Medical physics 1 radiation production Flashcards
Describe the electromagnetic properties of X rays
They have a very short wavelength and a high frequency
State the range in an x rays wavelength
Less than 100nm
What is the main difference between X rays and gamma rays?
X rays involve electron interactions
Gamma rays involve nuclear interactions
What are x rays?
They are a stream of photons with a wavelength of less than 100nm
What is a photon?
A photon is a quantum of energy measured in electron volts
How many joules in an electron volt?
1eV = 1.602 x 10^-19
State the relationship between frequency of wavelength, Planck’s constant and energy
Energy (E)= Planck’s constant (h) x frequency of wavelength (V)
E=h x V
State the relationship between wavelength, Planck’s constant, the speed of light and energy
Energy (E)= (Planck’s constant (h) x Speed of light (c)) /Wavelength (lambda)
E= hc / lambda
State Planck’s constant
6.626 x 10^-34 J per second
State the speed of light
2.998 x 10^8 m per second
What happens as wavelength decreases?
Photon energy increases so the he probability of an interaction decreases and photons tend to travel further through matter
How do X rays produce images?
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
Name the 2 mechanisms by which we can produce X rays in theory
- Bremsstrahlung
2. Characteristic spectrum
Interactions between what occur in the bremsstrahlung theory to produce x rays?
Interactions between the electron and nucleus
Describe how the bremsstrahlung theory produces x rays
- 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
Why are a range of X ray energies produces in the bremsstrahlung theory of x ray production?
As there are many pathways that the electron can take to get out of the atom
What determines the range of energies produced by the bremsstrahlung theory of x ray production?
The tube potential
Interactions between what occur in the characteristic radiation theory to produce x rays?
Interactions between the electron and another electron
Describe how the characteristic radiation theory produces x rays
- 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
What determines the energy produced by the characteristic radiation theory of x ray production?
Determined by the target material
What components make up an x ray tube?
- Cathode
- Anode
- Glass envelope
- Filters
- Cooling oil
- Shield
What does an x ray tube produce?
X rays in a vacuum
How are x rays produced in an x ray tube?
- 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
What is electron energy to proportional to in an x ray tube?
Electron energy is proportional to the voltage difference between anode and cathode
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
What does evacuated glass tube that makes up an x ray tube do?
- maximises x-ray production in the target because it stops electrons having interactions with other atoms
- provides insulation between the anode and cathode
- Produces tube arcing when gas is present in the tube
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
What is the window of the x ray tube made up of?
Beryllium
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
What is the maximum photon energy equal to?
Maximum photon energy is the same as tube potential
What happens to the relative photon output as the flow of electrons increases?
Increases
At what kV does the dental x ray tube operate at?
65kV
What is the flow of current across X-ray tube measured in?
The current measures in milliamperes (mA)
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
What is the total number of x rays produced dependent on?
on current & exposure time
What is the intensity of the x rays proportional to?
kV(p^2)
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
What is the purpose of a filter in the x ray tube?
To reduce the dose of x rays to a patient
What is filtration comprised of?
Inherent: Glass envelope within the tube
Added: aluminium
Total filtration = Inherent + Added
What does adding filtration do?
Adding filtration decreases overall beam intensity but increases mean energy
What does filtration affect?
affects both the quality and quantity of the x-rays
What should the filtration for a standard diagnostic x ray tube be?
At least 2.5mmAl
How do we get a sharp image?
By using a small focal point
What is the sharpness of an image affected by?
Sharpness is affected by size of the
focal spot
What happens if the focal point is too small?
The tube can get hot and catch fire
How do we achieve a small focal point?
- Focus electrons
2. Set target at steep angle
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
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
What does the build of heat init in an x ray tube?
limits the rate at which X-rays can be repeated
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
What do dental images need to show?
- Where things are: spatial information
2. What things are: contrast
How is contrast achieved in an image?
- Tissues have different atomic (Z) numbers
- All tissues are off a different thickness
- We Add other substances which are taken up by one tissue more than another.
What reduces he intensity of X rays as they travel through matter?
- Photoelectric absorption
2. Compton Scatter
What is the photoelectric effect?
When x rays are absorbed
What is the Compton effect?
When x rays are scattered
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
When does the probability of photoelectric absorption decrease?
As x ray energy increases
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
What is meant by the attenuation of x rays?
Attenuation is the total process of reducing the intensity of an x-ray beam
What happens when an X-ray is absorbed or scattered?
It is attenuated (weakened)
Why is scattering bad?
Leads the practitioner into thinking the organ is in a different area to its true place
What increases the probability of the photoelectric effect?
Increases with the cube of the atomic number of the absorbing material
What is the main source of contrast?
The photoelectric effect
Where is good contrast observed?
Between tissues of different atomic numbers §
What reduces contrast?
Compton scatter reduces contrast
Describe high energy x rays?
They are penetrating
They give a poorer contrast
Why do high energy x rays have a poor contrast?
As more of the interactions are Compton
Describe low energy x rays
They are more easily absorbed
They give off a better contrast
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
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
Name the 3 types of image receptors
- Film
- Computed radiography
- Digital radiography
What information can er not retrieve in a dimensional image?
We cannot retrieve information about depth
What do analogue images include?
Things like film screen radiography and fluoroscopy with image intensifiers
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
What type of image do computed radiography produce?
Produces an analogue latent image
What does a film require in order to produce an image?
An adequate x ray exposure
What is exposure determined by?
The number of electrons hitting anode (measured in mAs)
What is the film characteristic curve dependent on?
developer temp/time in developer/strength/age
How confirm characteristic curves be generated?
by exposing film to diff light intensities
What factors effect image quality?
- Contrast
- Unsharpness
3, Noise - Magnification
- Distortion
What does subject radiation contrast depend on?
The ratio of transmitted radiation through different materials
What is radiographic image contrast?
the difference between optical densities in different areas of the image
How can we influence the subject radiation contrast?
Changing the quality (average energy) of the radiation
How can we change the quality (average energy) of the radiation?
- Change the kV
- Change the filtration
- Change the target material
What effect does increasing the kV have on scatter?
- Absolute quantity reduced
as compton scatter decreases with increasing
kV so less primary radiation needed - Proportion of scatter reaching the image increases as scatter is more penetrating at higher kV
Give some practical ways to reduce scatter
- Reduce kV
- Change patient orientation
- Compression (reduce thickness thus reducing scatter volume)
- Use anti scatter grid
- Have an air gap
- Use a Cassette with high atomic number (Z) back
- Reduce field size thus reducing scatter volume
What problems are associated with reducing kV?
Means you need to increase dosage
What problems are associated with using an anti scatter grid?
Means you need to increase dosage
What problems are associated with using having an air gap?
Means you need to increase dosage
What causes unsharpness?
Light from the interaction spreading before it reaches the receptor
What is “noise” when we talk about x rays?
Statistical fluctuations in the number of photons reaching each part of the image
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
How can we reduce noise?
We need to increase the number of x-ray photons forming the image
How can we increase the number of x-ray photons forming the image and what does this reduce?
- Increasing the dose
- using a thicker screen to stop more of the photons
- use more photons per pixel
This all reduces noise
What effect does improving the contrast, resolution or noise usually have>
will normally mean either:
- Making one of the other aspects poorer
- Using more radiation dose
What does increasing the magnification lead to?
- A bigger image
- Means you need to give a bigger dose
- Results in more geometric unsharpness
What is distortion?
When an image is a different shape to the object
