The xray tube Flashcards
What are the components of an x-ray tube? Draw one and label it.
*Tungsten filament
*Rotating Anode
*Cathode
*Cathode Cup
*Vacuum Housing: lead lined (stop x-rays going everywhere/ shield against radiation), electrically insulated (it has high voltage), filled with oil (cool it down)
Why is heat such an issue? Bremsstrahlung is only 1% efficient . 99% converted to heat
*Oil
*Shielding
*Expansion bellows
What is the use of each component of the x-ray tube?
filament (also acts as cathode): boils off electrons by thermionic emission
target (also acts as anode): electrons strike to produce x-rays
Vacuum: provides clear path for electrons as they move from anode to cathode
Tube housing: provides electrical insulation and prtoects against leakage radiation
Focusing cup: focuses the electron cloud onto the target
Tungsten target: this is where interactions occur that produce x-rays
Tungsten filament: thermionic emission occurs here.
Tube filtration: removes x-rays that would not contribute to the image but would increase the dose to the patient
Rotating anode target: prevents damage due to overheating
What is thermionic emission and where does it occur in the x-ray tube
Emission of electrons from a heated metal (cathode)
How do we accelerate electrons across the x-ray tube?
High voltage is applied to the anode relative to the filament
Electrons from the space cloud will accelerate towards the positive anode, gaining kinetic energy as they do so.
The current (mA) and voltage (kV) can be adjusted across an x-ray tube.
Which one affects the number of electrons (and hence x-rays)?
Which one affects the kinetic energy of electrons (and hence the energy of the x-ray beam)?
number of electrons: mA
kinetic energy of electrons: KV
What are the two interactions by which x-rays are produced?
Characteristic radiation, bremsstrahlung
X-ray production steps
A current is passed through tungsten target
Electrons boil off during thermionic emission
Electrons are accelerated towards positively charged anode
Electrons collide with the anode
X-rays are produced via bremsstrahlung and characteristic radiation interactions
X-rays pass through tube window
What factors affect the intensity and/ or quality of x-ray spectrum?
Added filtration: will remove low energy x-rays and lower intensity of radiation across the spectrum
Higher tube current: increases number of electrons available to interact. Intensity of beam increases.
Distance from x-ray target: inverse square law applies
Voltage across x-ray tube: higher voltage will transfer more energy to electrons resulting in a higher energy beam
What is x-ray tube filtration?
Use a rotating target (anode is disk shaped, spins during exposure, heat is distributed all around the circumference of target, increases heat capacity of tube)
Tube housing filled with oil (cools and bellows)
Angle the anode relative to electron stream: target angle is between 7 and 17 degrees allowing electron beam to be spread over a larger surface area
Whys is x-ray tube filtration important?
Low energy x-rays are of no use in an image because they are completely absorbed by a patient
This creates a dose to the patient that is of no benefit
We need to remove these x-rays from the beam to minimize dose to the patient
What factors might affect the x-ray spectrum?
Inherent filtration: already built into the tube via its component parts (x-ray tube port, housing oil, field light mirror etc.)
Added filtration: used to remove remaining low energy x-rays that will not contribute to the image
tube current: If filament heat is increased –> more electrons available to cross the tube –> higher intensity
Tube voltage increased –> electrons gain more kinetic energy –> high energy x-rays
Relation between intensity of x-ray beam and filament current:
If filament current is increased, intensity of x-ray beam is also increased.
What happens to the x-ray energy when the voltage applied between anode and cathode increases?
It increases
What is a focusing cup?
The shape of the cup and its negative charge cause the electrons to be directed to a precise area on the node
Line focus principle- focal spot:
Actual focal spot: area of the anode struck by electron stream
Effective focal spot: vertical projection of actual focal spot (the smaller the effective focal spot, the sharper the image)
Which is larger, actual focal spot or effective focal spot?
Actual focal spot. (large focal spots allow for greater heat dissipation, small effective focal spot makes a sharper image)
