1. RAD 254 Chapter 10 X-ray interaction with matter
Five Interactions to know:
Coherent (classical, unmodified, Thompson)
Compton PE
Pair Production
Photodisintegration

2. 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”

3. CLASSICAL

4. 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

5. COMPTON

6. Compton items Compton reduces radiographic contrast
The greater the angle of the ejected “Compton electron” the more energy transferred from the incident photon

7. 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

8. PHOTOELECTRIC

9. 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)

10. 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)

11. 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)

12. PAIR PRODUCTION/ANNIHILATION

13. 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

14. 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

15. 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

17. 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

18. Contrast Agents
ARE ATOMS that can interact with the radiation beam to attenuate the photons it contains
Positive = iodine, barium
Negative = air or gas