3. What Happens When X-rays Hit You Or The Patient Flashcards
What happens when x-rays interact with matter?
When using our x-ray gun, we can shoot the photons into the patient and several different interactions can happen
There are 4 basic interactions that can occur
What x-ray interactions with matter are relevant in dentistry?
- absorption
- scattering with absorption
- BOTH INTERACTIONS OCCUR AT AN ATOMIC LEVEL
- With the photon interactions we use in dentistry, we don’t tend to get pure scatter
Summary of atomic structure
For next Q’s, HINT,
consider the interactions that take within the atoms of a patient when they are being X-rayed, using this simple diagrammatic illustration of the atom which shows the central nucleus surrounded by orbiting electrons
What is the atomic interaction resulting in absorption called?
Photo electric effect
How does photoelectric effect occur/ what are the stages?
ABSORPTION
Stage 1 :
- High energy X-ray photon collides with inner shell e-
- Electron knocked out + ejected
- ejected e- with high energy so can cause own interactions with tissues
- Incoming photon has used up all energy in collision + disappears
- = absorbed
- remaining atom has e- vacancy in inner orbit
Stage 2 :
- Vacancy of missing e- in inner shell
- Electron from adjacent shells drop in to fill vacancy (cascading rearrangement of e-)
- Dropping between energy levels cause energy to be emitted
- energy released as heat / light
Cascade - Free e- captured to make atom stable
- Photon COMPLETELY absorbed
- may result in damage to tissues
Why is energy given out during the photoelectric effect and in what form?
- energy which is given out within the patient’s tissues as a result of this cascading rearrangement of the electrons in their shells
- form of heat or light
Is this energy given out during the photo electric effect visible/ detectable?
- theoretically, if you were to x-ray your patients in the dark, they would glow if enough light was produced
- This obviously does not happen because not enough visible light is created
- if you were able to measure the tissue temperatures, there would be a very slight increase in temperature because of the heat which is being produced
What negative results can the photoelectric effect have?
- damage to the tissues
What increases the likelihood/ opportunity for the photo electric effect to happen?
Why?
- a higher density/ atomic number Z
WHY?
- there are more inner shell electrons that could be hit
EXTRA INFO
- in the photo electric effect, the incoming high energy x-ray photon has interacted with a bound inner shell electron
- As the density or atomic number Z of any material increases, the number of bound inner shell electrons also increases
- This means that there is more opportunity for the photo electric effect to happen, simply because there are more inner shell electrons that could be hit
What is the probability of photoelectric interactions happening proportional to?
proportional to the atomic number cubed
NOT JUST Atomic number
- More photoelectric interaction = more absorption
- absorption is proportional to density cubed
What does the atomic number tell you?
Atomic number = Z
the density of the atom
How does bone and tissue look on a radiograph?
Bone = white
Soft tissues = black
Why do bone and tissue look different on a radiograph?
Z = atomic number / density
Soft tissue Z approx = 7
hence probability of photoelectric interactions = 7 cubed = 343
Bone Z approx = 12
hence probability of photoelectric interactions = 12 cubed = 1728
Higher probability = more photoelectric interactions happening = more absorption
- much higher bone probability
- This accounts for why when we x-ray our patients there is less absorption in the soft tissues and more absorption in bone
- Soft tissues appear black as very little absorption has taken place because the x-rays have passed through them and hit the film
- bones absorb the x-ray photons, and stop them hitting the film, so that part of the film appear white.
What would happen to how we see bone and tissue on a radiograph if absorption was simply proportional to density and not density cubed?
- Just looking at the atomic numbers 7 and 12, which represent the actual density of bone and soft tissue, there is not a great difference
- If absorption was simply proportional to density, bone and soft tissue would look very similar radiographically
Why do x-ray images have vary in colour/ darkness? (black, grey and white)
Due to the variation in absorption
- less absorption = darker image
What happens during the Compton effect?
Incoming X-ray photon interacts with outer shell electron
Collision causes electron to be ejected
Some energy from photon lost / absorbed
Remaining energy of photon is deflected/ scattered in any direction
- undergo further collision in patient
- or deflected out of patient
interaction has created unstable atom with hole or vacancy in outer orbit
Free electron is captured to fill vacancy and allow stability
In the photoelectric effect and Compton effect interactions within the patient, what has happened to the atoms in the tissues
1.
What happens during this ionisation?
2.
can ionisation occur in dent
3.
why?
1.
- high energy x-ray photon fired into stable atom
- photon has knocked out e-
- ejected e- has neg charge = neg ion
- unstable atom remaining behind = overall pos charge = pos ion
= charged ions = more reactive = more likely to undergo chemical reactions
- reactive ions do not last long as free electron is captured and atom returns to stable state v quick
2.
yes
3.
- x-ray photons or bullets used in dent have enough energy to enter patient and disrupt the atomic structure of the tissues and are therefore capable of causing ionisation
Ionising rad photons vs non-ionising rad photons
- non-ionising radiation photons = do not have enough energy to disrupt the atomic structure of human tissue
- ionising radiation photons = have enough energy to displace electrons and create positive and negative ions
EXTRA INFO
- x-rays = part of electromagnetic spectrum
- above dotted line = non-ionising
- below dotted line = which included ionising radiation
What happens during direct damage?
- x-ray photons interact directly with and ionise vital biologic macromolecules such as DNA, RNA proteins and enzymes
- ionisation results in the breakage of the macromolecule’s chemical bonds, causing them to become abnormal structures, which may in turn lead to inappropriate chemical reactions
- Rupture of one of chemical bonds in DNA may sever one of the side-chains holding the nucleic acids together
- This type of injury = point mutation
Direct damage
FOLLOWING DNA DAMAGE, SUBSEQUENT CHROMOSOMAL EFFECTS COULD INCLUDE:
- cell death
- abnormal replication
- failure of transference of information
- only temporary damage – the DNA being repaired successfully before further cell division
EXTRA INFO
- If enough damage is caused to DNA, then cells are killed outright
- if nucleic acid bonds are broken and then rejoin together in an abnormal fashion, when the cell subsequently divides, then that abnormality will be repeated over and over again
↳ If the somatic cells are affected, a radiation induced malignancy may result
↳ If the genetic cells are affected, it could give rise to a congenital abnormality
During direct damage what factors affect what actually happens to the cell?
What happens during indirect damage?
During indirect damage what substances cause the damage?
The final effects of direct and indirect damage are classified as?
Acute effects following large whole body doses of radiation?
Recovery time?
Doses (Sv - sieverts)
0.25
0.25-1.0
1-2
2-6
6-10
>10
What are somatic stochastic effects?
How does the size of exposure of ionising radiation effect the severity of the damage induced?
- the size of the exposure to ionising radiation does not effect the severity of damage produced, only the probability of occurring
EXTRA INFO
EG
You may recall this picture of oral cancer. It could have been caused by this patient being x-rayed for dental reasons many years earlier. So, although we only use small doses of radiation in dentistry, IF, when we X-ray patients malignant cancer induction occurs, it could eventually be fatal.
