xray intro

Question 1

attenuation

Answer 1

removal of photons from the primary beam

Question 2

compton scatter

Answer 2

incident xray photon scattered by a loosely bound/free electron because binding energy < incoming xray energy

inelastic, energy of scattered xray much smaller

photons are scattered out of the beam with reduced energy

recoil electrons are emitted in a forward direction with kinetic energies up to Tmax. These ionise the material until energy is lost.

depends on density of material

Question 3

processes that can remove photons from initial beam

Answer 3

photoelectric
compton scatter
coherent/small angle scattering
pair production

Question 4

coherent scatter

Answer 4

elastic scattering from bound atomic electrons

confined to small angles

Question 5

photoelectric effect

Answer 5

photons incident on bound electrons (usually K shell) can be totally absorbed as the atom as a whole takes up recoil momentum

the excited electron is then emitted with energy T=hv-B where B is the binding energy of the electron

dominates at low energies

Question 6

when an ionised atom de-excites

electron ejected from atom followed by emission of …

Answer 6

characteristic xray:
electron drops down from higher shell
energy of xray depends on relative shell energies (difference in jump)

auger electron:
energy given to outer shell electron - emitted
lower atomic number materials

Question 7

atomic number effect on fluorescent yield

Answer 7

higher Z, higher yield

Question 8

absorption edges

Answer 8

discontinuities caused by absorption edges: binding energies of specific electron shells

low X materials, low K shell BE

as the photon energy increases, photoelectric absorption decreases

incident xray energy < binding energy of K shell electron, xray can only kick out L and M

when x-ray energy > K shell BE, attenuation coefficient for photoelectric absorption jumps up (xrays can kick out K shell electrons now)

Question 9

pair production

Answer 9

at high energies, an interaction with the electrostatic field of the nucleus produces a positron and electron pair

pair moves through material causing ionisation

positron annihilates giving 2 0.511MeV gamma rays

Question 10

how xrays are produced

Answer 10

bombard metal w electrons

when a charged particle accelerates, electromagnetic radiation is emitted

more abrupt, higher E

xrays produced when electrons interact with atoms

interactions are 99% heat, 1% xrays

Question 11

bremsstrahlung radiation

Answer 11

electrons slow down and emit radiation as a consequence

when a charged particle is accelerated it emits radiation

peak at low energies

Question 12

thin target theory

Answer 12

thin enough every electron undergoes only one radiative interaction when passing through

intensity/energy graph = flat distribution, uniform, same probability for all energies

Question 13

thick target theory

Answer 13

reality
number of thin tarhets superimposed, so repeat process but lower energy every time

triangular shaped spectrum
intensity/energy graph

Question 14

self-absorption in target material

Answer 14

xrays generated inside target material
some reabsorbed

lower energy, higher likelihood

Question 15

characteristic radiation

Answer 15

electrons ionise/excite atom

bombarding electron ionises the target atom

an electron from a higher shell drops down to fill the vacancy

photon energy given by difference in energy levels

69kv for tungsten
20kv for molybdenum

Question 16

x ray tube parts

Answer 16

rotating portion - heat dissipation over larger area (doesnt melt metal), short exposure times

glass envelope for vacuum
anode (W or Mo) emits xrays

Question 17

why anode is inclined

Answer 17

small focal spot to minimise unsharpness in image
large focal spot to handle heat from collisions

inclined so larger surface of electrons whilst reducing focal spot

Question 18

heel effect

Answer 18

xrays generate in the target exit the target along different path lengths

longer the path, more attenuation

lower energy xrays are attenuated easier than higher

so epctrum is less intense and harder where the path is longer, more intense and softer where the path is shorter

stronger heel effect if angle reduced as higher diff in path length

adv: smaller focal spot, better for high resolution imaging

Question 19

angular distribution of xrays

Answer 19

polar plot

low energies, xrays emitted in all directions
high, more forward distributed

Question 20

effect of kv and mA on xray spectrum

Answer 20

increasing kV
intensity increases
mean energy increases
relative weight of K lines change

increasing mA:
spectral shape doesnt change, but entire spectrum increases/decreases proportionally with mA value

Question 21

beam filtration

Answer 21

used to increase mean energy of the spectrum

to reduce dose to patient

beam quality:
high energy - hard
lower energy - soft

sheet of metal in front of exit window

tin, copper, Mo