Biomedical Engineering Reference
In-Depth Information
for metallic objects within the patient, the behavior of therapeutic
photon beams in all body parts is dominated by the physics of
Compton interactions.
Interactions with molecules
So far, the discussion has been confined to interactions of radiation
with atoms. Human tissue is, of course, made up primarily of
molecules - which themselves are constituted of atoms. By and large,
it is the interactions of radiation with atoms that are fundamental.
However, once an atom has been ionized or excited, its parent
molecule will be affected. If the atom has been ionized, some of the
bonds linking atoms within the molecule are likely to be disrupted
and the molecule thereby either broken up or substantially trans-
formed. Water, for example, which constitutes some 60% of our body,
may be transformed into free radicals which themselves are highly
chemo-active. The molecule may also take up energy through rotational
and vibrational excitations which ultimately appear as heat.
As we will soon see, the electrons set loose by the interactions of
photons with individual atoms play an important part in the way in
which photons cause dose to be deposited in bulk matter. So, we
must take a moment to address the electron's interactions.
T
HE
I
NTERACTIONS OF
E
LECTRONS WITH
I
NDIVIDUAL
A
TOMS
Just as photons exert forces on the constituents of atoms, so do
electrons. However, the mechanism is rather different, since electrons
carry a charge (1.60
10
−19
Coulomb per electron) while photons are
neutral. Any two charged objects, even when not moving, exert an
equal and opposite force on one another - called the Coulomb force.
2
The force is attractive when the objects are oppositely charged and
repulsive when they carry the same charge. The force is inversely
proportional to the square of the distance between the objects.
Thus, when an electron passes near to or through an atom, it exerts a
force on the orbiting electrons and on the nucleus, giving rise to four
2
Charles-Augustin de Coulomb was a French physicist working in the
second half of the 18
th
century who made many discoveries in the fields of
mechanics and of electricity and magnetism.
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