radiation physics

Question 1

definition of radiation (transfer of..)

Answer 1

transfer of energy through space and matter

Question 2

definition of ionising radiation (neutral atom..)

Answer 2

neutral atom loses an electron –> forms ion pair of position ion and electron –> cause downstream effects in body (radiation biology)

Question 3

2 types of radiation

Answer 3

1. particulate radiation
2. electromagnetic radiation

Question 4

particulate radiation: __ mass and ___ charge
examples

Answer 4

no mass and may have charge
alpha particle - helium, beta particle - electron, neutrons

Question 5

electromagnetic radiation: __ mass and __ charge
comprises of ___ radiation and non-___ radiation? describe?
Examples?

Answer 5

no mass and no charge
ionising radiation and non ionising radiation
non ionising radiation - long wavelength, low energy
ionising radiation - short wavelength, high energy = gamma and x ray

Question 6

10 characteristics of x ray

Answer 6

1. electromagnetic radiation
2. no mass
3. no charge
4. invisible
5. travel in str. lines
6. travel at speed of light 3.6x10^8 m/s
7. produce polyenergetic or heterogenous beam
8. scatter or absorbed by tissues of body
9. chemical or biologic damage
10. can’t be focussed by lens

Question 7

components of x ray unit
function of components in x ray tube

Answer 7

1. control panel
   - mAs (exposure time control) and kVp
2. suspensory arm
3. tube head
   –> power supply (2)
   - low voltage current to heat filament at cathode
   - high potential difference between cathode and anode

–> x ray tube:
√ tube housing with port cover (4)
- physical protection and electrical insulation
- has oil to dissipate heat
- made of metal (steel/aluminium) and lined w lead to minimise leakage
- port cover to allow x ray beam to exit

√ glass envelope with tube window (3)
- electrical insulation
- vacuum for x ray prdn
- tube window allow x ray beam to exit

√ cathode (-) has:
- nickel electron focussing cup: focus electron into narrow beam
- filament: thermionic emission occurs –> filament heated –> electrons boil off –> electrons accelerate towards anode

√ anode (+) w tungsten target (2):
- x ray photons produced due to bremsstrahlung and characteristic radiation
- inefficient conversion of kinetic energy to x ray photon –> 99% converted to heat energy

Question 8

2 ways x ray are produced

Answer 8

1. bremsstrahlung radiation
   - forms polyenergetic beam
   - electron from cathode electrostatically attracted to anode –> tungsten target nuclei deflects and decelerates electron –> electron loses kinetic energy –> converted to x ray photon energy
   - closer electron hit nuclei –> greater electrostatic attraction –> greater deflection and deceleration –> greater x ray photon energy
2. characteristic radiation
   - forms heterogenous beam
   - electron hits tungsten target nuclei –> inner orbit electron ejected out –> outer orbit electron fills vacancy –> diff in binding energy emitted as characteristic x ray photon
   - min 69.5keV

Question 9

x ray emission spectrum:
x axis?
y axis?
what does graph tell?
the curve is made of what 2 things?

Answer 9

y: photon quantity
x: photon quality (keV)

–> can tell the max kVp setting, mean energy
–> made of bremsstrahlung and characteristic radiation

Question 10

5 factors affecting x ray beam

Answer 10

1. increase mAs –> mean energy same, photon quantity increase, pass through origin
2. increase kVp –> mean energy increase, photon quantity increase, pass through origin
3. filtration - filter out low energy photons inherently or added filtration by manufacturer –> dont pass through origin, mean energy increase, photon quantity decrease
4. collimation: round or rectangular - rectangular lesser scatter radiation (no graph)
5. distance - distance x2, intensity x1/4 (no graph)

Question 11

how factors affecting x ray beam –> affect the image quality

Answer 11

1. mAs (current, exposure time)
   - increase mAs –> increase density –> dark
   - decrease mAs –> decrease density –> noisy/grainy
2. kVp (tube voltage)
   - increase kVp –> increase density, decrease contrast (more shades of grey)
   - decrease kVp –> decrease density, increase contrast (lesser shade of grey)
3. filtration
   - decrease density, decrease contrast
4. collimation
   - rectangle: increase contrast
   - round: decrease contrast
5. distance
   - increase distance –> decrease contrast

Question 12

types of x ray interactions

Answer 12

1. no interaction (9%)
2. coherent scatter (7%)
3. photoelectric absorption (27%)
4. comptom scatter (57%)

Question 13

describe coherent scatter

Answer 13

• does not cause ionisation –> minimal potential for biological damage
• minimal effect to image quality
• 7%
• low energy x ray photon (<15keV) interacts with atom –> excites atom –> vibrates electrons –> same energy x ray photon emitted in different direction

Question 14

describe comptom scatter

Answer 14

• cause ionisation –> biological damage
• affect image quality –> more scatter –> decrease contrast –> radiation fog
• useless information
• 57%
• intermediate energy x ray photon (>15keV) interacts with outer shell atom –> ejects electron –> remaining energy emitted as scatter photon

Question 15

describe photoelectric absorption

Answer 15

• cause ionisation –> biologic damage
• improve image contrast
• contributes to differential absorption
• 27%
• intermediate energy x ray photon (>15keV) interacts w inner shell electron –> ejects electron, leaves vacancy –> outer shell electron lose energy and fills vacancy –> energy lost emits as characteristic photon

Question 16

factors that affect attenuation (block) of x ray beam

Answer 16

1. thickness: thicker, receptor exposure decrease –> need increase mAs
2. density and atomic number: increase density and increase atomic number, decrease receptor exposure –> need increase mAs
3. energy (affected by tube voltage - kVp) : more energy –> more penetrating ability –> increase density/receptor exposure, increase comptom scatter –> decrease contrast