Interaction of xrays Flashcards
Which interactions of X-rays occur at the atomic level?
Photoelectric effect (pure absorption) Compton effect (inelastic scattering) Raleigh scattering (elastic scattering) Pair production
What happens in the photoelectric effect?
- Incoming x-ray photon interacts with a bound INNER shell electron of the tissue atom
- the INNER shell electron is ejected with considerable energy (now called a photoelectron) into the tissues which undergo further interactions
- Vacancy now exists in INNER shell = filled by electrons dropping from outer shells (energy loss = light/heat)
- Captures a free electron to achieve atomic stability
n.b. occurs differently in materials with different atomic number = occurs differently in different tissues
How does a photoelectron behave?
Like the original X-ray photon = many similar interactions -> ejecting electrons as it pass through tissues & ionisation interactions within tissue
What is the probability of a photoelectric event proportional to?
Atomic number ^3 (z^3)
1/ energy of the X-ray beam ^3 (1/keV^3)
How does density of tissue alter the probability of photoelectric effect occurring?
Denser material = greater probability photoelectric effect will occur
= denser material absorbs more photoelectrons = less hits the image receptor = paler patch on film
How does kilovoltage of x-rays alter the probability of the photoelectric effect occurring?
Lower kilovoltage = greater probability photoelectric effect will occur
How do intensifying screens work?
When exposed to X-rays the screens emit their excess energy as light -> affects film emulsion & produces the image (e.g. DPT)
What is the compton effect?
X-ray absorption and scattering
- Incoming X-ray photon interacts with free/loosely bound OUTER shell electron
- OUTER shell electron is ejected with some of the energy taken from the photon = recoil electron
- Remaining energy from photon is scattered (can travel in any direction)
n.b. NOT dependent on atomic number (occurs in all tissues equally) as X-ray photon does not distinguish between one electron and another
What does a recoil electron do?
- Always travels in a forward direction
- Has extra energy (behaves like photon & propelled forwards)
- No effect on diagnostic quality of image (increases dose and degrades image) = not good discrimination between tissues on the final radiograph
What happens to the proportion of compton interactions with high energy photons?
Proportion of compton interactions is higher with high energy photons
What do scattered x-ray photos go on to do?
- Undergo further compton interactions with the tissues & also photoelectric interactions
- Escapes the tissues (scattered radiation)
How do x-rays cause damage?
Much of this is theoretical Studies on mice/those exposed to high radiation)
At the moment we dont know any safe dose of radiation
= causes damage to macromolecules (DNA, RNA & proteins)
What is direct damage?
Where an x-ray photon / ejected high energy electron breaks nucleic acid bonds = defective chromosomes
What is the consequence of defective chromosomes?
Fail to pass on information
Replicate abnormally
Cause cell death
Cause temporary damage
What happens when a somatic cell is damaged directly?
Radiation induced malignancy
What happens when a stem cell is damaged directly?
congenital abnormality
Why is there less of a risk with dental x-rays?
Minimal exposure for dental x-rays
Not aimed at reproductive
BUT if there is not serious problem (i.e. pain/symptoms) with dentition we would rather not risk it and wait until next appointment
What is indirect damage?
Ionisation of intracellular water = OH- & H+ = recombine = H+, water & hydrogen peroxide
How does hydrogen peroxide damage the cell?
Breaks down the proteins (e.g. DNA)
Which factors influence radiation effects?
Host factors Species of animals Intrinsic resistance Type & sensitivity of tissue Rate of cell division (faster in young) Phase of cell cycle
What are the two biological effects of radiation?
Tissue reactions (deterministic/non stochastic) effects Stochastic (chance) effects
Give examples of early non-stochastic effects?
Reddening of skin
Ulceration
Cataracts
Give examples of late non-stochastic effects?
Osteroradionecrosis
Give examples of stochastic effects?
Cancer induction & heritable effects
List some radiation effects on the oral cavity (with therapeutic doses of radiation:
Mucositis Loss in taste Dry mouth Radiation caries (by gingival margin) Tooth defects if irradiated while teeth are developing Atrophic mucosa
What is the key thing to remember about stochastic effects?
Size of exposure has no effect on severity of the damage induced, only the probability of it occurring
Which type of effects are important in diagnostic medicine?
STOCHASTIC
Which heritable effects can occur?
Damage DNA in sperm / egg cells
May cause congenital abnormality -> reduction in IQ of foetus
No threshold dose
Risk estimates based on animal experiments
Dose of 0.5-1Sv double rate of spontaneous mutation
When is the most risky time for a foetus to be exposed?
During organogenesis
6-7 weeks
