1. Physics of Radiography
Interaction with matter

2. By the end of the first part of the session you should be able to:
Understand what can happen as x-ray interact with matter
Describe Compton scattering and the photoelectric effect
Describe the biological effects of ionizing radiation

3. What happens when the x-rays meet the patient?
The photons striking matter can:
Be scattered with no loss of energy
Be absorbed with total loss of energy
Be scattered with some absorption and loss of energy
Pass through unchanged
Key terms:
Scattering – change in direction of a photon
Absorption – energy removed from the photon beam
Attenuation – reduction in x-ray intensity
Ionization – removing an electron producing negative and positive ions

4. Refresh –
Ions have the same number of protons (defining the element) but different number of electrons giving an overall charge of the atom
Interractions depend on the energy of the incoming photon, including:
Photoelectric effect (pure absorption)
Pair production (pure absorption)
Rayleigh scattering (pure scatter)
Compton scattering (scatter and absorption)
The two ones of importance in dentistry are:

6. Xray photon interacts with inner shell electron
All energy gets absorbed and the inner shell electron (photoelectron) is ejected and goes on to interact with other atoms ejecting electrons
Outer shell electrons fill the vacancy emitting energy (light or heat)
Energy must be greater or equal to inner shell binding energy to eject it.
Higher density (higher atomic number) atoms have more bound inner electrons so probability of photoelectric interactions increases

7. Photoelectric absorption proportional to cube of the atomic number (Z) since greater number of inner bound electrons
Lead Z = 82 so good absorber of X-rays
Soft tissue Z = 7 Z3 = 343
Bone Z = 12 Z3 = 1728
This difference in radio-density accounts for contrast on radiographs
Predominates at low energies
Low energy = high absorption dose but good radiographs
Overall effect ionization, photoelectron can go on to interact with other atoms

8. Involving outer shell electrons
Electron is ejected energy difference between incident photon and Compton recoil electron emitted as a scattered photon
Result ionization
Incoming photon energy must be higher than binding energy of outer shell electron
Recoil electrons can continue to interact with other atoms

9. The energy of the incoming photon affects the angle of the scatter.
The atomic number of the material has no effect on the amount of Compton absorption so will not contribute to increased contrast.
The forward scatter could diminish the quality of the image so anti scatter grids are used.

10. Compton NOT dependent on atomic number (Z)
Pair production at around 1000keV producing an electron and positron pair
Compton NOT dependent on atomic number (Z)
Energy (1000keV)

11. Intensity – the number of photons in the x-ray beam
As distance away from the source increases, the intensity reduces by the square of the distance
At 2r away, the intensity would be ¼ the intensity than at r.
At 3r away, the intensity would be 1/9th the intensity than at r.
The thickness of the material affects how reduced the intensity is

12. Somatic deterministic Somatic Stochastic Genetic Stochastic
Biological effects:
3 main types:
Somatic deterministic
Somatic Stochastic
Genetic Stochastic
Somatic – relating to ‘normal’ cells of the body
Genetic – relating to future generations
Deterministic – will happen
Stochastic – can happen
Stochastic effects can be:
Acute/immediate or chronic/long term

13. Somatic deterministic effects:
These are effects on the body that WILL happen after dose of radiation has exceeded the threshold dose.
e.g. cataracts, reddening of skin
Somatic stochastic effects:
These are effects on the body that MAY happen when exposed to any dose of radiation (no threshold). Each exposure carries a possibility of inducing stochastic effect
lower dose = lower probability of damage
Stochastic effects can be:
Acute/immediate happening shortly after exposure or
Chronic/long term happening after a long period of time e.g. leukaemia

14. Genetic stochastic effects
Genetic mutations can happen at random, but MAY also be caused by ionizing radiation
affecting DNA in reproductive cells. There is no threshold dose.
Foetal x-rays are regulated by law due to the high sensitivity to deformation, particularly around 2-9 weeks gestation. Large doses can result in congenital deformation, lower doses can result in mental retardation.
DNA can be affected since the X-ray or high energy electron (e.g. Compton recoil electron or photoelectron) can ionize important molecules e.g. DNA, RNA, proteins & enzymes

17. Genetic code gives instructions about how to build cells
Genetic code gives instructions about how to build cells. Some coding doesn’t appear to have an effect or can be recessive, other mutations can alter the instructions and cause problems

18. Damage to DNA depends on:
Type and number of bonds broken
Intensity/type of radiation
Time between exposures
Cells ability to repair
Stage in cell cycle

19. Review:
What is it called when an incoming photon changes it’s direction?
What are the two main attenuating processes that affect x-ray absorption?
The photoelectric affect deals with which type of electron shells (inner or outer)?
The Compton effect absorbs energy (true or false)
The Compton effect involves which type of electron shells (inner or outer)?
Which of these depends on the atomic number of the material it is passing through? Photoelectric/Compton
Which process doesn’t improve contrast on a radiograph?
What happens to the intensity of the photon stream as the thickness of material increases?
What happens to the intensity of the photon beam as you move away from the source?