What happens when x-rays hit you or the patient? Flashcards
What are the 4 ways x-ray photons interact with matter
At what level do these interactions occur?
Transmitted unchanged (go through)
Absorbed (disappear)
Scatter + absorption (partially absorbed, some energy loss)
Pure scatter (no loss of energy)
Occur at atomic level
What are the shells of an atom, from innermost to outermost?
K L M N O
How is the density of any substance described?
By its atomic number
What is the atomic number?
It is the number of protons in the nucleus - the greater the density, the bigger the atom
What is the photo electric affect?
Absorption
Outline Stage 1 of the photoelectric effect (4 points)
- X-ray photon (high energy) collides with inner shell electron in patient’s atom
- Electron that has been hit by photon is ejected from atom (this makes atom unstable)
- The photon disappears (absorbed) as all the photon’s energy has been used in the collision
- Atom is left with a vacancy from ejected electron
Outline Stage 2 of the photoelectric effect (4 points)
- Electron from the next outer shell drops in to fill vacancy on the inner shell. As it drops from higher energy level, to lower energy level, energy is omitted.
- This stage repeats with outermost shell in order to fill the vacancy within the atom as the electrons rearrange.
- A free electron is captured to stabilise the atom.
- Energy is given off in the form of heat or light
What is the probability of photoelectric interactions proportional to?
Z cubed
Z being the atomic number
What is the atomic number (Z) for soft tissue?
7
What is the probability of photoelectric interactions for soft tissue?
Z cubed
Z for soft tissue is 7
7 cubed is 343
What is the Z value for bone?
12
What is the probability of photoelectric interactions for bone?
12 cubed = 1728
Why is there more absorption in bone than soft tissues when x-raying our patients, if they have similar atomic numbers?
Because it is proportional to the Z number CUBED
So the probability of photoelectric interactions in bone is much higher than soft tissues
Why does an x-ray image appear black, white and grey?
Because of varying levels of absorption of the x-ray photons in the body
What element is a good absorber of x-ray photons? What is it’s Z number?
Lead
It’s Z number is 82
This is why it is used for x-ray protection
What is the probability of photoelectric interactions inversely proportional to?
Photon energy cubed (keV)
Does low kV energy equipment mean more or less absorption of low energy photons?
More absorption of low energy photons
This sis because the lower the energy of the photons, the more they are absorbed
What is the Compton effect?
Absorption and scatter
Outline the steps involved in the Compton effect (6)
- Incoming photon interacts with outer shell electron
- Photon collides with electron, and it is ejected from atom
- Some energy absorbed from photon during collision
- Photon hasn’t lost all of its energy, so it is deflected and scattered
- Unstable atom with vacancy in outer orbit created
- Free electron captured and atom is stabilized
What has happened to the electrons in both the photelectric effect and the compton effect?
An electron has been ejected
What is the difference in the photon collision in the compton effect and the photoelectric effect?
The photoelectric effect involves an electron from an inner shell, whereas, the compton effect involves an electron from the outer shell
What is the name for the type of radiation that is high-energy enough to remove an electron from an atom?
Ionising
What charge does an atom have if an electron has been ejected from it?
It has a positive charge - it is a positive ion.
The electron that has been ejected is the negative ion
What is the difference between non-ionising radiation and ionising radiation?
Ionising radiation has enough energy to disrupt the atomic structure of human tissue, whereas, non-ionising radiation does not
Describe how tissues may be damaged by x-rays via direct damage
The DNA or RNA in the chromosome takes a direct hit from an x-ray photon which ionises and breaks the bonds between the nucleic acids = point mutation
Following DNA damage (e.g. by x-ray photons), what may the subsequent chromosomal effects include? (4)
- Cell Death > if enough damage is done initially, these cells can be killed outright
- Abnormal replication > (if nucleic acid bonds are broken, then rejoined together in an abnormal fashion, then these cells replicate, the abnormality will be replicated over and over again)
- Failure of transference of information
- Only temporary damage - the DNA being repaired successfully before further cell division
If somatic cells are affected by direct damage from x-ray photons, what may be the consequence?
Radiation may induce malignancy
If genetic cells are affected by direct damage from x-ray photons, what may be the consequence?
May result in congenital abnormality
What factors may affect what actually happens to a cell when irradiated by x-rays? (5)
- The type and number of nucleic acid bonds that are broken
- The intensity and type of radiation
- The time between exposures
- The ability of the cell to repair after damage
- The stage of the cell reproductive cycle when irradiated
Outline how x-ray radiation may cause damage indirectly
The x-ray photon ionises the water molecule - this produces ions and free radicals which can combine to form toxic substances which damage the DNA
What 3 further interactions may happen once water has been ionised from indirect damage?
High energy photon hits water molecule which produces a positive ion (H2O+) and a negative ion (e-) - these ions are more reactive.
- H2O+ > H+ + OH
- H2O + e- > H2O -
- H2O- > H + OH-
H & OH are free radicals which combine to form toxic substances like hydrogen peroxide or hydroperoxyl radical > which then damage tissues
In dentistry, which type of effects are we most concerned with?
Somatic stochastic effects
(somatic = involves body, stochastic = chance, governed by laws of probability….but this does include cancer induction)
Every exposure carries possibility of causing damage and no known threshold dose
