Radiation Physics 1 Flashcards
How do X-rays travel
X-rays travel in straight lines at the speed of light.
What can X-rays travel thru
materials and through a vacuum
What do X-rays behave as?
Sometimes x-rays behave like waves Sometimes x-rays behave like particles
What is a photon
A packet of energy
How much bigger are protons than e-
2000 times
What forms the nucleus of atom
Protons and neutrons (neutrons have almost exactly the same mass as protons)
How can you remove and e- from a shell?
Need enough energy to overcome strong attraction between neg charged e- and pos charged proton in nucleus
What is a binding energy
Amount of energy required to remove e- from shell
How do u increase binding energy
Inc protons in nucleus
Dec distance in e- and nucleus
Cathode
Source of e-
Negatively charged
Anode
Target for e- to hit
Positively charged
Describe thermionic emission
An electric current is passed through a filament (like the filament in a light bulb)
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This heats up the filament
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Some of the electrons within the filament get enough energy to sit in a ‘cloud’ surrounding it
How do we get our cloud of e- from cathode over to our anode
Make the anode positive! Put a very high voltage across the tube. Electrons are attracted to the positive anode.
They accelerate across the vacuum and collide with the anode.
Give effects of higher voltage
Higher voltage = higher energy electrons = higher energy x-rays
What does tube current, mA and mAs refer to
flow of electrons between the cathode and anode
What is current measured in?
measured in milli-amperes (mA)
What is time for which the current is flowing measured in?
Seconds
What is mAs?
The product of the current and the time
What is the effect of increasing current, the time of mAs?
It will increase the number of X-rays
What is energy, voltage and kV referred to?
Voltage between cathode and anode
What is voltage measured in?
kV- kilo volts
If the voltage is 70kV what will happen when the e- reach the anode
E- will gain energy of 70kV
What is the effect of increasing voltage
Increases number and energy of X-rays
What is anode made of?
Tungsten (W)
What is the focal spot?
The point that the electron beam hits anode is called the focal spot
Why is focal hot very hot?
A lot of energy is deposited into the focal spot so it gets very hot
How is X-ray production a “lossy” process?
The vast majority of energy is lost as heat
Electron beam intensity (100%) > splits into heat and light production (99%), and X-ray from anode surface (1%), which splits into remaining after fixed filtration (0.5%) and remaining after added filtration (0.1%)
What happens when incoming neg e- is attracted to positive nucleus
The electron slows down and changes direction. The electron has lost energy as a result.
Energy must be conserved. A photon with energy equal to that lost by the electron is produced.
What x-ray production recording to bremsstrahlung- in theory
The closer the electron passes to the nucleus, the more energy it will lose
The maximum energy loss comes from a direct collision with the nucleus
Very low chance this will happen, so very few x-rays with maximum energy
If tube voltage is 70kV, what will the max X-ray energy be?
70kV
What is x-ray production in bremsstrahlung- in reality
All x-ray tubes have some inherent filtration (e.g. exit window)
Reduces number of low energy x-rays
The lowest energy x-rays won’t get out of the tube at all
There is a low energy cut off
If the energy of the incoming e- is greater than the binding e- of the shell e- what happens?
The incoming e- knocks the e- out of its shell
Leaving a hole in the shell
An outer e- moves into hole in shell
E- in outer shell want to move closer to nucleus
To conserve e- xray photon is emitted
When an X-ray photon is emitted when an e- knocks an e- out of its shell, what happens to the energy of the photon and X-ray.
The energy of the photon is fixed - it is the difference in the binding energy of the two shells
These fixed energies x-rays are known as CHARACTERISTIC x-rays (cos because binding energies are characteristic of specific atoms)
What is a tungsten atom?
atomic number 74 (74 protons in nucleus and 74 electrons)
So there are many possible transitions between shells!
The only relevant transitions are when an electron is knocked out of the closest shell to the nucleus
The others characteristic x-rays are too low energy to make it out of the tube
What does a combined spectrum look like?
Characteristic peak after bremsstrahlung curve
What is the effect of kV on X-ray production
High voltage= more X-rays= higher energy of X-rays
Av energy is 1/3 to 1/2 of kVp
What is the binding energy of the inner shell of tungsten?
Approx 69 keV
If tube voltage is less then 69kV what will happen?
Won’t get characteristic X-rays
What is the effect of filtration
Removes lower energy X-rays and reduces patient dose
What is the advantage of removing lower energy X-rays?
These X-rays would be absorbed by the patient and not contribute to the image
What is the minimum AI filtration?
1.5mm
Why do we pick tungsten as our anode?
High atomic number means efficient Bremsstrahlung production
High melting point
Atomic number of tin?
50
What do we want to achieve in image formation in X-ray?
We want a ‘shadow’ image of the teeth
We want a wide range of contrast over the range of interest
White for the most dense objects
Black for the least dense objects
A wide range of shades of grey in-between!
How do X-rays interact with the bodyodyodyodypody
Transmitted, scattered, absorbed
-Transmitted
X-rays travel through patient and onto detector (these are the x-rays we want)
-Scattered
X-rays travel into patient and are scattered out at different angles. Onto detector & into space around patient (degrade image quality and contribute to staff dose)
-Absorbed
Energy deposited into the patient (give radiation dose to patient)
What is actually happening when the xray photons get to the atoms in our body?
- The photoelectric effect
- Compton scatter
What is the photoelectric effect?
X-Ray photon gives all of its energy to the electron (the photon ceases to exist)
incoming x-ray photon ejects electron out of shell (gives all of the energy to the e-)
What is the effect of atomic number of photoelectric effect (PE)
The higher the atomic number the more likely the PE is
What is the effect of energy on PE?
At high energy the PE become less likely
What is the effect of PE in the contrast of the image?
The PE effect increases contrast in image
What is the Compton scatter ?
Incoming xray photon= enough energy to knock out e- from shell
The photon does not cease to exist, its remaining energy scattered off in different directions
Effect of Compton scatter on contrasting in the image?
Comptons scatter reduces contrast in image
When and why is Compton scatter more likely ?
High energies cos PE is less likely
Effect of low X-ray energy?
less penetrating
• Higher dose to the patient
• More contrast, less scatter
Effect of high energy X-ray?
more penetrating
• Less dose to the patient
• More scatter, less contrast
What xray energy do we want?
All medical imaging is about compromise (as you’ll find out next time)
High enough that we get penetration through the tissue of interest Low enough that we can get high contrast
In dentistry what is the voltage and filtration measurements?
70kV with ~2mm Al filtration
What is the solution for the lack of depth in 2D planer imaging?
3D imaging- take images from many angles> construct into 3D image
Much higher radiation dose require > only justified if can’t get required information from a 2D image
