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BDS3 Radiology > physics basics > Flashcards

physics basics Flashcards

1
Q

radiographs

A

Images created by X-rays which have been projected through an object & then interacted with a receptor

The different shades of grey on the image correspond to the different types of tissue & thicknesses of tissue involved
- Enamel – white, soft tissue - greyer

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2
Q

use of radiographs

A

Provide ability to see structures 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, & monitoring

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3
Q

intraoral dental radiograph views

A

periapical

bitewing

occlusal

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4
Q

extraoral dental radiograph views

A

panoramic

lateral cephalograms

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5
Q

electromagnetic radiation

A

X-rays are a form of electromagnetic radiation

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

Schematically represented as a sine wave

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6
Q

properties of all EM radiation types

A

No mass

No charge
Always travels at “speed of light”
3x10^8 ms-1 = 671 million mph

Can travel in a vacuum

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7
Q

em spectrum

A

Consists of all the different types of electromagnetic radiation

Each type has different properties, dependent on its energy/wavelength/frequency

Typically divided into 7 main groups
- Gamma, X, ultravitolet, visible, infrared, microwave, radio

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8
Q

7 main EM groups

A

Gamma, X, ultravitolet, visible, infrared, microwave, radio

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9
Q

wavelength

A

distance over which the wave’s shape repeats

measured in m

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10
Q

wavelength measured in

A

metres, m

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11
Q

frequency

A

many times the wave’s shape repeats per unit time

Measured in hertz, Hz
One hertz = one cycle per second

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12
Q

frequency measures in

A

hertz, Hz

One hertz = one cycle per second

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13
Q

speed =

A

frequency x wavelength

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14
Q

speed of all electromagnetic radiation

A

3x10^8 ms-1

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15
Q

if frequency of EM increases what happens to wavelength

A

decrease

as
speed = frequency x wavelength
3x10^8 ms-1 = frequency x wavelength

shorter waves of greater frequency

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16
Q

if wavelength increases of EM what happens to frequency

A

decrease

as
speed = frequency x wavelength
3x10^8 ms-1 = frequency x wavelength

longer waves of less frequency

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17
Q

photon energy

A

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

Energy usually measured in electron volts, eV

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

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18
Q

1 eV =

A

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

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19
Q

energy usually measured in

A

electron volts, eV

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20
Q

history of X-rays

A

1895: officially discovered by German physicist Wilhelm Röntgen
- Awarded Nobel Prize in Physics

1896: X-rays used in medicine & dentistry

Named “X-rays” because of their unknown nature

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21
Q

X-ray photon energies

A

~124eV – 124keV

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22
Q

2 types of X rays

A

hard X-rays

soft X-rays

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23
Q

hard X-rays

A

higher energies

Able to penetrate human tissues

  • medical imaging
    (e. g. >5keV)
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24
Q

soft X-rays

A

lower energies

Easily absorbed

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25
Q

properties of X-rays (4)

A

Form of electromagnetic radiation
- No mass, no charge, very fast, can travel in a vacuum, etc.

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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26
Q

ionisation

A

displacement of electrons from atoms/molecules

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27
Q

gamma rays compared to X-rays

A

gamma rays are identical
to X-rays

except that they occur naturally (& generally have higher energies) whereas X-rays are man made

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28
Q

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) & heat

The X-ray photons released and are aimed at a subject

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29
Q

the atoms (BOHR model)

A

Atoms are the “building b locks” of matter

Central nucleus

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

Orbiting “shells”
- Electrons (-ve charge)

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30
Q

neutron

charge
mass
location

A

0
1
in nucleus

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31
Q

proton

charge
mass
location

A

+1
1
in nucleus

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32
Q

electron

charge
mass
location

A

-1
negligible (0)
orbiting shells

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33
Q

nucleus

A

Collection of nucleons

Protons & neutrons have similar mass

Overall positive charge

34
Q

atomic number (Z) =

A

number of protons

Unique to each element

35
Q

mass number (A) =

A

number of protons + neutrons

36
Q

hydrogen H atoms

atomic number
mass number

A 
Atomic Number (Z) = 1
Mass Number (A) = 1
37
Q

tungsten W atom

atomic number
mass number

A 
Atomic Number (Z) = 74
Mass Number (A) = 184
38
Q

what does the number of electrons determine

A

the chemical properties of an atom

39
Q

atom in ‘ground state’ is

A

neutral

Number of electrons = number of protons

40
Q

ionisation process

A

removing/adding electron(s) to an atom

Atom - e- -> positive ion

Atom + e- -> negative ion (negative e outweighs positive p)

41
Q

electron shells

A

Electrons spin around the nucleus in discrete orbits/shells
- Cannot exist between these shells

Each shell is labelled alphabetically
- Innermost shell is K, Then L, M, N, O, etc.

Electrons try to fill available spaces in the inner shells first

42
Q

maximum number of electrons per shell

A

= 2n^2

Where “n” is the shell number
K is 1, L is 2, etc.

Example: M shell is number 3 -> 2 x 32 = 18

43
Q

maximum number of electron in

K shell

A

1

2n^2
-> 1 x 1^2
= 1

44
Q

maximum number of electron in

L shell

A

8

2n^2
-> 2x 2^2
= 8

45
Q

maximum number of electron in

M shell

A

18

2n^2

  • > 2 x 3^2
  • > 18
46
Q

maximum number of electron in

N shell

A

32

2n^2
-> 2 x 4^2
= 32

47
Q

electrostatic force

A

Orbiting electrons are held within their shells by electrostatic force

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

48
Q

what holds orbiting electrons in their shells

A

electrostatic force

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

49
Q

binding energy

A

additional energy required to exceed electrostatic force

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

50
Q

name for specific amount of energy needed to remove an electron from its shell

A

binding energy

51
Q

closer electron is to nucleus then…

A

the greater the electrostatic force (& therefore binding energy)

K shell electrons have the highest binding energies
- Then L, then M, etc.

52
Q

the more positively charged the nucleus (i.e. high Z/atomic number) then

A

greater the electrostatic force

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

53
Q

electron movement between shells

A

The specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus) equals the difference in the binding energies of the 2 shells

Conversely, if an electron drops to a more inner shell then this specific amount of energy is released
- Possibly in the form of X-ray photons (if sufficient energy)

54
Q

The specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus) =

A

difference in the binding energies of the 2 shells

Binding energy of tungsten K Shell = 69.5 keV
Binding energy of tungsten L shell = 10.2 keV

69.5 – 10.2 = 59.3keV

55
Q

if an electron drops to a more inner shell then

A

energy is released

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

56
Q

5 basic components of dental X-ray unit

A 
Tubehead
Collimator
Positioning arm
Control panel
Circuitry
57
Q

3 fundamentals of electricity

A

Current
Voltage
Transformers

58
Q

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

Direction
- Direct current (DC) = constant unidirectional flow

  • Alternating current (AC) = flow repeatedly reverses direction
59
Q

alternating current (AC)

A

Flow periodically reverses direction

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

SI unit: hertz, Hz (cycles per second)

e.g. mains electricity (50Hz in UK)

60
Q

unit for alternating current

A

SI unit: hertz, Hz (cycles per second)

61
Q

direct current (DC)

A

constant unidirectional flow

e.g. batteries

62
Q

two directions of electricial current

A

direct current (DC)

alternating current (AC)

63
Q

unit for direct current

A

amp (or ampere), A

Measure of how much charge flows past a point per second

64
Q

rectification of current

A

X-ray production requires a unidirectional 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

65
Q

what type of current do X-rays need

A

unidirectional current

but powered by mains electricity (AC) so therefore need rectification of current by generators to mimic DC

66
Q

voltage

A

Difference in electrical potential between 2 points in an electrical field

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

SI unit: volt, V

Note: “potential difference” synonymous with voltage

67
Q

electrical supply to X-ray unit

A

mains electricity

Alternating current (≤13 amps)
220-240 volts
68
Q

voltage needed of dental X-ray unit

A

unidirectional (AC from mains rectified)

One as high as 10s of thousands of volts
One as low as around 10 volts

69
Q

transformers

A

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

70
Q

2 transformers needed for X-ray unit

A

Mains -> X-ray tube (cathode-anode)

Mains -> filament

71
Q

step-up transformer

A

↑ potential difference across X-ray tube

Usually 60,000-70,000 volts (60-70 kV)

Current reduced to milliamps (mA)

72
Q

step-down transformer

A

↓ potential difference across filament

~10 volts

~10 amps

73
Q

X-ray beam

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)

74
Q

X-ray beam intensity

A

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

↑ number &/or energy of photons = ↑ intensity

proportional to current in filament (mA) & potential difference across X-ray tube (kV)

75
Q

↑ number &/or energy of photons =

A

↑ intensity

76
Q

divergence of X ray beam

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)

77
Q

inverse square law

A

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

Intensity ∝ 1/distance^2

Therefore, doubling the distance will quarter the dose

78
Q

intensity ∝

A

1/distance^2

79
Q

e.g. 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

Double the distance twice (1 to 4m) so ¼ grays twice ((½)^2then (½)^2)

1 = intensity x distance^2
intensity(a) x distance(a)^2 = 1 & intensity(b) x distance(b)^2 = 1

intensity(a) x distance(a)^2 = intensity(b) x distance(b)^2

4 x 12 = ? x 42
4 = ? x 16
? = 4/16
? = 0.25 Gy

80
Q

other types of radiation

A

Alpha particles
Beta particles
Gamma rays

All produced by radioactive decay of unstable atoms
- Unlike X-rays which are directly man-made

BDS3 Radiology (14 decks)

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