Medical physics 1 radiation production Flashcards
1
Q
Describe the electromagnetic properties of X rays
A
They have a very short wavelength and a high frequency
2
Q
State the range in an x rays wavelength
A
Less than 100nm
3
Q
What is the main difference between X rays and gamma rays?
A
X rays involve electron interactions
Gamma rays involve nuclear interactions
4
Q
What are x rays?
A
They are a stream of photons with a wavelength of less than 100nm
5
Q
What is a photon?
A
A photon is a quantum of energy measured in electron volts
6
Q
How many joules in an electron volt?
A
1eV = 1.602 x 10^-19
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
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
9
Q
State Planck’s constant
A
6.626 x 10^-34 J per second
10
Q
State the speed of light
A
2.998 x 10^8 m per second
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
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
13
Q
Name the 2 mechanisms by which we can produce X rays in theory
A
- Bremsstrahlung
2. Characteristic spectrum
14
Q
Interactions between what occur in the bremsstrahlung theory to produce x rays?
A
Interactions between the electron and nucleus
15
Q
Describe how the bremsstrahlung theory produces x rays
A
- Electrons are accelerated through a voltage gradient in the atom
- 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 - There are many paths the electron can take so a range of x ray energies produced
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
17
Q
What determines the range of energies produced by the bremsstrahlung theory of x ray production?
A
The tube potential
18
Q
Interactions between what occur in the characteristic radiation theory to produce x rays?
A
Interactions between the electron and another electron
19
Q
Describe how the characteristic radiation theory produces x rays
A
- Electrons are accelerated through a voltage gradient in the atom
- Electron interacts with an outer orbiting electron & ejects it
- This leaves a gap in the orbital & electrons cascade down to fill opened up gaps
- As the electrons cascade down they release energy in the form of X-rays
20
Q
What determines the energy produced by the characteristic radiation theory of x ray production?
A
Determined by the target material
21
Q
What components make up an x ray tube?
A
- Cathode
- Anode
- Glass envelope
- Filters
- Cooling oil
- Shield
22
Q
What does an x ray tube produce?
A
X rays in a vacuum
23
Q
How are x rays produced in an x ray tube?
A
- Electrons are produced at a hot filament (cathode) & accelerated across a gap to anode
- Electrons collide with with material and x rays are given out in all directions
- A small hole is made in the housing to allow for electrons to leave
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
25
What does a high voltage supplied to in an x ray tube do?
Supplies the acceleration to the incident electrons – the higher the voltage, the greater the acceleration, the greater the energy of the incident electrons
26
What does evacuated glass tube that makes up an x ray tube do?
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
27
What does the shielded metal housing that makes up an x ray tube do?
It stops the x-rays being irradiated out of the tube in all directions
28
What is the window of the x ray tube made up of?
Beryllium
29
What does the oil in an x ray tube made do?
used to remove heat from the tube and works as insulation from the high voltage
30
What is the maximum photon energy equal to?
Maximum photon energy is the same as tube potential
31
What happens to the relative photon output as the flow of electrons increases?
Increases
32
At what kV does the dental x ray tube operate at?
65kV
33
What is the flow of current across X-ray tube measured in?
The current measures in milliamperes (mA)
34
What does the current in the dental tube affect?
• Current affects the quantity of x rays produced, but not the energy of x-rays produced
35
What is the total number of x rays produced dependent on?
on current & exposure time
36
What is the intensity of the x rays proportional to?
kV(p^2)
37
What does the filtration of x rays do?
Filtration is used to remove low energy X-rays
| It works by modifying the x-ray spectrum by removal of low energy photons
38
What is the purpose of a filter in the x ray tube?
To reduce the dose of x rays to a patient
39
What is filtration comprised of?
Inherent: Glass envelope within the tube
Added: aluminium
Total filtration = Inherent + Added
40
What does adding filtration do?
Adding filtration decreases overall beam intensity but increases mean energy
41
What does filtration affect?
affects both the quality and quantity of the x-rays
42
What should the filtration for a standard diagnostic x ray tube be?
At least 2.5mmAl
43
How do we get a sharp image?
By using a small focal point
44
What is the sharpness of an image affected by?
Sharpness is affected by size of the
| focal spot
45
What happens if the focal point is too small?
The tube can get hot and catch fire
46
How do we achieve a small focal point?
1. Focus electrons
| 2. Set target at steep angle
47
What effect does target angling have on the focal point?
If the anode is at a steeper angle then the focal spot will be smaller
48
Why is the removal of heal necessary in an x ray tube?
To prevent damage to focal spot and anode (Target)
| To prevent general overheating of the tube
49
What does the build of heat init in an x ray tube?
limits the rate at which X-rays can be repeated
50
What is used to cool down the anode?
A copper jacket is used to conduct the heat away from the anode and into the surrounding insulating oil
51
What do dental images need to show?
1. Where things are: spatial information
| 2. What things are: contrast
52
How is contrast achieved in an image?
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
What reduces he intensity of X rays as they travel through matter?
1. Photoelectric absorption
| 2. Compton Scatter
54
What is the photoelectric effect?
When x rays are absorbed
55
What is the Compton effect?
When x rays are scattered
56
When does photoelectric absorption occur?
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
When does the probability of photoelectric absorption decrease?
As x ray energy increases
58
When does Compton Scatter occur?
Photon collides with an electron and loses some of its energy
The photon changes direction and the electron leaves the atom
59
What is meant by the attenuation of x rays?
Attenuation is the total process of reducing the intensity of an x-ray beam
60
What happens when an X-ray is absorbed or scattered?
It is attenuated (weakened)
61
Why is scattering bad?
Leads the practitioner into thinking the organ is in a different area to its true place
62
What increases the probability of the photoelectric effect?
Increases with the cube of the atomic number of the absorbing material
63
What is the main source of contrast?
The photoelectric effect
64
Where is good contrast observed?
Between tissues of different atomic numbers §
65
What reduces contrast?
Compton scatter reduces contrast
66
Describe high energy x rays?
They are penetrating
| They give a poorer contrast
67
Why do high energy x rays have a poor contrast?
As more of the interactions are Compton
68
Describe low energy x rays
They are more easily absorbed
| They give off a better contrast
69
What is the problems associated with using high and low energy x rays?
High energy gives a lot of scatter
| But low energy needs to be given in higher doses
70
How do we balance the administration of high and low energy x rays to patients?
X rays need to be generated at 30KeV to 70KeV
| So we use: 70kV PD with 2mm Aluminium film in X ray tube
71
Name the 3 types of image receptors
1. Film
2. Computed radiography
3. Digital radiography
72
What information can er not retrieve in a dimensional image?
We cannot retrieve information about depth
73
What do analogue images include?
Things like film screen radiography and fluoroscopy with image intensifiers
74
What is an advantage of digital receptors?
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
What type of image do computed radiography produce?
Produces an analogue latent image
76
What does a film require in order to produce an image?
An adequate x ray exposure
77
What is exposure determined by?
The number of electrons hitting anode (measured in mAs)
78
What is the film characteristic curve dependent on?
developer temp/time in developer/strength/age
79
How confirm characteristic curves be generated?
by exposing film to diff light intensities
80
What factors effect image quality?
1. Contrast
2. Unsharpness
3, Noise
4. Magnification
5. Distortion
81
What does subject radiation contrast depend on?
The ratio of transmitted radiation through different materials
82
What is radiographic image contrast?
the difference between optical densities in different areas of the image
83
How can we influence the subject radiation contrast?
Changing the quality (average energy) of the radiation
84
How can we change the quality (average energy) of the radiation?
1. Change the kV
2. Change the filtration
3. Change the target material
85
What effect does increasing the kV have on scatter?
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
Give some practical ways to reduce scatter
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
What problems are associated with reducing kV?
Means you need to increase dosage
88
What problems are associated with using an anti scatter grid?
Means you need to increase dosage
89
What problems are associated with using having an air gap?
Means you need to increase dosage
90
What causes unsharpness?
Light from the interaction spreading before it reaches the receptor
91
What is "noise" when we talk about x rays?
Statistical fluctuations in the number of photons reaching each part of the image
92
Why does noise effect image quality?
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
How can we reduce noise?
We need to increase the number of x-ray photons forming the image
94
How can we increase the number of x-ray photons forming the image and what does this reduce?
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
What effect does improving the contrast, resolution or noise usually have>
will normally mean either:
1. Making one of the other aspects poorer
2. Using more radiation dose
96
What does increasing the magnification lead to?
1. A bigger image
2. Means you need to give a bigger dose
3. Results in more geometric unsharpness
97
What is distortion?
When an image is a different shape to the object
