Physics Basics

Question 1

Why are radiographs useful? What aid do they provide?

Answer 1

Can see healthy tissue and structures, particularly mineralised tissue

Can show normal anatomy and pathology

Can show abnormal anatomy and pathology

Aid in treatment planning, diagnosis and monitoring

Question 2

What is an x-ray?

Give some properties

Answer 2

EM radiation

Flow of energy created by simultaneous varying electrical and magnetic fields - presented as a sine wave

No mass or charge, travel at speed of light, travel in vacuum, ionising (displace electrons from atoms)

Question 3

How calculate wavelength?

Answer 3

Cycle number/distance travelled

E.g 8 cycles in 4m = 0.5m wavelength

Question 4

How calculate frequency?

Answer 4

Time taken to do 1 cycle/number of seconds

E.g. 1 cycle in 10 seconds = 0.1hz

5 cycles in 15s = 0.3333333hz

Question 5

Speed = ? X ?

Answer 5

Speed = hz x M

Speed is always constant at speed of light, so if wavelength increases Hz must go down

Question 6

How is energy measured in EM radiation?

Answer 6

Energy in photons, measured in electron volts eV

1eV = energy in joules gained by 1 electron moving across a potential difference of 1 volt.

Question 7

What eV / type of x ray is used for medical imaging?

Answer 7

Hard x ray

> 5keV

Question 8

What shell letters does an atom have starting from innermost? Hope can you calculate number of electrons per shell

Answer 8

K, L, M, N…

2n^2

Question 9

What is electrostatic force (and what influences this) and binding energy?

Answer 9

EF = force between -ve electron and +ve nucleus - influenced by distance to nucleus and also how large a positive charge the nucleus has

Binding force = energy required to overcome this

Question 10

How can electrons move between shells

Answer 10

To move to an outer shell = difference in binding energies between two shells

To drop down a shell, energy is released, possibly in form of x-ray photons

Question 11

What is current? What is its unit? What types of current are there?

Answer 11

Flow of charge, by movement of electrons.

Amp - measure of how much charge flows past a point per second

Direct current - constant unidirectional flow e.g. batteries

Alternating current - flow repeatedly reverses direction

Question 12

What is frequency with regards to AC?

Answer 12

Number of complete cycles (reverse + reverse back) per unit time

Mains in UK is 50Hz

Question 13

What current does X ray machine require? How is this achieved?

Answer 13

Requires DC, but mains is AC.

Unit has generators that modifies AC to mimic DC (rectification)

Question 14

What is voltage? What is its unit?

Answer 14

Potential difference between 2 points in an electrical field

Related to how forcefully a charge is pushed through an electrical field

Unit = V

Question 15

What electricity is needed for x-ray unit?

Answer 15

DC

2 voltages

1 as high as 10s thousands of V

1 as low as 10V

Question 16

How are the two different voltages produced in the x-ray unit?

Answer 16

Transformers

1. Mains to X-ray tube (step up to 60-70kV and milliamps current)
2. Mains - filament (step down to around 10V and 10 amps)

Question 17

What is x-ray intensity?

Answer 17

Quantity of photon energy passing through a cross sectional area of the beam per unit time

Increased number or energy of photons increases intensity

Question 18

How does beam divergence alter the cross sectional size of an x-ray when distance is doubled?

Answer 18

Beam becomes 4 times more dilated

E.g. dose of 4 grays at 1m

Dose at 4m?

1 doubled = 2, 2 doubled = 4

4 grays / 4 = 1

1 gray / 4 = 0.25g (as quarter every double distance)