HomeStretch CRACK Physics X-Ray Interactions/General/Detectors Flashcards
What are the three ways x-rays interact with the patient
- Classical (aka elastic aka coherent aka thomson) = happens at low energies, no ionization, just scattered x-ray
- Compton scatter
- Photoelectric effect
Probability of PE is inversely proportional to the =
IP Energy Cubed (1/E3)
Probability relative to Z (atomic number)?
Prob of P.E. is directly proportional to the atomic number (Z3)
High yield recap
Classic/elastic interaction
has 4 names. . . classic, coherent, elastc and thomson
low energy (<10keV)
No contribution to image
tiny contribution to dose
does not result in ionization (no e- lost)
High yield recap
Compton
Bad boy
Major contributor to scatter / image fog
Involves the outer shell electron (Compton drive by)
part of the e- energy is transferred
variable energy
doesn’t care about Z (doesn’t give a Compton shit)!
depends on density
dominates at above 30 keV
High yield recap
Photoelectric effect
Good boy
major contributor to image contrast
involves INNER shell e-
ALL of the photon energy is transferred
All or nothing!
depends on Z3
More important in higher Z elements
Dominates below 30 keV
Compton and PE are about equal at ____ keV within tissue
Compton and PE are about equal at ____ keV within bone
Compton and PE are about equal at ____ keV within barium and iodine
30 keV
50 keV
300 keV
Attenuation in tissue depends on what 3 things?
- Effective atomic number in tissue
- X-ray beam quality (energy average)
- tissue density
Linear attenuation coefficient
measure of the probability of the material to attenuate x-rays over a set distance
effected by the density of the material
Mass attenuation coefficient = LAC / Density
Mass attenuation coefficient
Mass attenuation coefficient = LAC / Density
Measure of the RATE OF ENERGY LOSS BY AN XRAY as it traves through a material
independent of density
depends only on atomic number and photon energy
half value layer
amount of material required to attenuate x-ray to half the original output
increasing energy (kVp) does what to linear attenuation coefficient and half-value layer?
decreases the LAC
increases the HVL
increasing density does what to linear attenuation coefficient and half-value layer?
increases LAC
decreases the HVL
What is entrance skin dose and what are the factors that determine it?
radiation ABSORBED by the skin as the beam strikes the patient
tube current (mA) = proportional
Tume of exposure = proportional
kVp = square
Distance = inverse square law (double distance = dose reduced by 4)
What is quantum mottle?
how do you reduce it?
it is most important source of random noise in imaging
noisy image because not enough x-rays are reaching the film/detector
reduce mottle by increasing more x-rays (mA) and more efficient detection
Scatter is maximized with what three things?
- high kVp
- thick parts (or people)
- large FoV
What does collimation do to FoV and noise?
increase collimation = decreased filed size and decreased noise
What does a grid do?
blocks off-angle xrays
decreases noise but increases dose
what is grid ratio?
ratio of the height of the grid to the distance between them
in regards to a grid, what does it do to dose ?
dose increases with use of a grid
higher grid ratio increases dose
if a grid is placed upside down, what do you see?
edges of image will be “cut-off” and the central portion is preserved (flip it around)
if given a choice to increase mA or kVp to decrease noise, you should increase which one?
increase mA
noise will increase as the distance between the tube and detector increases by what increment?
inverse square law
Quadrupaling the x-rays will decrease mottle by what factor?
mottle will be cut in half.
