radiology physics

Question 1

what is a photon

Answer 1

packet of energy

single particle of light or x ray

Question 2

transtitions of electrons to orbits

Answer 2

• a transition of an electron to a higher energy orbit will require the input of energy, in this case a photon
• a transition of an electron to a lower energy orbit will cause loss of energy, in this case a photon will be emitted
  If an eletron in orbit 1 (has E=10), and a photon with at least E=20 hits it, it can go up to orbit 2 as (E=30)
  If a space becomes available in orbit 1, an electron from orbit 2 will drop into it and give off a photon of E=20

Question 3

ionisation

Answer 3

If we give an electron enough energy, we can completely free it from the pull of the nuclear
- this gives a positive ion (the atom -1e) and one free electron

Question 4

nucleus contais

Answer 4

neutrons and protons

Question 5

particle radiation

Answer 5

• includes alpha and beta (electrons) particles from radioactive decay
• beams consist of many individual particles

Question 6

electromagnetic radiation

Answer 6

Shows the relationship between energy, frequency and wavelength for types of electromagnetic radiation

Question 7

gamma rays

Answer 7

An excited nucleus may emit a gamma ray in order to return to its ground state

• the emitted Y ray has no charge
• the Y ray is less ionising than the particular radiations
• The Y ray can be very penetrating, tens of mm Pb may be required to reduced their intensity
• Y rays are identical to X rays, except for the manner of production

Question 8

how are x rays produced

Answer 8

X rays are produced by accelerating electrons towards a metal target in high voltage, evacuated tubes

Question 9

steps of x ray production (diagnostic x ray tube

Answer 9

Filament is heated by electric current to release e-

• high voltage are between the anode and cathode
• e- are attracted to the anode
• glass envelope removes air
• anode (target) is typically tungsten
• e- hit the anode
• x rays are created in all directions
• lead shielding reduces radiation
• inefficient process, 99% becomes heat

Question 10

purpose of copper block

Answer 10

• copper block prevents heat damaged to anode, also needs high melting point to deal with heat

Question 11

rectification

Answer 11

AC - not suitable as anode/cathode will keep swapping over

Three way rectified(shifting phase to give 3 phases, can smooth to give more constant potential)

Question 12

braking radiaiton and how it works

Answer 12

• A continuous spectrum produced by rapid deceleration of electrons passing close to target nuclei
• atom in anode, e- comes in with lots of energy
• e- is slowed by the positive nucleus, energy lost is emitted as an xray photon
• electron interacts with the nucleus too
• if an electron looses all its energy, the photon given off is Emax (only one high energy photon)
• but if it looses little bits of energy, will get more photons with smaller amounts of energy

Question 13

characteristics x ray formation

Answer 13

An electron is knocked out of the K (inner shell) shell by an electron colliding with an e-, leaving a gap
An L shell electron falls into the gap (k shell) and emits energy as an x ray
- energy of photon emitted = (energy of electron in L shell) – (energy electron now has in K shell)

Question 14

atteunation

Answer 14

removing energy from the radiation beam

Question 15

comparison of attenuation between radiations

Answer 15

Patient = tissue
Beta is scattered and slowed in tissue

xray can travel long distances before being scattered
- depentant on tissue type ect

There is a reduction in intensity due to interaction with tissue
lower the energy more likely to stop, therefore you want to reduce the low energy photons much more than the high energy ones

Question 16

factors which affect x ray beam intensity, energy of photons quality act

Answer 16

• no of electrons hitting the anode is called the tube current (milliamp)
• Tube voltage between anode and cathode is what attracts the e- to the anode
• each e- has more energy, conversion efficieny improves so more photons and increase average energy of the photons so the beam quality is increased
• target atomic number affects the intensity but not quality of beam, intensity increases with the atomic number of the anode (tungsten has a high z 74 and high metling point to withstand het)
• rectification see power supply, reduced intensity via ripple

Question 17

half value layer and what happens after each layer is progressed through

Answer 17

amount of a given material that will reduce the given beam by half

• This means the average energy of photons in the beam increases after intenuation, as you have removed many photons but these are of lower energy, so therefore the average remaining energy of the photons is higher as the high energy ones have got through

Question 18

filtraiton use

Answer 18

Very low energy photons don’t make it out of the tube

• the next ones would make it to the patient but not through the patient so would not contribute to the imaging, only would contribute to the dose
• therefore these need to be removed from the beam usinga sheet of metal, minimum of 1.5mm Al
• this does not have much effect on the high energy electrons
• but too much filtration, reduce contrast in image
• also would need to start with more photons, therefore more heat production thus damaging the tube

Question 19

inverse square law

Answer 19

Radiation intensity falls off via the inverse square law

• assume all photons come from one point and spread from there
• number of photons per uni area decrease
• eg 9 photons at point s
• at distance r, all photons go through distance r
• the next distance is 2r, 4 squares
• the 9 photons are still passing through the area but the density is decreasing, by 9x by distance increase of 3x, hence inverse square law

Question 20

photo electric absorption in tissue

Answer 20

photo interacts with inner shell e-
photo energy will be absorped by the e-
if the energy absroptied exceeds the e- binding energy the electron is emitted from the atom and is known as photo electron

gap is filled from e- from higher shell, energy released is emitted as characteristic radiation

Question 21

Compton scatter

Answer 21

Energy required to remove an e- from the shell (binding energy)

• further away from nucleus,lower the binding energy
• photon bounces off from K shell e-, not losing much energy
• photon interacting with otuter e- may lose some enryg to the electron and ionise th atom
• photon will change direction and energy
• scattered photons will have a range of energies and direction, gives poor imaging
• some are still removed from the beam so we can still get some information
• more photons = more scatter

Question 22

x ray interactions and compromise

Answer 22

Absorption - Photoelectric Effect
Scattering - Fogging on films - Compton Effect
Most tissues are of similar density and Z; therefore need to choose low kV to optimise contrast.
But transmission increases with kV (so can reduce mA or time which decreases patient dose).
So we need to compromise between contrast and dose.
Highest absorption at low energies
Transmission increases with kV
Lower energies give greatest relative difference in attenuation between different materials