Week 1: Generation of X-Rays Flashcards
Draw a schematic of an X-ray tube and label each component.
(1)
List the key components of an X-ray tube.
Heated metal filament to provide electrons via thermionic emission.
Evacuated chamber across which a potential difference is applied.
Metal anode (target) with a high efficiency of conversion of electron energy into X-ray photons.
A window in the tube chamber that is largely transparent to X-rays.
Name the key features of the heated metal filament.
Also acts as a cathode.
Constructed of tungsten.
Thin, strong, high melting point.
Name the key features of the evacuated chamber.
Typically 30-100 kV.
Often the peak kilo-voltage (kVp) of a partially rectified applied voltage is quoted.
Voltage determines electron velocity and therefore the energy of the subsequently generated X-rays.
Name the key features of the metal anode.
Typically constructed of tungsten embedded in copper.
Tungsten has good X-ray producing properties.
Copper transmits heat away from the target.
Name the key features of the tube chamber window.
Allows X-ray photons to form a useful beam.
Sometimes materials are added to filter the X-rays.
State the basic process of operation for an X-ray tube.
- Filament is heated via a filament heating current.
- A high voltage is applied between the filament and the target.
- The electrons interact with the target to produce X-rays.
What happens when the filament is heated?
Thermionic emission leads to an electron cloud being formed close to the filament.
What happens when a high voltage is applied between the filament and the target?
The filament acts as the cathode and the target acts as the anode.
Electrons are accelerated towards the anode in a vacuum.
Typically the electron velocity is approximately 1/2 the speed of light when they hit the anode.
Why does the X-ray tube have casing with a window?
The casing prevents unwanted (potentially dangerous) undirected radiation.
The window in the casing allows the X-rays to escape and be targeted.
What are the two principal requirements of anode design?
To ensure a good spatial distribution of X-rays.
To allow heat to dissipate.
What are desirable properties of a chosen target material and why?
Metal - Good conductor of heat and electricity.
High melting point - Vital due to the large amount of heat generated during electron bombardment.
High atomic number (Z) - The production of Bremsstrahlung radiation is proportional to Z.
Sketch a front and side view of a rotating anode.
(2)
Why are rotating anodes preferred over stationary anodes?
The tungsten target is arranged as an annulus of material that rotates rapidly.
The rotation of the anode decrease the time for which any given part of the anode is exposed to electrons, therefore increasing the area over which heat can be absorbed and dissipated.
What does the bevel angle (α) govern?
- The focal spot size of the X-rays (f): A smaller angle will produce a better focussed X-ray beam, allowing for better image resolution.
- Coverage - The coverage will increase with α and distance of the patient from the anode.
What is the formula for the focal spot size of the X-rays?
f = Fsinα,
where F is the width of electron coverage on the anode.
What is the equation for the approximate coverage?
Coverage ~ 2d x tanα,
where d is the distance of the patient from the anode.
What is the anode heel effect? Draw a diagram to help describe it.
The electron will have to travel a certain distance into the anode target material before X-rays are produced.
As the electrons travel at different angles from the site of production, the length of target material that the X-rays travel through will be different.
Therefore the chance that the X-ray photons interact and are attenuated will be different.
This results in different intensities in the produced X-rays —> the anode heel effect.
(3)
What is the effect of changing the bevel angle on the anode heel effect?
Smaller bevel angles will result in longer path lengths, therefore longer path lengths, more X-ray attenuation and a larger anode heel effect.
Describe the three accelerated electron interactions.
A. Electron interacting with the outer shell atomic electron resulting in the generation of low energy electromagnetic radiation. No X-rays are produced and energy is dissipated as heat.
B. Electron passes very close to atomic nucleus resulting in the generation of Bremsstrahlung radiation.
C. Electron ejects inner shell electron from an atom. This results in the production of characteristic X-rays when the vacant state is filled by a higher energy state electron.
