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
dominant force below 30KeV
PE; low scatter
30 keV dominant interaction
probability of compton and PE are equal; increased probability of penetration and reduction in total attenuation
> 30 keV dominant interaction
reduced PE interaction (1/E^3) and compton scatter
Increased tissue mass density results in?
decreased penetration, increased compton interactions, increased photoelectric interactions
increase in atomic number
decreased penetration, no change in compton, increase in PE
attenuation definition
xrays that interact with matter (compton/PE) and do not travel through an object
linear attenuation definition
fraction of photons interacting per unit thickness; linear attenuating coefficient
Factors that influence attenuation
more: denser objects, higher Z, at K-edge
less: higher kVp
applied absorption theory
low keV the contrast is greatest, but photoelectric effects predominate at lower keV and increases absorption
what is entrance skin dose
radiation absorbed by skin as beam strikes patient
factors influencing entrance skin dose
tube current (mA), time of exposure (s), peak kilovoltage (squared), distance (inverse squared)
