Two methods of x-ray production
what percent is typical for general purpose x-ray?
Bremsstrahlung (90%) or characteristic(10%)
What steps/components do you need to generate medical x-rays? (x4)
- A source of electrons - the cathode
- A means of accelerating the electrons - the applied kilovoltage
- A means of converting the electron energy into x-rays - electron interactions in the anode material
- A means of cooling the anode - 99% of electron energy is converted into heat
Multiple Tungsten Filaments
- Electrons produced by thermionic emission and boil off
Focussing Cups
Why do we need them?
What do we have to be careful of?
- Structure of cathode and applied charge forces electrons together
- Ensure electron beam hits focal spot in small area (reduce penumbra, increase heating)
Rotating Anode (Stator & rotor)
Whats it made out of?
How does it rotate?
- Don’t want to overheat, small focal spot - Tungesten track, Mo disk. High z, high mpt, dissipate heat.
- Stator: stationary, current flowwing induces magnetic field, windings energised sequentially so induced magnetic field rotates on axis of stator
- Anode rotor rotates
Two ways to reeduce penumbra (geometric)
Smaller focal spot
Smaller distance to image plane
What two things affect size of focal spot on anode?
- Size of incident electron beam (focussing cups)
- Angle of annode (larger angle = greater effective focal spot)
What is Anode-Heel effect?
Variation in X-ray intensity across the field of view due to self absorption within the target. This is worse for older tubes with more electron damage pits but may also limit minimum target angle in certain circumstances (e.g. radiotherapy)
What is light beam diaphragm?
Enables radiographer to visualise and control size of x-ray beam
What is a taget rating chart?
Used by manufacturers to provide maximum beam current for any kV and exposure time (so you don’y nack the tube)
