Physics of RadiographyInteraction with matter

By the end of the first part of the session you should be able to:

1. Understand what can happen as x-ray interact with matter

2. Describe Compton scattering and the photoelectric effect

3. Describe the biological effects of ionizing radiation

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 unchangedKey terms:Scattering – change in direction of a photonAbsorption – energy removed from the photon beamAttenuation – reduction in x-ray intensityIonization – removing an electron producing negative and positive ions

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:Compton scattering (scatter and absorption)Photoelectric effect (pure absorption)

http://www.ammrf.org.au/myscope/analysis/eds/xrayintensity/photoelectric.php

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

- 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)

The number of interactions by Photoelectric absorption is proportional to cube of the atomic number.

Recall: Atomic number = number of protons, denoted by the letter Z, and equal to the number of electrons in a neutral atom. This relationship is due to the greater number of inner bound electrons

Soft tissue Z = 7 Z3 = 343Bone Z = 12 Z3 = 1728

Lead Z = 82 Z3 = 551368so good absorber of X-rays

Number of photoelectric interactions related to (atomic number)3

• 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

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

Compton scatter

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.

Energy (1000keV)

Pair production at around 1000keV producing an electron and positron pair

Compton NOT dependent on atomic number (Z)

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.

Intensity – the number of photons in the x-ray beam

The thickness of the material affects how reduced the intensity is

Summary

Main processes involved in attenuation:• Photoelectric effect (pure absorption, inner shell electrons,

photoelectron emitted, dependent on atomic number, predominates at low energies)

• Compton Scatter (Scatter and absorption, outer shell electrons, compton recoil electron emitted, NOT dependent on atomic number, predominates at energies above those where photoelectric effect dominates, back scatter GREATER at lower energies)

Main sources of attenuation:• Distance from source• Thickness of material through which radiation is passing• Composition of material through which radiation is passing

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

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 orChronic/long term happening after a long period of time e.g. leukaemia

Genetic stochastic effects

Genetic mutations can happen at random, but MAY also be caused by ionizing radiationaffecting 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

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

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

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?