X Ray Tube Construction Flashcards
What is the difference between x rays and gamma rays
X rays can be switched on and off as there produced electronically
Gamma rays can’t as they are produced by unstable atoms
What are the components of the x ray tube
Cathode assembly
Vaccine
Anode disc
Anode stem
Rotor
Bearings
Focal track filament , and glass envelope
Filament set In focus
Give me a brief overview of what takes place in the x ray tube
1) cathode filament is heated
Electron are released by thermionic emissions
Electrons are accelerated by tube potential - at a high voltage
Electrons crash into anode and are slowed down - this deceleration emits x rays
Heat is produced along side 2 types of x rays
Heat must be dissipated
After emissions x rays are collimated and filtered before they reach patients
Describe the cathode
Is negative
Filament has a source of electrons
Made up of tungsten
What happened at the cathode In step 1
Cathode in heated which increases energy of atoms and electrons are emitted
why is a focusing cap needed
We need a focusing cap due to filaments which produce a cloud of electrons.
As theses both are opposite in charge the electrons go outwards when we need them to be closely packed together so we use focusing cap
What is a focusing cap
It is negative and on the surface of the cathode
It concentrated the electron beams towards the focal spot of the anode
Made up of nickel
As it is negative it helps accelerate the electrons towards the anode
Focal cap is coiled which gives it a high surface area helping with the coverage of the electrons onto the focal point -(otherwise the electrons would spread out so only some would reach the target)
Why do we need a vaccume
As the electrons are accelerate ,if there are particles in the way the electrons will lose energy and scatter
So we remove the particles
Why is there a glass envelope in the tube
Contains a vaccume
What are the 2 types of x rays
Bremstrahhlung radiation
Characteristic radiation
What does the anode look like and why does it look like this
Disk
Slightly angles
Rotates - spreads the area the electrons are hitting so that the heat is distributed - too much heat will damage the tube and melt the wires
At the end of x ray production what is more formed
Less than 1% of energy is converted to c rays
And as heat can damage tubing heat dissipation is built into the design- e.g shape of anode
What is the equation for heat
W x kVp Xmas
KVe- effective kV- potential difference
W - waveform of voltage
KVp- peak kV
MAs- current exposure time product
What is heat capacity and what is desirable for the anode
Temperature rises with heat input
Temp rise = energy applied/ heat capacity
High heat capacity desirable for anode so it wont melt and cool down to another shape
What increases heat capacity
Removing more heat
What effects heat capacity in terms of equipment in the tube
Rotating anode:
Disk diameter- wider the diameter the larger the circumference you are spreading the electrons
Speed of disk rotation
Target angle
Oil
What materials are we looking for when constructing the tube and why
Tungsten
High atomic mass- more electrons to interact with
High melting point
High thermal conductivity- to dissipate heat
Low vapour pressure at working temp- reduces sublimation which damages tube ( transfer of substances from solid state to gas)
When d we see a stationery anode
Dental x rays so electrons only hit one area
Why does target angle help with heat distribution
Angle increases the area of heat distribution
The focal spot is small as possible - more precise x ray beam so increases image quality
However a small focal spot we won’t dissipate heat as well as a large focal spot
What is the target stem ……..
It is a molybdenum disk with tungsten target
This molybdenum stem is a poor heat conductor -present heat from escaping or entering temperature sensitive areas like the bearing
Why is the x ray tube submerged in and why
Is housed in oil which dissipates heat
Can leak
Why is the casing of the x ray tube important
Protects tube and shields it
So x ray is directed into one specific and wanted place
What is the casing made out of
Aluminium or steel
Heat and electrical insulator
Shields from radiation going in wrong direction
Protects components
Why is filtration needed
No all energy of x ray beam is useful
So this filtered useless energies
These photons can contribute to dose received to patient but not the image produced as they would be absorbed by patients
So the low ebergues are filtered
