X-Ray Interactions Flashcards
what is Rayleigh Scattering?
incident photon interacts with the whole atom, which it excites, scattered photon is emitted with no loss of energy
what energy x-rays does Rayleigh scatter usually occur at?
rayleigh scatter occurs with low energy x-rays (15-30keV)
what is compton scatter?
incident photon interact with loosely bound or “free”/valence shell electrons.
in compton scatter, what must the incident photon exceed?
the incident photon’s energy must greatly exceed the binding energy of the valent electron
what does compton scatter result in?
a scattered x-ray photon and an ejected compton electron
how is the incoming photon changed when it comes out the other side?
it comes out slightly lower in energy, this is because the compton electron has absorbed some of the energy
do the scattered photons and the compton electron appear separately or simultaneously?
simultaneously
what is the probability of compton scatter proportional to?
the probability of compton scatter is proportional to the density of the material
what is the range of angle that the scattered photon can be scattered at?
anywhere from 0-180 degrees
which is higher energy forward or backward scatter?
forward scatter has higher energy
what other interactions can occur after compton scatter?
scattered photons can further interact with other atoms, leading to further compton scatter and reach the image receptor
what effect does scatter have on image quality?
scatter can reduce tissue contrast in an image without contributing to overall image
compton scatter probability equation
σ ∝ 𝛒/E
what is photoelectric absorption?
when an incident x-ray photon interacts with a tightly bound electron
what shell(s) are the tightly bound electrons located that that can be involved in photoelectric absorption
usually the K or the L shell
what must the photon energy be for photoelectric absorbtion?
the photon energy must be at least equal to or greater than the binding energy of the ejected electron
what happens to the atom when an electron is ejected?
it becomes ionised
when an electron from an inner shell is ejected what can happen?
an outer shell electron can fall back to fill the vacancy
what are auger electrons?
auger electrons (also known as characteristic x-rays) are the difference in binding energy between the shells
why would a number of different energy characteristic x-rays be released?
there can be an electron cascade within atoms where electrons drop from shell to shell to fill vacancies
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?
the kinetic energy of this auger electron is = to the energy of the characteristic x-ray photon minus the binding energy of the electron
what happens to characteristic radiation produced by PE in soft tissue?
in soft tissue, when characteristic radiation is produced by PE it is of low energy and does not exit the patient
what radiography is PE important for? and why?
PE is important for mammography as low energies are used (less than 50keV)
