RAD 254 Chapter 10 Xray interaction with matter

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RAD 254 Chapter 10X-ray interaction with matter

• Five Interactions to know
• Coherent (classical, unmodified, Thompson)
• Compton
• PE
• Pair Production
• Photodisintegration

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Coherent

• Incoming photon interacts with the nuclear force
  field of the atom and changes direction, BUT has
  NO CHANGE in energy just direction thus the
  name unmodified

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CLASSICAL
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Compton

• Incoming photon ejects an orbital electron
  (losing some of its on-board energy to
  overcome the binding energy of the electron it
  ejects), and continues on in a deviated direction
  MINUS the energy required to overcome the binding
  energy of the ejected electron

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COMPTON
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Compton items

• Compton reduces radiographic contrast
• The greater the angle of the ejected Compton
  electron the more energy transferred from the
  incident photon

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Photo Electric

• Incident photon must be at or slightly higher
  energy of the orbital electron it ejects. THE
  INCIDENT PHOTON CEASES TO EXIST.
• Any and all of its energy is used to overcome
  the binding energy of the electron it ejects
• ANY SURPLUS energy the incident photon has is
  held by the ejected electron in the form of
  potential energy

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PHOTOELECTRIC
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PE Tidbits

• As kVp increase, the likelihood of P.E. DECREASES
• P.E. can occur at any orbital level BUT the
  MAXIMUM energy exchange is always at the K-shell
  (k-shell is the highest binding energy of any
  atoms shells)

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Useful for Diagnostic X-ray

• ONLY P.E. and COMPTON are useful in the
  diagnostic energy range (Coherent is too low of
  energy to be of use and Pair Production and
  Photodisintegration are too high of energy to be
  in the diagnostic range energy)

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Pair Production

• The incident photon (with min. of 1.02 MeV)
  interacts with the FORCE FIELD of the nucleus
• The photon disappears and two electrons (one
  negative and one positive POSITRON) appear
  EACH with HALF of the original photon energy
  (i.e. .51 MeV each)

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PAIR PRODUCTION/ANNIHILATION
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The Positron produces Annihilation Radiation

• The positron will interact with an electron,
  converting the mass of BOTH the electron and
  positron to a photon with an energy of the SUM of
  the two particles energy
• Useful in PET scanning energy range

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Photodisintegration

• Photons with energy over 10 MeV can interact with
  the FORCE FIELD of an atom and be ABSORBED by the
  nucleus.
• The photon Ceases to exist and a NUCLEON or
  NUCLEAR FRAGMENT is emitted

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Differential Absorption

• The varied absorption of x-rays and resulting
  actions from P.E. and COMPTON as well as
  NON-INTERACTING photons can reach the film
  depending on the amount of attenuation
  (absorption) in the body producing varied
  densities on the image
• kVp must be appropriate to allow the proper
  amount of differential absorption to produce
  varied shades of grays (contrast)
• COMPTON scattered photon contributes NO USEFUL
  information to the film/receptor

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Exponential Absorption

• Theoretically there is always some residual
  radiation the product of absorption and
  scattering is NEVER zero.
• There is always a bit of a bit of radiation left

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Contrast Agents

• ARE ATOMS that can interact with the radiation
  beam to attenuate the photons it contains
• Positive iodine, barium
• Negative air or gas