MAPPING ANATOMY - Radiation Oncology: A Physicist's-Eye View - page 39

M AGNETIC R ESONANCE I MAGING (MRI)

Principle of operation

This is not the place for a description of what makes magnetic
resonance imaging (MRI) possible and how it is performed - nor
would I be a good guide to these matters. However, I will make a
very few broad comments, principally to underline the ways in which
CT and MRI differ from the point of view of their use in radiation
oncology.
One of the properties of atomic nuclei is that they act like tiny
magnets, their magnetic field arising from the fact that the charged
nucleus has a spin and a moving charge creates a magnetic field
(which is the basis of operation of electromagnets). The nucleus of
interest in MRI is the proton 5 - abundant in tissue not least because it
is the nucleus of hydrogen which, in the form of water, constitutes a
large fraction of the human body.
A proton's spin, and hence the direction of its magnetic field, is
quantized so that it can align in one of two possible directions -
crudely speaking “up” and “down.” Under normal conditions, the
spins are equally distributed between the “up” and “down” directions.
However, when a magnetic field is applied to a body of tissue, the
proton spins have a slight tendency to align with the magnetic field -
thus creating a slight excess of “up” as opposed to “down” protons.
The difference in the numbers is very small - typically about one part
per million - but is enough to form the basis of MRI. Not only do the
spins tend to align with the magnetic field, they also rotate (precess)
about it, much in the manner in which a spinning top wobbles

(precesses) around the vertical due to the influence of gravity. The

frequency of this precession is known as the Larmor frequency and

is proportional to the strength of the applied magnetic field. For

protons, the Larmor frequency in a 1 T field is 43 MHz. At, say,
higher field strengths, the Larmor frequency is proportionately higher.

If a radiofrequency (rf ) field of precisely the Larmor frequency is

applied to tissue within a magnetic field, the spins tend to be forced
into the more energetic down state - and, after the rf field is removed,
the spins gradually return to their equilibrium distribution. As they do

5

Except in the realm of magnetic resonance spectroscopy, where other

nuclei than protons may be imaged.