X Ray Interaction With Matter And Biologic Effects Flashcards
Describe the beam leaving the x ray unit?
Focused stream of x ray photons going in the same direction (collimated to the shape of receptor).
With a range of energies present (lower, non-diagnostic photons removed by aluminium filtration).
These x ray photons travel in a straight line until they interact with something
What are the 3 types of interactions with matter?
Transmission
Absorption
Scatter
What is attenuation?
Reduction in intensity of the x ray beam (absorbed and scattered)
How is x ray beam intensity measured?
The quantity of photon energy passing through a cross-sectional area of the beam per unit time.
This is proportional to energy (potential difference across X-ray tube- photons have more kinetic energy when reaching tungsten atom, more converted into x rays) and number of photons (can be increased by increasing the current)
What is the photoelectric effect?
Complete absorption
Photon in x ray beam interacts with inner shell electron in subject, resulting in absorption of the photon and creation of a photo electron.
When energy oil incoming electron is equal to or greater than binding energy of inner shell electron.
This prevents x ray photons reaching the receptor
What is the Compton effect?
Photon in x ray beam interacts with outer shell electron in subject, resulting in partial absorption and scattering of the photon and creation of a recoil electron (ejected electron).
When energy of incoming electron is much greater than binding energy of electron
Can result in fogging of the image
How is scatter reduced?
Decrease surface area irradiated
Decrease volume of irradiated tissue
Decrease number of scattered photons produced in the tissue
Decrease scattered photons interacting with receptor
Decrease loss of contrast on radiopaque image
What is the effect of lowering kV on X-ray unit?
Increases contrast between tissues, increases dose absorbed by pt
What is the effect of increasing kV on x ray unit?
Decreases photoelectric interactions
Decreases dose absorbed by pt
Decrease contrast between tissues
What is the most significant effect of ionising radiation?
Damage to DNA
Direct - interacts with molecules of DNA
Indirect - interacts with water in the cell, producing free radicals (highly reactive, unstable molecules) which cause DNA damage.
DNA mutation can be repaired/ cause cell death/ cause mutation which could cancer cancer
What will the biologic effect depend on?
Type of radiation
Amount of radiation (dose)
Time over which the dose is received (dose rate)
Tissue or cell type irradiated
What is tissue radiosensitivity?
Depends on the function of the cell and if they are actively dividing.
What is x ray dose a measure of?
Absorbed dose is a measure of the energy deposited by radiation - measured in Gray (Gy).
Equivalent dose is a derived quantity multiplied by a weighting factor depending on the type of radiation
What is LNT?
Linear No Threshold model
Estimates the long term biological damage from radiation
What are the 2 types of effects of radiation?
Deterministic - tissue reactions only occurring above a certain threshold (severity related to dose)
Stochastic - the probability of occurrence related to the dose
