Principles of radiation

Labelling a tube head

What is a cathode

• Coiled wire filament

• Transports electrical current

• Negatively charged

What is an anode

• AKA target anode

• Stationary or rotating

• Positively charged

What is a focusing cup

• Contains cathode

• Narrows and concentrates the beam

What is the function of the tube head

Contains a light diaphragm to change collimation

What is the structure of an x-ray tube head

• Anode, cathode and part of copper stem are housed in an airtight glass envelope 

• Glass vacuum and everything in it is bathed in oil to insulate it and help conduct heat away from the anode 

• Then it is earthed and encased in a lead lined metal casing 

• These stationary anode machines are limited by the amount of heat they produce so they are only able to produce a primary beam of low intensity 

• More powerful machines require a rotating anode in order to produce enough x rays to penetrate large areas 

 

Photons recap

• They do not require a medium for transmission 

• They travel in straight lines but in any direction 

• They interact with matter by either scattering or being absorbed  

• When any electromagnetic radiation is emitted, the intensity of the radiation will decrease, the further away it travels from its source   

 

How are x-ray photons produced

• Photons are produced by electron interaction

• Referred to as x-ray photons

• Produced by high voltage acceleration of electrons

• The negative charge created by the cathode will always move from the negative to the positive charge of the anode

• There must be a step down filament

• There must be a high tension filament transformer

What are the properties of a stationary anode

• The anode target is set at an angle of 15-20 degrees to increase the surface area where the electrons hit the anode to distribute the heat 

• Made from Tungsten.  

• This is called the actual focal spot 

• Some machines offer a choice of focal spots (fine focus vs broad focus)

What does fine focus mean

• Smaller cathode

• Finer cross section of electrons

• More detail 

What does broad focus mean

• Big cathode

• Larger cross section of electron

• Less detail, achieving a smaller effective focal spot is called the line focus principle   

 

What are the properties of a rotating anode

• Rotating anodes are more effective at removing the heat created by the Electrons 

• The anode is a disk with a bevelled edge set at 15-20 degrees which is driven by a motor.  

• The rest of the tube head is the same  

• During exposure the anode disk rotates rapidly (3,000-9,000) so the target that the electrons hit is constantly changing therefore the surface area of the target is much larger than in a stationary anode 

What are the properties of the glass envelope

• Made of Pyrex 

• Tolerate high heat 

• Immersed in oil (heat conduction)

Focusing cup

Why is a focusing cup needed

• Needed as it tries to control, manage and direct the stream of electrons from the cathode across to the target anode 

• Without a focusing cup we wouldn't have a fine level of focus/high resolution 

Cathode

What are the properties of a cathode

• Cathode filament sits within the focusing cup and creates electrons 

• Electrons are created through the heat created via an electrical current to the cathode--> subject to mAs 

How does an x-ray machine produce radiation (part 1)

• Electric passes from the mains BUT is too strong, so before it enters the cathode it is stepped down with a filament transformer  

• When the electrical current passes through the cathode it heats up and releases a cloud of electrons (the size of which is dictated by the mAs) known as thermionic emission 

• In order for this to travel across to the anode at speed there needs to be a high electrical potential difference between the cathode (-ve) and anode (+ve) 

How does an x-ray machine produce radiation (part 2)

• This process takes up more electricity than the mains voltage can supply and so must be stepped up by a high tension transformer which is measured in Kilovolts (kV) 

• The cloud of electrons (-ve) is repelled by the cathode (-ve) and is attracted to the anode (+ve) 

• Electrons hit the target anode at speed to produce the x-ray photons via the release of their energy 

How does an x-ray machine produce radiation (part 3)

• X-ray photons then form the primary beam and exit the tube head via the tube window  

• Heat is a by-product of this process therefore both the cathode and anode of made from tungsten alloy as is has a high atomic number (74) and a high melting point (3380C). This results in 99% of the energy released from the electrons when they hit the target being lost in heat 

• This excess heat is absorbed by a copper block by conduction and travels via a copper stem to be cooled in an oil bath 

What does mAs stand for

Milliamps x seconds

What does mAs control

• Radiographs consist of quality and quantity 

• Controls the quantity of photons, blackening or density 

• MAs controls the amount of electrons created 

• This increases or decreases the radiation dose 

What does KV stand for

Kilovolts

What does KV control

• Controls the quality, contrast and grey scale 

• When increased the speed that electrons move across the tube head increases 

• Higher penetrability to move through tissue 

• Reduces the controls in tissues with different compositions