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ODDR- Dental physics > Medical physics 1 radiation production > Flashcards

Medical physics 1 radiation production Flashcards

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1
Q

Describe the electromagnetic properties of X rays

A

They have a very short wavelength and a high frequency

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2
Q

State the range in an x rays wavelength

A

Less than 100nm

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3
Q

What is the main difference between X rays and gamma rays?

A

X rays involve electron interactions

Gamma rays involve nuclear interactions

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4
Q

What are x rays?

A

They are a stream of photons with a wavelength of less than 100nm

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5
Q

What is a photon?

A

A photon is a quantum of energy measured in electron volts

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6
Q

How many joules in an electron volt?

A

1eV = 1.602 x 10^-19

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7
Q

State the relationship between frequency of wavelength, Planck’s constant and energy

A

Energy (E)= Planck’s constant (h) x frequency of wavelength (V)
E=h x V

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8
Q

State the relationship between wavelength, Planck’s constant, the speed of light and energy

A

Energy (E)= (Planck’s constant (h) x Speed of light (c)) /Wavelength (lambda)
E= hc / lambda

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9
Q

State Planck’s constant

A

6.626 x 10^-34 J per second

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10
Q

State the speed of light

A

2.998 x 10^8 m per second

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11
Q

What happens as wavelength decreases?

A

Photon energy increases so the he probability of an interaction decreases and photons tend to travel further through matter

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12
Q

How do X rays produce images?

A

Photons interact with any tissue they met and as X rays are high energy the ohitins in x rays will penetrate tissues but will not pass through all medium

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13
Q

Name the 2 mechanisms by which we can produce X rays in theory

A
  1. Bremsstrahlung

2. Characteristic spectrum

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14
Q

Interactions between what occur in the bremsstrahlung theory to produce x rays?

A

Interactions between the electron and nucleus

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15
Q

Describe how the bremsstrahlung theory produces x rays

A
  1. Electrons are accelerated through a voltage gradient in the atom
  2. Electrons interact with atom and chooses the path that misses all electrons & comes back around and out the atom
    3 As the electron changes direction it changes speed, this releases an X-ray
  3. There are many paths the electron can take so a range of x ray energies produced
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16
Q

Why are a range of X ray energies produces in the bremsstrahlung theory of x ray production?

A

As there are many pathways that the electron can take to get out of the atom

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17
Q

What determines the range of energies produced by the bremsstrahlung theory of x ray production?

A

The tube potential

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18
Q

Interactions between what occur in the characteristic radiation theory to produce x rays?

A

Interactions between the electron and another electron

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19
Q

Describe how the characteristic radiation theory produces x rays

A
  1. Electrons are accelerated through a voltage gradient in the atom
  2. Electron interacts with an outer orbiting electron & ejects it
  3. This leaves a gap in the orbital & electrons cascade down to fill opened up gaps
  4. As the electrons cascade down they release energy in the form of X-rays
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20
Q

What determines the energy produced by the characteristic radiation theory of x ray production?

A

Determined by the target material

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21
Q

What components make up an x ray tube?

A
  1. Cathode
  2. Anode
  3. Glass envelope
  4. Filters
  5. Cooling oil
  6. Shield
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22
Q

What does an x ray tube produce?

A

X rays in a vacuum

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23
Q

How are x rays produced in an x ray tube?

A
  1. Electrons are produced at a hot filament (cathode) & accelerated across a gap to anode
  2. Electrons collide with with material and x rays are given out in all directions
  3. A small hole is made in the housing to allow for electrons to leave
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24
Q

What is electron energy to proportional to in an x ray tube?

A

Electron energy is proportional to the voltage difference between anode and cathode

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25
Q

What does a high voltage supplied to in an x ray tube do?

A

Supplies the acceleration to the incident electrons – the higher the voltage, the greater the acceleration, the greater the energy of the incident electrons

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26
Q

What does evacuated glass tube that makes up an x ray tube do?

A
  1. maximises x-ray production in the target because it stops electrons having interactions with other atoms
  2. provides insulation between the anode and cathode
  3. Produces tube arcing when gas is present in the tube
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27
Q

What does the shielded metal housing that makes up an x ray tube do?

A

It stops the x-rays being irradiated out of the tube in all directions

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28
Q

What is the window of the x ray tube made up of?

A

Beryllium

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29
Q

What does the oil in an x ray tube made do?

A

used to remove heat from the tube and works as insulation from the high voltage

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30
Q

What is the maximum photon energy equal to?

A

Maximum photon energy is the same as tube potential

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31
Q

What happens to the relative photon output as the flow of electrons increases?

A

Increases

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32
Q

At what kV does the dental x ray tube operate at?

A

65kV

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33
Q

What is the flow of current across X-ray tube measured in?

A

The current measures in milliamperes (mA)

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34
Q

What does the current in the dental tube affect?

A

• Current affects the quantity of x rays produced, but not the energy of x-rays produced

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35
Q

What is the total number of x rays produced dependent on?

A

on current & exposure time

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36
Q

What is the intensity of the x rays proportional to?

A

kV(p^2)

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37
Q

What does the filtration of x rays do?

A

Filtration is used to remove low energy X-rays

It works by modifying the x-ray spectrum by removal of low energy photons

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38
Q

What is the purpose of a filter in the x ray tube?

A

To reduce the dose of x rays to a patient

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39
Q

What is filtration comprised of?

A

Inherent: Glass envelope within the tube
Added: aluminium
Total filtration = Inherent + Added

40
Q

What does adding filtration do?

A

Adding filtration decreases overall beam intensity but increases mean energy

41
Q

What does filtration affect?

A

affects both the quality and quantity of the x-rays

42
Q

What should the filtration for a standard diagnostic x ray tube be?

A

At least 2.5mmAl

43
Q

How do we get a sharp image?

A

By using a small focal point

44
Q

What is the sharpness of an image affected by?

A

Sharpness is affected by size of the

focal spot

45
Q

What happens if the focal point is too small?

A

The tube can get hot and catch fire

46
Q

How do we achieve a small focal point?

A
  1. Focus electrons

2. Set target at steep angle

47
Q

What effect does target angling have on the focal point?

A

If the anode is at a steeper angle then the focal spot will be smaller

48
Q

Why is the removal of heal necessary in an x ray tube?

A

To prevent damage to focal spot and anode (Target)

To prevent general overheating of the tube

49
Q

What does the build of heat init in an x ray tube?

A

limits the rate at which X-rays can be repeated

50
Q

What is used to cool down the anode?

A

A copper jacket is used to conduct the heat away from the anode and into the surrounding insulating oil

51
Q

What do dental images need to show?

A
  1. Where things are: spatial information

2. What things are: contrast

52
Q

How is contrast achieved in an image?

A
  1. Tissues have different atomic (Z) numbers
  2. All tissues are off a different thickness
  3. We Add other substances which are taken up by one tissue more than another.
53
Q

What reduces he intensity of X rays as they travel through matter?

A
  1. Photoelectric absorption

2. Compton Scatter

54
Q

What is the photoelectric effect?

A

When x rays are absorbed

55
Q

What is the Compton effect?

A

When x rays are scattered

56
Q

When does photoelectric absorption occur?

A

When an X ray photon interacts with an outer electron

The X ray photon gives all its energy to the electron & causes it to leave

57
Q

When does the probability of photoelectric absorption decrease?

A

As x ray energy increases

58
Q

When does Compton Scatter occur?

A

Photon collides with an electron and loses some of its energy
The photon changes direction and the electron leaves the atom

59
Q

What is meant by the attenuation of x rays?

A

Attenuation is the total process of reducing the intensity of an x-ray beam

60
Q

What happens when an X-ray is absorbed or scattered?

A

It is attenuated (weakened)

61
Q

Why is scattering bad?

A

Leads the practitioner into thinking the organ is in a different area to its true place

62
Q

What increases the probability of the photoelectric effect?

A

Increases with the cube of the atomic number of the absorbing material

63
Q

What is the main source of contrast?

A

The photoelectric effect

64
Q

Where is good contrast observed?

A

Between tissues of different atomic numbers §

65
Q

What reduces contrast?

A

Compton scatter reduces contrast

66
Q

Describe high energy x rays?

A

They are penetrating

They give a poorer contrast

67
Q

Why do high energy x rays have a poor contrast?

A

As more of the interactions are Compton

68
Q

Describe low energy x rays

A

They are more easily absorbed

They give off a better contrast

69
Q

What is the problems associated with using high and low energy x rays?

A

High energy gives a lot of scatter

But low energy needs to be given in higher doses

70
Q

How do we balance the administration of high and low energy x rays to patients?

A

X rays need to be generated at 30KeV to 70KeV

So we use: 70kV PD with 2mm Aluminium film in X ray tube

71
Q

Name the 3 types of image receptors

A
  1. Film
  2. Computed radiography
  3. Digital radiography
72
Q

What information can er not retrieve in a dimensional image?

A

We cannot retrieve information about depth

73
Q

What do analogue images include?

A

Things like film screen radiography and fluoroscopy with image intensifiers

74
Q

What is an advantage of digital receptors?

A

A digital image is an array of numbers they can be manipulated
This allows images to be enhanced and copied, or transferred to other viewing systems

75
Q

What type of image do computed radiography produce?

A

Produces an analogue latent image

76
Q

What does a film require in order to produce an image?

A

An adequate x ray exposure

77
Q

What is exposure determined by?

A

The number of electrons hitting anode (measured in mAs)

78
Q

What is the film characteristic curve dependent on?

A

developer temp/time in developer/strength/age

79
Q

How confirm characteristic curves be generated?

A

by exposing film to diff light intensities

80
Q

What factors effect image quality?

A
  1. Contrast
  2. Unsharpness
    3, Noise
  3. Magnification
  4. Distortion
81
Q

What does subject radiation contrast depend on?

A

The ratio of transmitted radiation through different materials

82
Q

What is radiographic image contrast?

A

the difference between optical densities in different areas of the image

83
Q

How can we influence the subject radiation contrast?

A

Changing the quality (average energy) of the radiation

84
Q

How can we change the quality (average energy) of the radiation?

A
  1. Change the kV
  2. Change the filtration
  3. Change the target material
85
Q

What effect does increasing the kV have on scatter?

A
  1. Absolute quantity reduced
    as compton scatter decreases with increasing
    kV so less primary radiation needed
  2. Proportion of scatter reaching the image increases as scatter is more penetrating at higher kV
86
Q

Give some practical ways to reduce scatter

A
  1. Reduce kV
  2. Change patient orientation
  3. Compression (reduce thickness thus reducing scatter volume)
  4. Use anti scatter grid
  5. Have an air gap
  6. Use a Cassette with high atomic number (Z) back
  7. Reduce field size thus reducing scatter volume
87
Q

What problems are associated with reducing kV?

A

Means you need to increase dosage

88
Q

What problems are associated with using an anti scatter grid?

A

Means you need to increase dosage

89
Q

What problems are associated with using having an air gap?

A

Means you need to increase dosage

90
Q

What causes unsharpness?

A

Light from the interaction spreading before it reaches the receptor

91
Q

What is “noise” when we talk about x rays?

A

Statistical fluctuations in the number of photons reaching each part of the image

92
Q

Why does noise effect image quality?

A

If fluctuations in the number of photons reaching each part of the image are as great as the variations in contrast, you can no longer detect contrast differences

93
Q

How can we reduce noise?

A

We need to increase the number of x-ray photons forming the image

94
Q

How can we increase the number of x-ray photons forming the image and what does this reduce?

A
  1. Increasing the dose
  2. using a thicker screen to stop more of the photons
  3. use more photons per pixel
    This all reduces noise
95
Q

What effect does improving the contrast, resolution or noise usually have>

A

will normally mean either:

  1. Making one of the other aspects poorer
  2. Using more radiation dose
96
Q

What does increasing the magnification lead to?

A
  1. A bigger image
  2. Means you need to give a bigger dose
  3. Results in more geometric unsharpness
97
Q

What is distortion?

A

When an image is a different shape to the object

ODDR- Dental physics (3 decks)

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