Transcript RAD 254 Chapter 12 X-ray interaction with matter

RAD 254 Chapter 10
X-ray interaction with matter
Five Interactions to know:
Coherent (classical, unmodified,
Thompson)
Compton
PE
Pair Production
Photodisintegration
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”
CLASSICAL
Compton
• Incoming photon ejects an orbital electron
(losing some of it’s “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
COMPTON
Compton items
• Compton reduces radiographic contrast
• The greater the angle of the ejected
“Compton electron” the more energy
transferred from the incident photon
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 it’s 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
PHOTOELECTRIC
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 atom’s shells)
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)
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)
PAIR
PRODUCTION/ANNIHILATION
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
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
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
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
Contrast Agents
• ARE ATOMS that can interact with the
radiation beam to attenuate the photons it
contains
• Positive = iodine, barium
• Negative = air or gas