X Ray Interaction with Matter Flashcards
photons in diagnostic x ray beam can interact with matter in 3 ways
- transmission
- absorption
- scatter
attenuation
reduction in intensity of x ray beam
x ray beam intensity
intensity proportional to energy & number of photons
- energy of x ray beam affected by potential difference across x ray tube (kV) i.e. increase kv = increase av photon energy = increase max photon energy
- number of photons in x ray beam primarily affected by current in filament (mA) i.e. increase mA = increase in no of photons
1 line summary of photoelectric effect
photon in x ray beam interacts with inner shell electron in subject resulting in absorption of photon & creation of photoelectron
important points about photelectric effect
- occurs when energy of incoming photon is equal to or just > than binding energy of inner shell electron
- so photoelectric effect predominates with lower energy photons
- this photon energy causes inner shell electron to be displaced &now be a photoelectron
- any excess energy becomes kinetic energy of photoelectron
- photoelectron can ionise & potentially damage adjacent tissue
- vacancy in inner shell is filled by cascade of outer shell electrons
impact of photoelectric effect on image
absorption by photoelectric effect prevents x ray photons reaching the receptor so leads to lighter area on radiographic image
1 line summary of compton effect
photon in x ray beam interacts with outer shell electron in subject resulting in partial absorption & scattering of the photon & creation of a recoil electron
important points of compton effect
- occurs when energy of incoming photon is much greater than binding energy of electron
- compton effect predominates with higher energy photons & outer shell electrons (which are loosely bound)
- some photon energy transferred to electron to overcome binding energy & provide kinetic energy
- electron rejected & called ‘recoil’ electron
- recoil electron can ionise & potentially damage adjacent tissues
- photon loses energy & changes direction i.e. scattered so can undergo photoelectric effect & further compton effect interactions
direction of scatter
high energy = forwards scatter
low energy = backwards scatter
majority from x ray beam produced by x ray tube operating at 70kV is forwards scatter
to reduce scatter
collimation
impact of collimation
- decreases SA irradiated
- decreased volume of irradiated tissues
- decreased no of scattered photons produced in the tissue
- decreases scattered photons interacting with receptor
- decreases loss of contrast on radiographic image
- reduces ptx radiation dose & amount of radiation being released into surroundings
impact of photoelectric / compton on radiation dose
PE = deposition of all x ray photon energy into tissue, increases ptx dose but is necessary for image formation
C = deposition of some x ray photon into tissue, increases ptx dose but scattered photons do not contribute to image usefully & can increase dose to operators from back scatter
effect of lowering kV on x ray unit
lower x ray tube potential difference (kV)
overall lower energy photons produced
increased photoelectric effect interactions
increased contrast between tissues with different Z
increased dose absorbed by ptx
effect of raising kV on x ray unit
higher x ray tube potential difference (kV)
overall higher energy photons produced
decrease photoelectric effect interactions & increased forwards scatter
decreased dose absorbed by ptx
but decreased contrast between tissues with different Z
comparison of interactions in x ray process
continuous & characteristic = occur in x ray tube at target, electrons interacting with tungsten atoms, leads to production of x ray photons
PE & C = occur in ptx/receptor/shielding, x ray photons interacting with atoms, leads to attenuation of x ray beam
