2. XR Interactions with People Flashcards
Types of x-ray interactions
classical/coherent, compton, photoelectric, pair production, photodisintigration
What energy level does compton scatter occur compared to classical/coherent interaction?
higher energy
Describe the compton scatter interaction
- incident X-ray frees an electron (compton e-)
- atom is ionized (missing e-)
- incoming x-ray is deflected (scattered photon)
dominant force contributing to scatter/fog?
compton interactions, >100 keV
major source of occupational exposure?
compton
probability of compton depends on?
density of material or hydrogenous material/people (materials rich in hydrogen have increased probability)
decreased with increased x-ray energy
mechanism of photoelectric interactions
x-ray strikes inner shell electron, displacing it and energy is absorbed; Auger electron is released
- characteristic radiation
- negative ion (photoelectron)
- positive ion (atom missing an electron)
what energy level does photoelectric interactions take place?
low energy; although present throughout diagnostic range (20-120)
probability of PE?
- inversely proportional to incident photo energy 1/E^3
- must have minimum incident photon energy to free K shell electron
- probability increases as binding energy/incident photon energy are similiar
- proportional to the atomic number cubed (Z^3)
how does Z and PE affect contrast?
small differences in Z (bone > soft tissue) are amplified (cubed); better soft tissue delineation
why do xrays appear overexposed?
kVP is too high; not enough photoelectric effect/contrast
lead and the photoelectric effect
lead has high Z; more absorption
k shell of calcium
4 keV
role of barium/iodine in secondary radiation
higher Z elements; generate secondary x-rays that can leave patient/fog radiation
k-edge
photoelectric energy peaks around binding energy for the inner shell electron
optimal kvP for contrast agents
65-90 kVp as barium (k edge 47), iodine (k edge 33)
downside of PE interactions?
generate more dose than other types of radiation because ALL the energy of incident x-ray photon is absorbed
x-ray production increases how with mA vs kVP
linearly with mA; increase kVp will double the intensity
tactic to visualize low contrast objects
keep kVp constant, increase mAs; optimize PE
lower dose but maintain exposure?
raise kVp 15%, lower mAs 50%
optimal kVp for contrast agents
double the binding energy of contrast agent; if K edge of Iodine is 33, you want kVp of 66; barium kVp is 74 (k edge is 37 kev)
what energy photon is needed to create pair production?
1.022 MeV, high energy photon on something with high Z
mechanism of pair production
high energy photon interacts with nucleus and is absorbed; production of 1 e/positron. Positron will interact with an electron and annihilate giving off 2 511 keV photons 180 degrees apart
what is the mechanism of photodisintegration
high energy photons (>10 MeV) strke nucleus and cause ejection of alpha particles