X-Ray Interactions

Question 1

what is Rayleigh Scattering?

Answer 1

incident photon interacts with the whole atom, which it excites, scattered photon is emitted with no loss of energy

Question 2

what energy x-rays does Rayleigh scatter usually occur at?

Answer 2

rayleigh scatter occurs with low energy x-rays (15-30keV)

Question 3

what is compton scatter?

Answer 3

incident photon interact with loosely bound or “free”/valence shell electrons.

Question 4

in compton scatter, what must the incident photon exceed?

Answer 4

the incident photon’s energy must greatly exceed the binding energy of the valent electron

Question 5

what does compton scatter result in?

Answer 5

a scattered x-ray photon and an ejected compton electron

Question 6

how is the incoming photon changed when it comes out the other side?

Answer 6

it comes out slightly lower in energy, this is because the compton electron has absorbed some of the energy

Question 7

do the scattered photons and the compton electron appear separately or simultaneously?

Answer 7

simultaneously

Question 8

what is the probability of compton scatter proportional to?

Answer 8

the probability of compton scatter is proportional to the density of the material

Question 9

what is the range of angle that the scattered photon can be scattered at?

Answer 9

anywhere from 0-180 degrees

Question 10

which is higher energy forward or backward scatter?

Answer 10

forward scatter has higher energy

Question 11

what other interactions can occur after compton scatter?

Answer 11

scattered photons can further interact with other atoms, leading to further compton scatter and reach the image receptor

Question 12

what effect does scatter have on image quality?

Answer 12

scatter can reduce tissue contrast in an image without contributing to overall image

Question 13

compton scatter probability equation

Answer 13

σ ∝ 𝛒/E

Question 14

what is photoelectric absorption?

Answer 14

when an incident x-ray photon interacts with a tightly bound electron

Question 15

what shell(s) are the tightly bound electrons located that that can be involved in photoelectric absorption

Answer 15

usually the K or the L shell

Question 16

what must the photon energy be for photoelectric absorbtion?

Answer 16

the photon energy must be at least equal to or greater than the binding energy of the ejected electron

Question 17

what happens to the atom when an electron is ejected?

Answer 17

it becomes ionised

Question 18

when an electron from an inner shell is ejected what can happen?

Answer 18

an outer shell electron can fall back to fill the vacancy

Question 19

what are auger electrons?

Answer 19

auger electrons (also known as characteristic x-rays) are the difference in binding energy between the shells

Question 20

why would a number of different energy characteristic x-rays be released?

Answer 20

there can be an electron cascade within atoms where electrons drop from shell to shell to fill vacancies

Question 21

if the energy is transmitted to an orbital electron in the atom and this electron is ejected, what is the kinetic energy of that Auger electron equal to?

Answer 21

the kinetic energy of this auger electron is = to the energy of the characteristic x-ray photon minus the binding energy of the electron

Question 22

what happens to characteristic radiation produced by PE in soft tissue?

Answer 22

in soft tissue, when characteristic radiation is produced by PE it is of low energy and does not exit the patient

Question 23

what radiography is PE important for? and why?

Answer 23

PE is important for mammography as low energies are used (less than 50keV)