Radiology Basic Physics

Question 1

x rays are a form of

Answer 1

electromagnetic radiation
schematically represented as a sine wave

Question 2

properties of EM radiation

Answer 2

no mass
no charge
travels at speed of light i.e. 3x10 8 ms-1
can travel in a vacuum

Question 3

photon energy

Answer 3

EM radiation involves movement of energy as photons measured in electron volts i.e. eV
1 eV = energy (J) gained by 1 electron moving across a potential difference of 1 volt

Question 4

basic production of x rays

Answer 4

electrons fired at atoms at very high speed
on collision, kinetic energy of electrons is converted to EM radiation & heat
x ray photons are then aimed at a subject

Question 5

atomic number & mass number

Answer 5

atomic no = number of protons
mass no = number of protons & neutrons
this is 74 and 184 respectively in a tungsten atom

Question 6

no of electrons of an atom

Answer 6

determines chemical properties of atom

Question 7

ionisation

Answer 7

removal or addition of electron(s) to an atom making it a positively or negatively charged ion

Question 8

electron shells

Answer 8

electrons orbit the nucleus in shells
fill available spaces in inner shells first with each shell only holding a maximum number of electrons
held here due to electrostatic force from -ve charge of electrons and overall +ve charge of nucleus

Question 9

binding energy

Answer 9

closer to the nucleus or the more positive the nucleus, the greater the electrostatic force experienced by electrons so the greater the binding energy required ( additional energy required to exceed electrostatic force)

Question 10

what is current & what types are there

Answer 10

current =flow of electric charge, usually by movement of electrons
measured in amps
1. DC = direct current = constant unidirectional flow e.g. batteries
2. AC = alternating current = flow repeatedly reverses direction = measured in Hz = mains electricity (50Hz)

Question 11

x rays & current

Answer 11

x ray production required DC but mains is AC so x rays units have generators which modify AC so it mimics constant DC - process known as rectification

Question 12

voltage

Answer 12

difference in electrical potential between 2 points in an electrical field measured in V
mains in UK = 220-240V
dental x ray unit required 2 diff voltages

Question 13

transformers

Answer 13

alter voltage (& current) from 1 circuit to another

Question 14

transformers & dental x ray unit

Answer 14

2 separate transformers required
1. mains -> x ray tube
2. mains -> filament
step up transformer = increases potential difference across x ray tube, usually 60-70kV, current reduced to mA
step down transformer = decreases potential difference across filament to approx 10V & 10amps

Question 15

x ray beam

Answer 15

photons travel from x ray source
they diverge
beam intensity = quantity of photon energy passing through cross sectional area of beam per unit time; increase no/energy of photons and this will increase intensity
proportional to current in filament (mA) & potential difference across x ray tube (kV)

Question 16

inverse square law & x rays

Answer 16

intensity of x ray beam is inversely proportional to square of the distance between x ray source & point of measurement
X2 distance will 1/4 dose