ENTC 4390 MEDICAL IMAGING

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ENTC 4390MEDICAL IMAGING

• RADIOACTIVE DECAY

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Nuclear Particles Radiation

• Only a few of the many different nuclear
  emanations are used in medicine.
• In order of importance, the entities are
• x-rays
• Electromagnetic waves of very short wavelength
  that behave in many ways like particles.
• Gamma (g) rays
• Electromagnetic waves similar to x-rays, but of
  even shorter wavelength.
• Neutrons
• Actual particles that are produced during the
  decay of certain radioacitve materials.

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Radioactivity
Dont be confused by this picture! A single
radioactive source does not emit all three types
a, b and g.
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3 Types of Radioactivity
a particles helium nuclei
Easily Stopped
b particles electrons
Stopped by metal
g photons (more energetic than x-rays)
penetrate!
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• The nucleus of an atom consists of neutrons and
  protons, referred to collectively as nucleons.
• In a popular model of the nucleus (the shell
  model), the neutrons and protons reside in
  specific levels with different binding energies.

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Materials Science Fundamentals

• The structure of an atom

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Materials Science Fundamentals

• 2. Elements/Atomic Number (Z) Atomic Masses
• Key Chemical Behavior Determined by Z and
  Ionization

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Materials Science Fundamentals

• Atomic Number of Protons
• Mass Number of Protons and Neutrons
• Atomic Weight Total Mass of Atom

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• If a vacancy exists at a lower energy level, a
  neutron or proton in a higher level may fall to
  fill the vacancy
• This transition releases energy and yields a more
  stable nucleus.
• The amount of energy released is related to the
  difference in binding energy between the higher
  and lower levels.
• The binding energy is much greater for neutrons
  and protons inside the nucleus than for electrons
  outside the nucleus.
• Hence, energy released during nuclear transitions
  is much greater than that released during
  electron transitions.

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• If a nucleus gains stability by transition of a
  neutron between neutron energy levels, or a
  proton between proton energy levels, the process
  is termed an isometric transition.
• In an isomeric transition, the nucleus releases
  energy without a change in its number of protons
  (Z) or neutrons (N).
• The initial and final energy states of the
  nucleus are said to be isomers.
• A common form of isomeric transition is gamma
  decay (g) in which the energy is released as a
  packet of energy (a quantum or photon) termed a
  gamma (g) ray
• An isomeric transition that competes with gamma
  decay is internal conversion, in which an
  electron from an extranuclear shell carries the
  energy out of the atom.

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• It is also possible for a neutron to fall to a
  lower energy level reserved for protons, in which
  case the neutron becomes a proton.
• It is also possible for a proton to fall to a
  lower energy level reserved for neurons, in which
  case the proton becomes a neuron.
• In these situations, referred to collectively as
  beta (b) decay, the Z and N of the nucleus
  change, and the nucleus transmutes from one
  element to another.
• In beta (b) decay, the nucleus loses energy and
  gains stability.

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• In any radioactive process the mass number of the
  decaying (parent) nucleus equals the sum of the
  mass numbers of the product (progeny) nucleus and
  the ejected particle.
• That is, mass number A is conserved in
  radioactive decay

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• In alpha (a) decay, an alpha particle (two
  protons and two neutrons tightly bound as a
  nucleus of helium ) is ejected from the
  unstable nucleus.
• The alpha particle is a relatively massive,
  poorly penetrating type of radiation that can be
  stopped by a sheet of paper.

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• An example of alpha decay is

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ENTC 4390MEDICAL IMAGING

• DECAY SCHEMES

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• A decay scheme depicts the decay processes
  specific for a nuclide (nuclide is a generic term
  for any nuclear form).
• Energy on the y axis, plotted against the
• Atomic number of the nuclide on the x axis.

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• Given a generic nuclide, there are four
  possible routes of radioactive decay.
• a decay to the progeny nuclide by emission of a
  nucleus.
• (a) b (positron) decay to progeny nuclide
  by emission of positive electron from the
  nucleus.
• (b) b- (negatron) decay to progeny nuclide by
  emission of negative electron from the nucleus.
• g decay reshuffles the nucleons releasing a
  packet of energy with no change in Z (or N or
  A).

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Atomic Number
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ENTC 4390

• BETA DECAY

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• Nuclei tend to be most stable if they contain
  even numbers of protons and neutrons and least
  stable if they contain an odd number of both.
• Nuclei are extraordinarily stable if they contain
  2, 8 ,14, 20, 28, 50, 82, or 126 protons.
• These are termed nuclear magic numbers and
• Reflect full occupancy of nuclear shells.

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• The number of neutrons is about equal to the
  number of protons in low-Z stable nuclei.
• As Z increases, the number of neutrons increases
  more rapidly than the number of protons in stable
  nuclei.

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• Can get 4 nucleons in each energy level-
• lowest energy will favor NZ,
• But protons repel one another (Coulomb Force) and
  when Z is large it becomes harder to put more
  protons into a nucleus without adding even more
  neutrons to provide more of the Strong Force.
• For this reason, in heavier nuclei NgtZ.

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ENTC 4390

• ISOMERIC TRANSITIONS

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• Isomeric transitions are always preceded by
  either electron capture or emission of an a or b
  ( or -) particle.
• Sometimes one or more of the excited states of a
  progeny nuclide may exist for a finite lifetime.
• An excited state is termed a metastable state if
  its half-life exceeds 10-6 seconds.

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• An isometric transition can also occur by
  interaction of the nucleus with an electron in
  one of the electron shells.
• This process is called internal conversion.
• The electron is ejected with kinetic energy Ek
  equal to the energy Eg released by the nucleus,
  reduced by the binding energy Eb of the electron
• The ejected electron is accompanied by x rays and
  Auger electrons as the extranuclear structure of
  the atom resumes a stable configuration.

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• The rate of decay of a radioactive sample depends
  on the number N of radioactive atoms in the
  sample.
• This concept can be stated as
• where DN/Dt is the rate of decay, and the
  constant l is called the decay constant.

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• The decay constant has units of time .
• It has a characteristic value for each nuclide.
• It also reflects the nuclides degree of
  instability
• a larger decay constant connotes a more unstable
  nuclide
• i.e., one that decays more rapidly.
• The rate of decay is a measure of a samples
  activity.

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• The activity of a sample depends on the number of
  radioactive atoms in the sample and the decay
  constant of the atoms.
• A sample may have a high activity because it
  contains a few highly unstable (large decay
  constant) atoms, or
• because it contains many atoms that are only
  moderately unstable (small decay constant).

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• The SI unit of activity is the becquerel (Bq.)
  defined as
• 1 Bq 1 disintegration per second (dps)
• An older, less-preferred unit of activity is the
  curie (Ci), defined as
• 1 Ci 3.7 x 1010 dps

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Example
a. has a decay constant of 9.49 x 10-3 hr
-1. Find the activity in becquerels of a sample
containing 1010 atoms.
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Example

• How many atoms of with a decay constant
  of 2.08 hr -1 would be required to obtain the
  same activity in the previous problem.
• More atoms of than of are required to
  obtain the same activity because of the
  difference in decay constants.

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• Note that the equation
• can be written as

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• Rearranging and solving for N,
• where No is the number of atoms at time to .

natural log format
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• The physical half-life, T1/2, of a radioactive
  nuclide is the time required for decay of half of
  the atoms in a sample of the nuclide.

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Example

• The half-life is 1.7 hours for 113mIn (Indium).
• A sample of 113mIn has a mass of 2mg.

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ENTC 4390MEDICAL IMAGING

• DECAY SCHEMES

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X-Rays

• Strong or high energy x-rays can penetrate deeply
  into the body.
• Weak or soft x-rays are used if only limited
  penetration is needed
• The energy of x-rays, as well as other nuclear
  particles is measured in
• electron-volts (ev)
• thousands of electron-volts (kev)
• millions of electron-volts (Mev)

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• The diagnostic use of x-ray depends on the fact
  that various types of absorbs x-rays to a greater
  or lesser degree.
• Absorption by bone is quite high,
• Absorption by fatty tissue is low.
• This allows the use of the x-ray beam for
  delineating the details of body structure.

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Gamma Rays

• X-rays are generally produced electrically.
• g_rays are the result of a radioactive transition
  in a substance that has been activated in a
  nuclear reactor.
• Once again, energy is measured in
• electron-volts (ev)
• thousands of electron-volts (kev)
• millions of electron-volts (Mev)

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• The higher energy g-rays penetrate all human
  tissue quite easily.
• g-rays are used in conjunction with scanning
  systems to detect anomalies due to disease or
  neoplastic growth.

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Neutrons

• Neutron applications in medicine are limited.
• Again, energy is measured in
• electron-volts (ev)
• thousands of electron-volts (kev)
• millions of electron-volts (Mev)

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Preflight - Gamma Ray Emission

• Gamma rays are emitted due to electrons making
  transitions to nuclear energy levels.
• true
• false

No, gamma rays are high energy photons emitted
when nucleons make transitions between their
allowed quantum states.
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Preflight - Nuclear Beta Decay

• Beta rays are produced when the atom
  spontaneously repels all its electrons from its
  orbits.
• true
• false

Beta particles are electrons. However, the atom
does not emit its atomic electrons.
Beta electrons are emitted by a nucleus along
with a neutral weakly interacting particle called
the neutrino when one of the neutrons in the
nucleus decays.
Free neutrons are unstable - they decay.
Sometimes in atoms with large numbers of
neutrons, one of its neutrons may be loosely
bound - spontaneous decay!
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Preflight - Positrons

• Beta particles are
• Always negatively charged.
• Always positively charged.
• Some beta decays could produce positively charged
  particles with properties similar to those of
  electrons.

Some radioactive elements emit a positively
charged particle which is in all other respects
similar to an electron! Anti-matter!! Positrons!!!
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Preflight - Alpha Particles

• Alpha particles are
• Electrons
• Protons.
• Nuclei of Helium atoms
• Nuclei of Argon atoms

Some nuclei have lots of protons and many more
protons. Lowest energy bound-states require about
equal numbers of protons and neutrons. Those
nuclei emit most tightly bound nuclear matter,
i.e., Helium nuclei with two protons and two
nuclei.