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BDS3 - 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 and then interacted with a receptor

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2
Q

what do the different shades of grey in an x-ray represent

A

The different shades of grey on the image correspond to the different types of tissue and thickness of tissue involved

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3
Q

why are radiographs useful

A

• Provide ability to see structures within the body, particularly mineralised tissues - thing that would not be visible with the naked eye
○ Many dental related conditions affect the mineral content of tissues

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

name common intra-oral radiographs

A

Periapical
Bitewing
Occlusal

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5
Q

name common extra-oral radiographs

A

Panoramic / OPT

Lateral cephalograms

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6
Q

what form of radiation are x-rays

A

electromagnetic (EM) radiation

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7
Q

what is electromagnetic radiation

A

The flow of energy created by simultaneously varying electrical and magnetic fields

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8
Q

how is electromagnetic radiation represented schematically

A

as a sine wave

s shaped curve

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9
Q

what are the properties of EM radiation

A
  • No mass
  • No charge

• Always travels at “speed of light”
○ 3x10^8ms-1 = 671 million mph

• Can travel in a vacuum

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10
Q

what is the EM spectrum

A

Consists of all the different types of electromagnetic radiation

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11
Q

each of the different types of EM radiation have different properties which are dependent on…

A

energy
wavelength
frequency

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12
Q

how many groups is the EM spectrum divided into

name them

A

7

  • gamma ray
  • X-ray
  • ultraviolet light
  • visible light
  • infrared
  • microwave
  • radio waves
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13
Q

gamma rays are found at the beginning of the EM spectrum; what can be expected in terms of wavelength, frequency and energy

A

shorter wavelength
higher frequency
higher energy

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14
Q

radio waves are at the other end of the spectrum, over the course what happens to the wavelength, frequency and energy

A

longer wavelength
lower frequency
lower energy

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15
Q

what is frequency

A

how many times the waves shape repeats per unit time

measured in hertz, Hz
one hertz = 1 cycle per second

look up diagram to see what a cycle is cos i would only confuse you hahahhaah

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16
Q

what is wavelength

A

the distance over which the waves shape repeats

measured in metres, m

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17
Q

what is speed

A

frequency X wavelength

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18
Q

what is the speed for all EM radiation?

how does this affect frequency and wavelength

A

speed is constant: 3 x 10^8 ms-1

therefore is frequency increases then wavelength must decrease
and vice versa

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19
Q

what is photon energy

A

EM radiation involves the movement of energy as “packets of energy” known as photons

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20
Q

what is used to measure energy

A

Energy usually measured in electron volts, eV

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21
Q

what is meant be 1eV

A

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

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22
Q

what is the range of x-ray photon energies

A

X-ray photon energies ~ 124eV - 124keV

124 to 124 thousand x-ray volts

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23
Q

what are the types of X-rays

A

○ Hard X-rays (higher energies)
§ Able to penetrate human tissues

○ Soft X-rays (lower energies)
§ Easily absorbed

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24
Q

what short of x-rays does medical imaging mostly use

A

Medical imaging mostly uses hard X-rays (eg >5keV)

Need to x-rays to be able to pass through to get to the receptor

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25
Q

what are properties of X-rays

A 
• Form of electromagnetic radiation
○ No mass
○ No charge
○ Very fast
○ Can travel in a vacuum etc
  • Undetectable to human senses
  • Man-made

• Cause ionisation
○ Ie displacement of electrons from atoms / molecules
○ It is this process that causes the dangerous side effects that humans can have when they are exposed to too much radiation

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26
Q

are gamma rays similar to x-rays

A

gamma rays are identical except that they occur naturally (& generally have higher energies)
Gamma rays are created by radioactive decay

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27
Q

explain the basic production of X-rays

A

• Electrons fired at atoms at very high speed

• On collision, the kinetic energy of these electrons is converted to electromagnetic radiation (ideally X-rays) and heat
○ Depending on their speed they can form other types of electromagnetic radiation
○ Heat is a side effect

• The x-ray photons are aimed at a subject

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28
Q

what are atoms

A

building blocks of matter

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29
Q

what do atoms consist of

A

○ Central nucleus
§ Protons - positive charge
§ Neutrons - neutral

○ Orbiting “shells”
§ Electrons - negative charge

30
Q

what is the relative mass of neutrons, protons ad electrons

A

neutrons and protons have a relative mass of 1 (they have equal mass)

electrons essentially have no mass at all

31
Q

what is the nucleus

A

collection of nucleons
Protons and neutrons have similar mass
Overall positive charge

32
Q

what is the atomic number (Z)

A

number of protons
Unique to each element

Eg all carbon atoms will have 6 protons

33
Q

what is the mass number (A)

A

number of protons and neutrons
This number can change even for the same element

Even though the element has the same number of protons, can have different numbers of neutrons in the same atom

34
Q

what does the number of electrons determine

A

the chemical properties of an atom

35
Q

what is an atom in its “grounded state”

A

An atom in its “grounded state” is neutral

Number of electrons = number of protons

36
Q

what is ionisation

A

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

Atom removing an electron = positive ion

Atom gaining an electron = negative ion

37
Q

what is an electron shell

A

Electrons spin around the nucleus in discrete orbits / shells

electrons cannot exist between these shells

Electrons try to fill available spaces in the inner shells first

38
Q

how is each shell labelled

A

Innermost shell is K

Then L, M, N, O, etc

39
Q

what is the maximum number of electrons each shell can hold

A 
K = 2
L = 8
M = 18
N = 32

• Maximum number = 2n^2
Where “n” is the shell number
§ K = 1
§ L = 2 etc

Example: M shell is 3 –> 2 x 3^2 = 18

40
Q

how are orbiting electrons held within their shells

A

electrostatic force

Negative charge of electrons attracted to overall positive charge of nucleus

41
Q

how do can an electron be removed from its shell

A

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

42
Q

what is binding energy

A

Binding energy = additional energy required to exceed electrostatic force

The closer the electron is to the nucleus, the greater the electrostatic force (& therefore binding energy)

The more positively-charged the nucleus (ie higher Z), the greater the electrostatic force

○ Carbon (Z=6): K shell binding energy = 0.28keV
○ Tungsten (Z=74): K shell binding energy = 69.5keV

43
Q

how can you work out the amount of energy needed to move an electron to a more outer shell (away from the nucleus)

A

The amount of energy required to move an electron to a more outer shell equals the difference in the binding energies of the 2 shells

44
Q

what happens if an electron drops to a more inner shell

A

if an electron drops to a more inner shell than this specific amount of energy is released

ie the difference in the binding energies of the 2 shells

This is a different number from element to element due to the different number of protons in the nucleus

45
Q

name the different parts of the X-ray unit

A
  • Tube head
  • Collimator
  • Positioning arm
  • Control panel
  • Circuitry
46
Q

what does the tube head do

A

Creates the x-rays

47
Q

what is the collimator

A

Metal window which the X-rays pass through

48
Q

what does the positioning arm do

A

Lets you hold the tube head and position it as close to the patient as possible

49
Q

what does the circuitry do

A

Required to power the whole thing

50
Q

what is current

A

Flow of electric charge, usually by the movement of electrons
SI unit: amp (or ampere), A

Measure of how much charge flows past a point per second

51
Q

explain the directions of current

A

○ Direct current (DC) = constant unidirectional flow (one direction)
§ Eg batteries

○ Alternating current (AC) = flow repeatedly reverses direction
[Electron going one way then back again]

It doesn’t matter what direction the current is going in as long as there is current going

52
Q

what is alternating current

A

Flow periodically reverses direction

Number of complete cycles (reverse and reverse back) per unit time is the frequency
○ SI unit: hertz, Hz (cycles per second)
○ 1 hertz = 1 cycle per second

Eg: mains electricity (50Hz in UK)

current goes in forward direction, becomes more and more powerful before starting the become weaker
current then flips and goes in a backward direction and gets more and more powerful before then becoming weaker before it flips again

53
Q

what is rectification of current

A

• X-ray production requires a unidirectional current (DC current)

But x-ray units are powered by mains electricity (AC)

• X-ray units have generators which modify the AC so that it mimics a constant DC
= Process is known as rectification

54
Q

what is voltage

A

Difference in electrical potential between 2 points in an electrical field
{Basically the difference between a negatively charged point in one place and a positively charged point in another}

Related to how forcefully a charge will be pushed through an electrical field

SI unit: volt, V

55
Q

what can voltage also be called but essentially means the exact same thing

A

“potential difference”

56
Q

explain the electrical supply to an X-ray unit

A

• Mains supply in UK
○ Alternative current (<13 amps)
○ 220-240 volts

• Dental X-ray unit
○ Requires direct current

57
Q

what are transformers

A

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

58
Q

what are the 2 separate transformers needed for the X-ray unit

A

○ Mains –> X-ray tube (cathode-anode)
§ Takes the mains supply and changes it so it is appropriate for the x-ray tube

○ Mains –> filament
Takes the mains supply and changes it so it is appropriate for the filament

59
Q

what is a step-up transformer

A

Increase potential difference across X-ray tube
Usually 60,000-70,000 volts (60-70kV)
Current reduced to milliamps (mA)

60
Q

what is a step-down transformer

A

○ Decrease potential difference across filament
§ Around 10 volts
§ Around 10 amps

61
Q

what is an x-ray beam

A

Made up of millions of X-ray photons directed in the same general direction

62
Q

how do photons travel

A

Photons effectively travel in straight lines but diverge from the X-ray source (ie do not travel in parallel)

63
Q

what is the strength of the x-ray beam known as

A

its intensity

64
Q

what is the x-ray beam intensity

A

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

65
Q

how can the intensity be increased

A

Increase number of photons and / or energy of photons = increase intensity

66
Q

what is the intensity proportional to

A

Proportional to current in filament (mA) and potential difference across X-ray tube (kV)

  • Intensity is related to the current in the filament (as increasing the current will result in more photons)
  • Also related to the potential difference / voltage across the X-ray tube as this will result in photons with higher energies
67
Q

what happens to the dose as the distance from the x-ray tube increases

A

Dose decreases with distance from X-ray source

68
Q

what is the inverse square law used for

A

To help work out how much you can cut down the radiation received from a source by standing further away

69
Q

explain the inverse square law

A

Intensity of X-ray beam is inversely proportional to the square of the distance between the X-ray source and the point of measurement

therefore, doubling the distance will quarter the dose

i dont know how to do that little symbol that is acc in the square law so look it up sorry

its intensity {funny symbol} 1/distance^2

70
Q

Example: if a patient standing in the X-ray beam gets a dose of 4 grays at a distance of 1 metre (from the X-ray source) what will the dose be at 4 metres

A
  • Doubling the distance away from the source will quarter the dose received
  • Starting at 1metre from source then moving 4metres from the source the patient has doubled their distance twice
    ○ Doubled from 1 to 2 metres
    ○ And again from 2 to 4 metres
  • Double distance twice = quarter the dose twice
    ○ Start at 4 grays and quarter this once to get to 1 gray
    ○ Then quarter 1 gray to get to 0.25 grays

Answer: 0.25 Gy (or 250 mGy)

[maths way]
Intensitya X distancea^2 = 1 AND
intensityb X distanceb^2 = 1

Intensitya X distancea^2 = intensityb X distanceb^2 
○ 4 X 12 = ? X 4^2
○ 4 = ? X 16
○ ? = 4/16
? = 0.25 Gy
71
Q

what are other types of ionising radiation

A 
Alpha particles
(can't pass through paper)
Beta particles 
(can pass through paper, can't pass through aluminium)
Gamma rays
(can pass through paper, aluminium and lead)
72
Q

how are these other types of ionising radiation produced

A

radioactive decay of unstable atoms

BDS3 - radiology (19 decks)

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