X-Ray Interactions with Matter Flashcards
what are the 3 ways that photons interact with matter
transmission, absorption, scatter
what is transmission
photon passes directly through tissue unchanged
what is absorption
energy fully deposited into tissue and photon ceases to exist
what is scatter and absorption
photon deflected by tissue
partial deposition of energy into tissue
photon continues in new direction
what is attenuation
reduction in intensity of x-ray beam (can include absorption and scatter)
what is x-ray beam intensity
quantity of photon energy passing through a cross-sectional area of the beam per unit time
what is intensity proportional to
energy and number of photons
what does minimal attenuation give
black image
what does partial attenuation give
grey image
what does complete attenuation give
white image
what are predictions of x-ray photon interactions based on
physical properties of matter being exposed
what are the 2 specific attenuation interactions
photoelectric effect and compton effect
what is the photoelectric effect
photon in x-ray beam interacts with inner shell electron in subject, resulting in absorption of the photon and creation of photoelectron
when does the photoelectric effect take place
when energy of incoming photon is equal to or greater than the binding energy of inner shell electron
when does the photoelectric effect predominate
with lower energy photons
what can a photoelectron do to tissues
ionise and damage adjacent tissues
why does photoelectric effect lead to lighter areas on radiographs
it prevents x-ray photons reaching the receptor
what is the photoelectric effect proportional to
atomic number cubed
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
when does the compton effect occur
when energy of incoming photon is much greater than binding energy of electron
when does the compton effect predominate
with higher energy photons and outer shell electrons
what can a recoil electron do to tissues
ionise and damage adjacent tissues
what is scatter influenced by
energy of incoming photon
where are higher energy photons scattered
more forward - forward scatter
where are lower energy photons scattered
more backward - back scatter
what is the majority of the scatter from an x-ray beam
forward scatter
what is the reason for the controlled area to surround the patient
scatter
what is the probability of the compton effect occurring
proportonal to the density of material
what does collimation do
decrease surface area irradiated
decrease volume of irradiated tissues
decrease number of scattered photons produced in tissue
decrease scattered photons interaction with receptor
decrease loss of contrast on radiographic image
what is the photoelectric effect on radiation dose
increase patient dose but is necessary for image formation
what is the compton effect on radiation dose
increased patient dose but scattered photons do not contribute usefully to image
what is the effect of lowering kV on x-ray unit
increased contrast between tissues with different atomic numbers but increased patient dose
what is the effect of raising kV on x-ray unit
decreased dose absorbed by patient but decreased contrast between tissues with different atomic numbers
what is the UK guidance for range of kV in intraoral units
between 60-70kV
