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BDS3 Dental Radiology > Physics Basics > Flashcards

Physics Basics Flashcards

1
Q

What are radiographs?

A
  • Images created by x-rays which have been projected through an object & then interacted with a receptor on the other side
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2
Q

What do the different shades of grey on a radiograph correspond to?

A
  • The different shades of grey on the image correspond to the different types of tissue & thickness of tissue involved
  • The x-rays pass through a tooth - the very dense enamel will have a whiter area on the image if the x-rays only pass through the skin of the cheek we will have a very dark image as these tissues aren’t very dense
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3
Q

Why are radiographs useful in dentistry? (3)

A

Provide the ability to see structured within the body, particularly mineralised tissues - many dental-related conditions affect the mineral content of tissues

  • Can show normal anatomy & pathology
  • Aid diagnosis, treatment planning and monitoring
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4
Q

What are the 3 common intra-oral radiographs?

A
  • Periapical
  • Bitewing
  • Occlusal
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5
Q

What are the 2 common extra-oral radiographs?

A
  • Panoramic

- Lateral cephalograms

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6
Q

What are x-rays a form of?

A

Electromagnetic Radiation

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7
Q

What is electromagnetic radiation?

A
  • The flow of energy created by simultaneously varying electrical & magnetic fields (schematically represented as a sine wave)
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8
Q

What are the main properties of electromagnetic radiation? (4)

A
  • No mass
  • No charge
  • Always travels at the speed of light (3x10^8 ms-1 = 671 million mph)
  • Can travel in a vacuum
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9
Q

What is the speed of light?

A
  • 3x10^8 ms-1 = 671 million mph
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10
Q

What does the EM spectrum consist of? (3)

A
  • Consists of all different types of Electromagnetic radiation
  • Each type has different properties, dependent on its energy/wavelength/frequency
  • Typically divided into 7 main groups
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11
Q

How many main groups is the EM spectrum typically divided into?

A

7

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12
Q

What are the 7 main groups in the EM spectrum?

A
  • Gamma ray
  • X-ray
  • UV
  • Visible
  • Infrared
  • Microwave
  • Radio
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13
Q

How do we determine wavelength?

A

= divide the number of cycles over the length

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14
Q

How do we determine the frequency?

A

= divide the number of cycles by the number of seconds

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15
Q

What is frequency?

A

Frequency = how many times the wave’s shape repeats per unit time

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16
Q

What unit is frequency measured in?

A

Measured in hertz, Hz

one hertz = one cycle per second

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17
Q

What is wavelength?

A

Wavelength = the distance over which the waves shape repeats

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18
Q

What unit is wavelength measured in?

A

Measured in metres, m

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19
Q

What does speed equal?

A
  • Frequency x wavelength
  • BUT speed of all electromagnetic radiation is constant (3x10^8ms-1 - speed of light)
  • Therefore if frequency increases then wavelength must decrease (& vice versa)
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20
Q

What is energy proportional to?

A

Frequency

- So if something has a high frequency it also has a high energy

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21
Q

WM radiation involved the movement of energy as ‘packets of energy’. What are these packets of energy known as?

A

Photons

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22
Q

What unit is energy usually measured in?

A
  • Energy usually measured in electron volts, eV
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23
Q

What does 1eV mean in relation to energy?

A

= energy (in joules) gained by 1 electron moving across a potential difference of 1 volt

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24
Q

Why were x-rays names x-rays?

A

Because of their unknown nature

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25
Q

For electromagnetic radiation to be classed as x-ray energy it has to be in a range. What is this range?

A

Around 124eV - 124keV

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26
Q

What are the 2 different types of x-ray?

A
  • Hard and soft
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27
Q

What are hard x-rays?

A
  • Have higher energies

- Able to penetrate human tissues

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28
Q

What are soft x-rays?

A
  • Have lower energies
  • Easily absorbed
  • (don’t want soft x-rays if we are taking a radiograph as we want some of the x-rays to pass through the body and reach the receptor therefore medical imaging mostly uses hard x-rays)
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29
Q

Which type of x-ray do we usually use for medical imaging?

A
  • Mostly use hard x-rays (e.g. >5keV)
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30
Q

What are the properties of x-rays? (4)

A

Form of electromagnetic radiation:
- No mass, no charge, Very fast, can travel in a vacuum

Undetectable to human senses

Man-made
- note: gamma rays are identical except that they occur naturally (& generally have higher energies)

Cause ionisation:
- i.e. displacement of electrons from atoms/molecules

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31
Q

Which property of x-rays is this process which is what causes the dangerous side effects that humans can have when they are exposed to too much radiation?

A
  • The fact x-rays cause ionisation
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32
Q

What is the basic production of x-rays? (3)

A
  • Electrons fired at atoms at very high speed
  • On collision, the kinetic energy of these electrons is converted to EM radiation (ideally x-rays) & heat
  • The x-ray photons are aimed at a subject
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33
Q

What are the ‘building blocks’ of matter?

A

Atoms (they make up many of the things in the universe)

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34
Q

What do atoms consist of? (3)

A

Central nucleus:

  • Protons (+ve charge)
  • Neutrons (neutral)

Orbiting ‘shells’:
- Electrons (-ve charge)

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35
Q

What is the relative charge of a neutron?

A

0

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36
Q

What is the relative charge of a proton?

A

+1

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37
Q

What is the relative charge of an electron?

A

-1

38
Q

What is the relative mass of a neutron?

A

1

39
Q

What is the relative mass of a proton?

A

1

40
Q

What is the relative mass of an electron?

A

Negligible (0)

41
Q

Where is the location of a proton?

A
  • Nucleus
42
Q

Where is the location of a Neutron?

A
  • Nucleus
43
Q

Where is the location of an electron?

A
  • Shells
44
Q

What is in the nucleus of an atom?

A
  • Collection of nucleons:

- Protons and neutrons have a similar mass

45
Q

What is the overall charge of the nucleus of an atom?

A

Positive

46
Q

What does the atomic number (Z) equal?

A

Atomic number = number of protons (unique to each element)

47
Q

What does the mass number (A) equal?

A

Mass number = number of protons + neutrons (this can change as can have different numbers of neutrons in an atom without changing elements)

48
Q

What does the number of electrons in an atom determine?

A
  • Determines the chemical properties of the atom
49
Q

An atom in its ‘ground state’ is neutral. What does this indicate?

A

Number of electrons = number of protons

50
Q

What does ionisation mean?

A

Ionisation = removing/adding electron(s) to an atom

  • Atom - e- = positive ion
  • Atom +e- = negative ion
51
Q

Where are electrons positioned in atoms?

A
  • Electrons spin around the nucleus in discrete orbits/shells
  • Cannot exist between these shells
52
Q

What is the name of the innermost electron shell?

A
  • K
  • Then L, M, N, O etc
  • Electrons try to fill available spaces in the inner shells first
53
Q

What is the maximum number of electrons that can be held in the first shell (K)?

A

2

54
Q

What is the maximum number of electrons that can be held in the second shell (L)?

A

8

55
Q

What is the maximum number of electrons that can be held in the third shell (M)?

A

18

56
Q

What is the maximum number of electrons that can be held in the fourth shell (N)?

A

32

57
Q

How do we work out what the maximum number of electrons in a shell can be?

A
  • Maximum number = 2n^2
  • Where n is the shell number

Example: M shell is number 3 -> 2 x 3^2 = 18

58
Q

What force are orbiting electrons held within their shells by?

A

Electrostatic force

59
Q

What is electrostatic force?

A

-ve charge of electrons attracted to overall +ve charge of nucleus

60
Q

To remove an electron from its shell, a specific amount of energy is required to overcome this attraction. What is this energy called?

A

Binding energy = additional energy required to exceed electrostatic forces

61
Q

The closer the electron is to the nucleus, the greater the electrostatic force (& therefore the binding energy). So, which shell electrons have the highest binding energy?

A

K shell electrons

62
Q

How do we calculate the specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus)?

A

This equals the difference in the binding energies of the 2 shells

63
Q

If an electron drops to a more inner shell, what happens?

A
  • This specific amount of energy is RELEASED

- Possibly in the form of x-ray photons (if sufficient energy)

64
Q

What is included in a dental x-ray unit? (5)

A
  • Tubehead (creates the x-rays)
  • Collimator (the x-rays pass through here)
  • Positioning arm (holds the tube head and allows you to position it)
  • Control panel
  • Circuitry (to power it)
65
Q

What is ‘current’?

A
  • Flow of electric charge, usually by the movement of electrons
66
Q

What unit is current measured in?

A
  • SI unit: amp (or ampere), A

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

67
Q

Current can be defined by its direction. What are the 2 different directions?

A

Direct current = constant unidirectional flow

Alternating current = flow repeatedly reverses direction

68
Q

Give an example of something that uses direct current?

A

Batteries

69
Q

What is alternating current?

A
  • Flow periodically reverses direction
70
Q

What is the frequency of alternating currents?

A
  • Number of complete cycles (reverse + reverse-back) per unit time is the frequency
71
Q

What unit is used for the frequency of alternating current?

A

SI unit: hertz, Hz (cycles per second)

72
Q

Give an example of something that uses alternating current?

A

Mains electricity (50Hz in UK) - so the current reverses and reverses-back 50 times every second

73
Q

What is rectification of current?

A
  • X-ray production requires a unidirectional current BUT x-ray units are powered by mains electricity which are alternating currents
  • X-ray units have generators which modify the AC so that it mimics a constant unidirectional current (process known as rectification)
74
Q

What is ‘voltage’?

A

Difference in electrical potential between 2 points in an electrical field (difference between a negatively charged point in one place and a positively charged point in another)

75
Q

What is voltage related to?

A
  • Related to how forcefully a charge will be pushed through an electrical field (if it is between these 2 points)
76
Q

What is the unit used for voltage?

A

SI unit: volt, V

77
Q

What is another name for voltage?

A

Potential difference

78
Q

What is the mains supply in the UK like?

A

Alternating current (< or equal to 13 amps)

79
Q

What is the voltage of the mains supply in the UK?

A

220-240 volts

80
Q

Dental x-ray units require 2 different voltages. What are these?

A
  • One as high as 10s of thousands of volts

- One as low as around 10 volts

81
Q

What is a transformer?

A
  • Transformers alter the voltage (& current) from one circuit to another

Two separate transformers required for x-ray unit:

  1. Mains -> x-ray tube (cathode-anode)
  2. Mains -> filament
82
Q

What do ‘step-up’ transformers do? (3)

A
  • They increase the potential difference across x-ray tube
  • Usually 60,000-70,000 volts (60-70 kV)
  • Current reduces to milliamps (mA) (because we can’t create electricity out of nothing we also have to reduce the current)
83
Q

What do ‘step-down’ transformers do? (3)

A
  • decrease the potential difference across filament to approximately:
  • 10 volts
  • 10 amps
84
Q

What is an x-ray beam made up of?

A
  • Made up of millions of x-ray photons directed in the same general direction
  • Photons effectively travel in straight lines but DIVERGE from the x-ray source (i.e. do not travel in parallel)
85
Q

What is the strength of the x-ray beam called?

A
  • The intensity
86
Q

What is the x-ray beam intensity measured as?

A
  • Measured as quantity of photon energy passing through a cross-sectional area of the beam per unit time
87
Q

If you increase the number &/or energy of photons in an x-ray beam what are you also increasing?

A

The intensity

88
Q

What is x-ray beam intensity proportional to?

A

Proportional to current in filament (mA) & potential difference across x-ray tube

89
Q

What happens to dose as we move away from the x-ray source?

A
  • Dose decreases with distance from x-ray source

- Ensure staff stand a sufficient distance from patient (& not in the direction of the primary x-ray beam)

90
Q

What is the inverse square law?

A
  • To work out how much you can cut down the radiation you might receive from a source by standing further away we can use this law
  • Remember that if you double the distance that you are standing away from a source you will quarter the dose
91
Q

What are 3 other types of ionising radiation?

A
  • Alpha particles
  • Beta particles
  • Gamma rays

(these are all produced by radioactive decay of unstable atoms unlike x-rays which are directly man-made)

BDS3 Dental Radiology (13 decks)

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