X-ray Generation Flashcards
What is radon gas also known as?
Natural gas
What is the average dose of radiation to the population from natural gas?
2.6 Sv p.a
Why does electromagnetic radiation having different properties depend on?
The wavelength of the waves that form the radiation
Why are x-rays and gamma rays ionising radiation?
Because they have enough energy to knock electrons off atoms and cause damage
Who discovered x-rays?
Wilhelm Rotegan
How did Wilhelm Rotegan discover x-rays?
(4)
He covered a crooked tube with black cardboard
He darkened the room and powered on the tube
He saw a faint glow on a barium planticocyanide screen 8 feet away
He then realised that an invisible ray was passing through the cardboard
When x-ray were first used, what was the radiation dose like?
Very high- much higher than now
What are the differences between x-rays are gamma rays?
(2)
X-rays can be turned off
Gamma rays can’t be turned off
X-rays are produced by unstable atoms decaying and releasing energy
Gamma rays are produced by electromagnets
What are bearings in x-ray tubes made of?
Silver
What does HT cable in x-ray tubes stand for?
High tension cable
How do x-ray tubes work?
(11)
Cathode filament heated
Gets hot
Atoms gets lots of energy
Atoms move faster
Electrons released
Electrons accelerated by tube potential
Electrons crash into anode
Electrons lose kinetic energy and stop moving
Heat produced
Radiation produced
Heat must be dissipated (or tubes would melt(
What is produced when electrons crash into the anode?
(2)
Heat
Radiation
What 2 types of radiation are produced when the electrons in the x-ray tube crash into the anode?
Bremstrahhlung radiation
Characteristic radiation
What happens when the 2 types of radiation have ben produced in the x-ray tube?
(3)
The radiation is produced in all directions
The radiation needs to be collimated
The radiation is then detected
Which end of the x-ray tube is the cathode?
The negative end
What does the filament in the x-ray tube do?
It’s the source of the electrons
What is the filament in the x-ray tube made of?
A coil made of Tungsten
What does the filament in the x-ray tube contain?
What does that do?
It contains 1% thorium
It increases the release of electrons via thermionic emission
What is thermionic emission?
The liberation (release) of electrons from an electrode by virtue of its temperature
Why is the x-ray filament a coil?
Because it’s a controlled source of electrons that are used to produce x-ray beams
What is the focussing cup in the x-ray tube?
It’s a negatively charged shallow depression that’s found on the surface of the cathode
What does the focussing cup in the x-ray tube do?
It concentrates the electron beam towards the focal spot of the anode
What is the focussing cup in the x-ray tube mainly made of?
Nickel
What does the negative charge of the focussing cup in the x-ray tube help to do?
It helps to accelerate the electrons towards the anode
What does the shape of the focussing cup in the x-ray tube help to do?
It helps to converge (connect) the electrons onto the focal point
What would happen if the focussing cup in the x-ray tube didn’t have its particular shape?
The repulsion between the negatively charged electrons would cause the electrons to spread out as they reach the anode, causing only some of the electrons to reach the anode
What is the charge of electron?
Negative
What does increasing the mean photon energy do?
It causes the release of fewer photons of lower energy
What does an increased x-ray output do?
It allows the x-ray to stay closer to the maximum for longer
What does a shorter exposure of x-rays do?
Since the output is higher, it allows the exposure to be shorter in time but still getting the same output
What does a lower patient dose do?
It allows for an increased mean energy, so fewer low energy photons contribute to the patient’s dose, but they don’t contribute to the final image
Between the rectified current and the 3 phase supply, which has the higher average energy?
3 phase supply
What do transformers doing the x-ray tube?
They provide the high voltages that are used in x-ray production
How many volts are the mains electricity at home?
230 volts
How many volts is a chest x-ray exposure?
125 000 volts
What does a vacuum in the x-ray tube do?
It prevents electrons from losing energy
Why do we need a vacuum in the x-ray tube?
(5)
There’s a high potential pulling the electron to the anode
But they’re particles in the way, and the electrons will end up interacting with them
This would cause the electrons to lose energy
The electrons would get scattered
So we need to remove all the particles in the way
What does the anode look like?
(3)
It’s a disk
It’s at a slight angle
It rotates
Why is the anode a rotating disk?
It allows the spreading of electrons, so more electrons can hit more of the disk,which allows for the heat to be distributed over a larger area
What is the equation for heat?
Heat (J) = w x KvP x mAs
What is heat measured in?
Joules (J)
What does w stand for in the heat equation?
It’s the waveform (shape of the wave) of the voltage through the x-ray tube
The more uniform (consistent) the waveform…
(Heat production)
The lower the heat production
What does the heat capacity show?
It shows how much the temperature rises with heat input
What is the equation for temperature rise?
Energy applied / heat capacity
Is a higher or lower heat capacity desirable for anodes?
Higher
Why is a higher heat capacity desirable for anodes?
So the anode doesn’t become too hot and melt
What does increasing the heat removed from the system increase?
Heat capacity
How is heat capacity represented?
By tube rating charts
What 3 factors allow for a safer heat capacity?
Lower kV
Lower mA
Shorter time
What is the advantage of the anode rotating?
It allows the parts of the anode that have been hit by electrons time to cool down, so they don’t melt
What are the features of the target material of the anode?
(5)
It’s usually tungsten
It has a high atomic number
It has a high melting point (doesn’t melt)
It has a high thermal conductivity
It has a low vapour pressure at working temperature
What does the target material of the anode man?
It means what the anode is made of
Why is it an advantage of the target material of the anode that it has a high atomic number?
It allows for more electrons to interact with the target material
Why is it an advantage of the target material of the anode that is has a high melting point?
It won’t melt
Why is it an advantage of the target material of the anode that it has a high thermal conductivity?
It dissipates heat (takes away heat well)
Why is it an advantage of the target material of the anode that it has a low vapour pressure at working temperature?
It prevents sublimation, which can damage the tube
What are the 2 different types of anodes?
Stationary
Rotating
Where are stationary anodes mainly found?
(2)
In dental radiology and
Radiotherapy systems
How do stationary anodes work?
The electron beam constantly hits one small area
Why are stationary anodes used in dental radiology?
Because the electron beam is going through thinner/less material
What charge does a rotating anode disk have?
Positive
Why does a rotating anode disk have a positive charge?
To attract electrons (which have a negative charge) towards it
Why does a rotating anode disk rotate?
To dissipate (release) heat
How much does the rotating anode disk rotate?
50 Hz
What is the temperature of the rotating anode disk?
2000 degrees
Is a larger rotating anode disk better or worse for heat transfer?
Better
Why is the rotating anode disk’s target angled?
(2)
To increase the area of heat distribution, so we can get the biggest surface area for the heat being dissipated (released)
So it can compromise between image and heat dissipation (release)
What is the focal point in the x-ray tube?
The area of the target that’s impacted by the electrons where x-rays are produced
What does the target being at an angle increase?
The effective focal point
What does a smaller area of the focal point cause?
A more precise x-ray beam, but it means there’s an increase in heat
What is the angle of the anode?
Between 7-20 degrees
What does the angle of the anode affect?
(2)
Effective focal point size
Heel effect
What does a larger anode angle increase?
Effective focal point size
Why might different examinations require different anode angles?
Because the angle depends on the power needed for the heat and resolution
What is the target stem?
A molybdenum disk with a thin tungsten target
Is a molybdenum stem a good or poor heat conductor?
Poor heat conductor (it conducts little heat)
What does the target stem (molybdenum stem) include?
(2)
Silver lubricated bearings
Blackened rotor
Why does the molybdenum stem have a blackened rotor?
For heat transfer
What is the disk also known as?
The anode
Why is it important to minimize heat conduction to the bearings in the molybdenum stem?
To prevent the bearings from melting and causing damage
What is the x-ray tube housed in?
Why?
Oil
To dissipate heat
What is a problem with the x-ray tube being housed in oil?
It can leak
What does the anode cooling chart show?
It shows how long it takes for the anode to cool from maximum heat
Why is the anode cooling charge important?
Because it’s used to prevent anode damage between exposures
What could a worn anode lead to?
(2)
Inconsistent output
Inconsistent image quality
What can the casing on the x-ray tube be made of?
(2)
Aluminium or
Steel
What is the casing of the x-ray tube an insulator for?
(2)
Electrical insulator
Heat insulator
Why is the casing of the x-ray tube important?
(2)
It shields radiation from going in the wrong direction
It protects the components of the x-ray tube
What is an issue with the casing of the x-ray tube?
It can cause head injuries
Are all the energies of the x-ray beam useful?
No
What does filtration do?
It cuts out some of the useless energies
Why are low energy photons bad?
They can contribute towards the dose received by the patients but not to the image, because those photons would be absorbed by the patient
What does filtration do to the low energy photons?
The low energy photons are absorbed by the filters
What does the anode heel effect say?
There’s a higher intensity of x-rays at the cathode, so larger materials are put at the cathode. Therefore, more x-rays are absorbed at the anode. This shows that the x-ray beam isn’t the same across the whole area
What are the 2 types of collimation?
Cylindrical collimation
Rectangular collimation
Which type of collimation is used now?
Rectangular collimation
Which type of collimation isn’t used anymore?
Cylindrical collimation
What does collimation reduce?
It reduces the risk of radiation
Why is cylindrical collimation not used anymore?
The radiation around the edge isn’t recorded by the detector, but the patients are exposed to that radiation, meaning that they’re exposed to more radiation
What do collimators do?
They restrict the span of the x-ray beam. So they block part of the beam- they don’t focus it
What are collimators typically made of?
Lead shutters
What does the light beam diaphragm do?
It allows us to see where the x-ray beam is being directed and how it’s collimated
What does decreasing the area (amount) of the x-ray beams being exposed result in?
A higher quality image
Why is the light beam diaphragm good?
(2)
It reduces the amount of scattered radiation
It reduces the dose to the patient
What is an issue with the light beam diaphragm?
It affects post processing
How many lines of collimation does a good x-ray have?
4 lines of collimation
How many lines of collimation does a good x-ray have?
4 lines of collimation
