
p = h I /2 S
µ = J p