Transcript Document 7636287

Medical Imaging
Dr. Hugh Blanton
ENTC 4390
X-rays
• How does an x-ray machine work?
• We first accelerate electrons with a high
voltage (several thousand volts).
• We then allow the high speed electrons
to smash into a target.
• As the electrons slow down on collision,
they can emit photons - via
• photoelectric effect or
• Compton scattering.
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• An electron gun inside
the tube shoots high
energy electrons at a
target made of heavy
atoms, such as tungsten.
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X-rays
• However, the maximum energy of the
electrons limits the maximum energy of
any photon emitted.
• In general glancing collisions will give less
than the full energy to any photons
created.
• This gives rise to the continuous spectrum for
x-ray production.
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X-rays
• If an electron knocks out an inner shell
electron, then the atom will refill that
missing electron via normal falling of
electrons to lower levels.
• This provides a characteristic emission of
photons, and depends on the target
material.
• For the inner most shell, we can use a
formula similar to the Bohr atom
formula:
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X-rays
• For the inner most shell, we can use a
formula similar to the Bohr atom
formula:
• ionization = 13.6 eV * (Z-1)2
• where the -1 comes from the other inner shell
electron.
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X-rays
• If the electrons have this ionization
energy, then they can knock out this
inner electron, and we can see the
characteristic spectrum for this target
material.
• For iron, the ionization energy is:
13.6 eV * (26-1)2 = 1e * 8500 volts.
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X and  ray penetration
• High energy photons interact with
material in three ways:
• the photoelectric effect (which dominates
at low energies),
• Compton scattering, and
• pair production (which dominates at high
energies).
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X and  ray penetration
• But whether one photon interacts with
one atom is a probablistic event.
I = Io e-x
• where  depends on the material the x-ray is
going through.
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X and  ray penetration

pair
production
total
photoelectric
effect
Compton
Scattering
1 MeV
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Energy
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Measuring Health Effects
• Gamma rays (high energy photons) are very
penetrating, and so generally spread out
their ionizations (damage).
• Beta rays (high speed electrons) are less
penetrating, and so their ionizations are
more concentrated.
• Alphas (high speed helium nuclei) do not
penetrate very far since their two positive
charges interact strongly with the electrons
of the atoms in the material through which
they go.
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Bremsstrahlung
• When the electrons strike the dense metal target,
strong Coulomb forces are created between the
negative electron particles and the strongly
positive nuclei of the metal.
• This interaction causes the electron to slow down
(brake), or change directions, very quickly. Thus,
bremsstrahlung.
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• Bremsstrahlung is easier to
understand using the
classical idea that radiation
is emitted when the velocity
of the electron shot at the
tungsten changes.
• The electron slows down after
swinging around the nucleus
of a tungsten atom and loses
energy by radiating x-rays.
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• Due to the conservation of energy principle, this
loss of kinetic energy has to be compensated for
and is done so by the production of a photon of
electromagnetic energy.
• We call this photon an X-ray.
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• X-rays are just like any other kind of
electromagnetic radiation.
• They can be produced in parcels of energy
called photons, just like light.
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• There are two different atomic processes
that can produce x-ray photons.
• One is called Bremsstrahlung, which is a
fancy German name meaning "braking
radiation."
• The other is called K-shell emission.
• They can both occur in heavy atoms like tungsten.
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• In the quantum picture, a lot of photons of
different wavelengths are produced, but
none of the photons has more energy than
the electron had to begin with.
• After emitting the spectrum of x-ray radiation
the original electron is slowed down or
stopped.
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• The K-shell is the lowest energy state of an atom.
• The incoming electron from the electron gun can give a
K-shell electron in a tungsten target atom enough
energy to knock it out of its energy state.
• Then, a tungsten electron of higher energy (from an
outer shell) can fall into the K-shell.
• The energy lost by the falling electron shows up in an emitted
x-ray photon.
• Meanwhile, higher energy electrons fall into the vacated
energy state in the outer shell, and so on.
• K-shell emission produces higher-intensity x-rays than
Bremsstrahlung, and the x-ray photon comes out at a single
wavelength.
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• The energy lost by the falling electron shows up
in an emitted x-ray photon.
• Meanwhile, higher energy electrons fall into the
vacated energy state in the outer shell, and so on.
• K-shell emission produces higher-intensity x-rays
than Bremsstrahlung, and the x-ray photon comes
out at a single wavelength.
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• In the same way that x-rays are deflected in the
target crystal, they are deflected by atoms in our
body.
• When radiation strikes an atom it has the ability to
knock electrons out of the orbiting shells.
• Once these atoms are ionized, they seek out other atomic
particles or ions to make themselves more stable.
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