X-Ray Interaction with Matter Flashcards
what are the three types of ways in which photons interact with matter
transmission - passes through matter unaltered
absorption - stopped by matter
scatter - changes direction
what happens when a photon is both scattered and absorbed
photon is deflected by tissue but there is partial dposition of energy into tissue
what is attenutation
reduction in intensity of x-ray beam due to absorption or scattering
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 of x-ray beam and number of photons
how can you increase the energy of the x-ray beam
increase average photon energy
what colour will tissue that have minimal attenuation be
black
what colour will tissue be that has partial attenuation
grey
if there is complete attenuation what colour will show up on the image
white
what are the two specific attenuation interactions
photoelectric effect - complete absorption
compton effect - absorption and scatter
what is the photoelectric effect
photon in x-ray bea, interacts with inner shell electron in subject, resulting in absorption of the photon and creation of photoelectron
what energy do photo-electrons move off with
kinetic
when does the photoelectron effect occur
when energy of incoming photon is equal to or just greater than the binding energy of inner shell electron
what does the absorption by radiooelectric effect cause on the x-ray image
it prevents photons reaching the receptor and causes a lighter area on the image
what is the relation between the photoelectric effect and atomic number
small increases in atomic number lead to large jumps in absorption
what is the photoelectric effect proportional to
the atomic number cubed
why are gold and lead goo shields
high atomic numbers = huge proportion of photoelectric effect
what is the compton effect
photon in x-ray beam interacts with outer shell electrons in subject, resulting in partial absorption and scattering of the photon and creation of recoil electron
when does compton effect occur
when the energy of incoming photon is much greater than binding energy of electron
what occurs with the recoil electron from compton effet
can ionise and potentially damage adjacent tissues
in which direction do higher energy photons deflect
forward scatter
in which direction do lower energy photons deflect
back scatter
why does the control area need to completely surround the patient
due to scatter
how does scatter effect the radiographic image
photons scattered backwards, sideways or obliquely will not reach receptor
photons scattered slightly forwards may still reach receptor but interact with the wrong area which causes fogging of image
why should decreasing photon energy reduce fogging of images
because then there will not be forward scatter - only back scatter which does not reach the receptor and does not affect image quality
what are the 5 aspects of collimation
reduce surface area irradiated
reduce volume of irradiated tissue
reduce number of scattered photons produced in the tissue
reduce scattered protons interacting with receptor
reduce loss of contrast on radiographic image
what effect does the photoelectric effect have on radiation dose
deposition of all x-ray photons energy into tissue
increases patient dose but necessary for image formation
what effect does the compton effect have on radiation dose
deposition of some x-ray photon energy into tissue
increase patient dose but scatter photons do not contribute usefully to image
what is the average effective dose of periapical/ bitewing radiographs
4 microSv
what happens when you decrease the kilovoltage on x-ray unit (kV)
lower x-ray tube potential difference which causes overall lower energy photons produced so there is increase in photoelectric effect and increase contrast between tissue but the patient also absorbs a lot
what occurs when you raise the kilovoltage (kV) dose on x-ray unit
higher x-ray tube potential difference which causes overall higher energy photons produced which decreases photoelectric effect and causes forward scatter - decreases dose absorbed by patient but image quality is poor
