Physics Ch 2 XR Interactions Flashcards
XR –> interact w ppl –> 5 ways?
- classical
- compton
- photoelectric
- pair production
- photodisintegration
what is classical (elastic, coherent, thomson) interaxn?
xray –> strike orbital electron –> bounce off –> change direction
classical (elastic, coherent, thomson) –> what energy level?
low energy –> <10 keV
classical (elastic, coherent, thomson) –> cause ionization –> T/F?
F: too low energy
classical (elastic, coherent, thomson) –> how much contribute to…
- image?
- pt rad dose?
image: no sig
pt rad dose: tiny/small
Compton scatter –> what energy level?
high
what is compton scatter?
XR –> strike outer shell electron –> eject electron –> XR lose some energy –> change direction
compton scatter interaxn –> main results? (3)
- freed electron –> compton electron
- atom ionized
- XR –> lose energy & change direction –> “scattered photon”
compton scatter interaxn –> what happen to…
- image?
- pt rad dose?
image –> foggy
rad dose: increase
diag energy (>100keV) –> soft tissue –> MC interaxn?
compton
diag energy range –> what can decrease compton scatter?
inc XR energy
image –> scatter/fog –> MC contributor?
compton scatter
occupational exposure –> MC source?
compton scatter
inc atomic# (Z) –> decrease compton scatter –> T/F?
F: compton scatter not depend on Z
inc materal density –> inc compton scatter –> T/F?
T
photoelectric interaxns –> occur at what energy?
diag range (20-120)
low energy –> photoelectric interaxn vs compton scatter –> which predominates?
photoelectric
photoelectric interaxns –> MOA?
xray –> strike inner shell electron –> eject electron
photoelectric interaxns –> what 3 products?
- charact xrays or Auger electron
- ejected electron (photoelectron)
- positive ion (original atom –> now missing 1 electron)
photoelectric interaxns –> highest probability –> at what energy level?
near binding energy
probability of photoelectric interaxn relative to kVp energy (incident photon energy) –> formula?
1/E^3
probability of photoelectric interaxn relative to atomic# –> formula?
Z^3
which interaxn develops image contrast?
photoeletric
probability of photoelectric interaxn –> affected by what 2 factors?
- kVp
- Z
which interaxn contributes most to pt rad dose?
photoeletric
general practice –> select highest kVp possible –> why?
photoelectric interaxn:
- higher kVP –> less absorbed –> less rad dose to pt
- but not too high kVp –> enough image contrast –> diagnostic image
more photoelectric interaxn –> what happen to contrast?
increase
what is K-edge?
energy at which there is peak photo-electric
to visualize low-contrast tissues –> how to change…
- kVp
- mA
- kVp constant
- mA inc
to lower dose but maintain constant exposure –> how to change…
- kVp
- mA
why?
- kVp inc 15%
- mA dec 1/2
higher kVp –> more penetrate –> less absorbed by pt (less photoelectric)
iodine –> K-edge 33 keV –> minimum kVp should be?
66 kVp
barium –> K-edge 37 keV –> minimum kVp should be?
74 kVp
classic (coherent, elastic) –> causes ionization –> T/F?
F
too low energy to cause ionization
differeniate: probability of photoelectric vs compton –> at <30 keV?
- PE: high prob
- compton: low
differeniate: probability of photoelectric vs compton –> at 30 keV?
PE & compton –> equal prob
differeniate: probability of photoelectric vs compton –> at >30 keV?
- PE: sig dec prob (1/E^3)
- compton: dec prob (but not as much as PE)
differeniate: probability of photoelectric vs compton –> inc tissue mass density?
- PE: inc
- compton: inc
differeniate: probability of photoelectric vs compton –> inc Z?
- PE: inc
- compton: no change
relationship bw mass attenuation & linear attenuation –> formula?
mass attenuation coefficient = linear atten coeff / density
what is linear attenuation coefficient?
material’s probability to attenuate XR beam over a set distance
what is mass attenuation coefficient?
rate of XR energy loss as travel thru material
linear attenuation coefficient –> affected by what factor? (1)
material density
mass attenuation coefficient –> affected by what factor? (2)
- Z
- photon energy
inc kVp –> what happen to…
- linear atten coeff
- half value layer
- linear atten coeff –> dec
- half value layer –> inc
inc density of tissue –> what happen to…
- linear atten coeff
- half value layer
- linear atten coeff –> inc
- half value layer –> dec
entrance skin dose –> affected by what factors? (4) by how much?
- mA –> proportional
- time of exposure (s) –> proportional
- kVp –> ^2
- distance –> 1/^2
if dose = 28 mR at 25mA –> what is dose at 35mA?
28 x 35 / 25
if dose = 20mR at 75kVp –> what is dose at 125kVp?
20 x 125^2 / 75^2
