5 principle interactions of x-rays with matter
-low to high
- coherent scattering
- photoelectric effect
- Compton scattering (effect)
- pair production
- photo disintergration
coherent scattering
- what does it do
- happens at
- eliminated by
- changes the direction of the x-ray without changing its energy
- at lowest energy
- filtration
photoelectric effect
- definition
- 3 steps
- probability goes down as
- probability goes up rapidly as
- why is it important
- absorption of x-rays by ionization
- x-ray kicks out an inner shell (K or L)
- x-ray energy completely absorbed by e-
- e- is ejected
- x-ray kicks out an inner shell (K or L)
- as x-ray energy increases
- rapidly with increases in atomic number
- best for medical
properties of photoelectric effect
- lead is a good shield
- no scatter x-rays
- good to image dense vs soft tissue
- must be filtered to remove lowest energies
- relatively large distance (dense vs soft)
Compton effect
- definition
- type of x-ray
- 3 steps
- scattering of x-rays that results in ionization and loss of energy
- lower energy x-ray
- x-ray interacts with outer shell electron
- electron is ejected
- x-ray loses some energy and is scattered
- x-ray interacts with outer shell electron
probability of Compton effect
- lowest at
- highest at
- drops off rapidly at
- differences?
- low energies
- moderate energies (diagnostic)
- above 1Mev
- not as much difference between bone and soft tissue4
implication of Compton effect
- very likely in higher energy diagnostic x-rays
- creates scatter radiation primarily from patient
- source of most occupational dose to staff
pair production
-interaction between the x-ray and the nuclear electric field that causes the x-ray to disappear and that causes 2 electrons to take its place
photo disintergration
-process by which very high-energy x-rays can escape interactions with electrons and the nuclear electric field can be absorbed directly by the nucleus
differential absorption
difference between x-rays absorbed and x-rays transmitted
what situations will produce fogging on an x-ray
Compton scattered x-rays
what situations will provide info to the image
photoelectric effect and Compton effect (when x-ray is partially absorbed)
what settings does of kVp and mAs can result in better differential absorption
kVp = low; mAs = high
-patient’s dose increases
mass density
the quantity of matter per unit volume
how does mass density play a role in imaging bones
when mass density is doubled, chance for x-ray interaction is doubled. therefore, nearly twice as many x-rays would be absorbed and scattered in bone as soft tissue. bone would be imaged
how does mass density play a role in imaging lungs
imaged because of differences in mass density
-three times as many x-rays interacts with soft tissue
how do contrast agent enhance mass density
they are used to image the organs
-higher atomic number and greater mass density than soft tissue
