Ch. 9 X-ray interaction with matter Flashcards
Name 5 ways x-rays interact with matter.
- coherent scattering
- compton scattering
- photoelectric effect
- pair production
- photodisintegration
Of the 5 way x-rays interact with matter, what are the 2 that are important in making an x-ray image/
- compton scattering
2. photoelectric effect
Electromagnetic radiation interacts with structures that are…
similar in size to the wavelength of the radiation
What kind of wavelengths do x-rays have?
very short wavelengths
The higher the energy of an x-ray, the….
shorter the wavelength
What do low energy x-rays interact with?
whole atoms
What do moderate energy x-rays usually interact with?
electrons
What do high energy x-rays interact with?
nuclei
At what keV do x-rays interact with matter through coherent scatter?
below 10 keV
What are other names for coherent scattering?
Thompson scattering or classical scattering
Who was the 1st physicist to describe coherent scattering?
J.J. Thompson
How does coherent scatter occur?
- the incident x-ray interacts with a target atom, causing it to be excited
- the target atom releases this excess energy as a scattered x-ray with wavelength equal to that of the incident x-ray
- the direction of the scattered x-ray is different from that of the incident x-ray
What is the result of coherent scattering?
a change in direction of the x-ray without a change in its energy
In coherent scattering, why is there no ionization?
because there is no energy transfer
What direction does coherent scatter usually go?
forward
Is coherent scatter important in diagnostic xray?
no. it has little importance
Coherent scatter has ____ energy x-rays.
low
At 70 kVp, how does coherent scatter affect a diagnostic x-ray image?
it contributes a little to image noise
What is compton scattering?
when x-rays undergo an interaction with outer shell electrons that not only scatters the x-ray but reduces its energy and ionizes the atom
What is a compton electron?
the ejected electron that is ejected from the atom during compton scatter
In compton scatter, the x-ray continues in a ____ direction with ____ energy
different ; less
The energy of the compton scattered x-ray is equal to what?
it is equal to the difference between the energy of the incident x-ray and the energy of the ejected electron.
What is the energy of the ejected electron equal to?
its binding energy plus the kinetic energy with which it leaves the atom
What is the equation for the compton effect?
incident energy=scattered energy (binding energy x kenetic energy)
During compton, what retains most of the energy, the scattered x-ray or the compton electron?
the scattered x-ray, but both may have enough energy to undergo additional ionizing interactions
Ultimately, the scattered x-ray is absorbed how?
photoelectrically
What happens to the compton electron?
it loses all of its kinetic energy through ionization and excitation and drops into a vacancy in an electron shell created by some other ionizing event.
How many degrees can a compton scattered x-ray go?
up to 180 degrees from the incident x-ray
During compton scatter with a deflection of 0 degrees from the incident x-ray, how much energy is transferred?
none
As the angle of deflection increases in compton scatter, what happens?
more energy is transferred tot he compton electron but even at 180 degrees, the scattered x-ray keeps at least 2/3 of its original energy.
What is backscatter radiation?
x-rays that are scattered back in the direction of the incident x-ray beam
How could the possibility of compton scatter decrease?
as the x-ray energy increases, the possibility of compton scatter decreases
the probability of compton scattering is ____ proportional to x-ray energy and _____ of atomic number
inversely; independent
T or F? the likelyhood of compton scatter does not depend on the atomic number of the atom involved.
T. Any x-ray is just as likely to undergo compton scatter with an atom of soft tissue as with an atom of bone
Compton scatter reduces…
image contrast
During what exams can compton be bad?
During radiography and especially fluoro.
Scattered radiation from the patient during fluoro leads to what for the radiographer?
the most of the occupational radiation exposure that radiographers receive.
What is photoelectric effect?
X-rays in the diagnostic range that also undergo ionizing interactions with inner shell electrons. The x-ray is not scattered, but totally absorbed.
Who earned the nobel peace prize for discovering the photoelectric effect? year?
Albert Einstein in 1921
Explain how PE works.
the electron removed from the atom, which is called the photoelectron, escapes with kinetic energy equal to the difference between the energy of the inident x-ray and the binding energy of the electron.
What is the mathematical equation for PE?
incident energy = electron binding energy + kinetic energy of the electron
What is total x-ray absorption called?
the photoelectric effect
What tissue has low atomic number electrons to interract with?
soft tissue
When are characteristic xrays produced?
after a photoelectric interaction
When an electron is pushed out of the K-shell and replaced by one of an L shell, what also happens?
it is accompanied by the emission of an x-ray whose energy is equal to the difference between binding energies of the shells involved.
Secondary radiation behave in the same manner as..
scatter, but do not penetrate the IR
The probability of the photoelectric effect is inversely proportional to…
inversely proportional to the third power of the x-ray energy.
A photoelectric interaction cannot occur unless…
the incident x-ray has energy equal to or greater than the electron binding energy
The probability of the photoelectric effect is directly proportional to…
the third power of the atomic number of the absorbing material
A photoelectric interaction is much more likely to occur with _____ than with _____
high Z atoms; low Z atoms
Explain the process of pair production
If an incident x-ray has sufficient energy, it may escape interaction with electrons and come close enough to the nucleus of the atom to be influenced by the strong nuclear field. The interaction between the x-ray and the nuclear field causes the x-ray to disappear, and in its place, 2 electrons appear, one positively charged (positron) and one negatively charged.
pair production does not occur during…
x-ray imaging
Pair production occurs with x-rays that have energies greater than…
1.02 MeV
What modality does pair production matter?
PET positron emission tomography in nuke med
Describe photodisintegration
when x-rays with energy above 10 MeV can escape interaction with electrons and the nuclear field and be absorbed directly by the nucleus and the nucleus is raised to an excited state and instantly emits a nucleon or other nuclear fragment.
Photodisintegration does not occur during….
diagnostic imaging
Of the 5 ways an x–ray can interact with tissue, only 2 are important to radiology…
compton scatter and photoelectric effect
only 2 methods of x-ray production are important in diagnostic x-ray. They are…
bremsstrahlung and characteristic
Why does differential absorption occur?
because of compton scattering, photoelectric effect, and x-rays transmitted through the patient
photoelectric absorption of x-rays produce which part of the radiograph?
the light areas such as bone
describe differential absorption…
an image results from the difference between those x-rays absorbed photoelectrically in the patient and those transmitted to the IR. The difference in x-ray interaction is called differential absorption
Differential absorption increases as …
kVp is reduced
With higher x-ray energy, fewer interactions occur, so…
more x-rays are transmitted without interation
At low energies, most x-ray interactions with tissue are…
photoelectric
At high energies, what predominates?
compton scattering
As energy is increased, the chance of any interaction at all….
decreases
To image small differences in soft tissue, one must use…
low kVp to get maximum differential absorption
Do you use high or low kVp for barium studies?
high kVp
Differential absorption in bone and soft tissue result from why type of interactions?
photoelectric
What do the crossover energies of 20 and 40 keV refer to?
monoenergetic x-ray beam
What is monoenergetic x-ray beam?
a beam containing x-rays that have the same energy
What is polyenergetic?
a beam containing x-rays are emitted over an entire spectrum of energies. (clinical x-rays)
What is mass density?
the quantity of matter per unit volume, specified in units of kg per cubic meter
The interaction of x-rays with tissue is proportional to the
mass density of the tissue regardless of the type of interaction
What is the atomic number of barium?
56
What is the atomic number of iodine?
53
What technique produces excellent high contrast radiographs of the organs of the GI tract when doing contrast studies?
low kVp (less than 80 kVp)
When would you use higher kVp (above 90) during a contrast study?
When you want to outline the organ or also to penetrate the contrast medium so the lumen of the organ can be visualized
What is a double contrast exam?
when air is also used with barium or iodine as a contrst
Attenuation is the product of..
absorption and scattering
The total reduction in the number of x-rays remaining in an x-ray beam after penetration through a given thickness of tissue is called..
attenuation
