X-ray Production Flashcards
Methods of X-ray production
Bremsstrahlung
Characteristic radiaton
Bremsstrahlung
Braking radiation - electron beam scattered, continuous x-rays given off
Characteristic radiation
Primary beam scatters inner shell electrons, characteristic radiation given off as electron moves down to inner shell
What will high energy electron interactions generate?
Bound electron interaction: characteristic
At nucleus: high energy brem.
Near nucleus: high energy brem
Further away: low energy brem.
How are most clinical x-rays generated?
Bremsstrahlung (~90%)
[except mammography which is roughly the opposite]
What do we need to generate x-rays?
A source of electrons (cathode)
A way to accelerate electrons (applied kv)
A way to convert electrons to x-rays
A way to cool the anode
Source of electrons?
Cathode containing multiple tungsten fibres (only one used at a time). Produced by thermionic emission.
How are electrons generated?
Thermionic emission - electrons ‘boil off’ and hover in ‘charge space’.
Rotating anode details
Stator is around glass envelope - windings on it are energised sequentially so induced magnetic field rotates on axis.
Tungsten focal point, high Z and high melting point. Molybdenum disc and graphite help dissipate heat.
Heel effect
Variation in x-ray intensity across field of view due to self absorption in target. May limit minimum target angle in certain circumstances - radiotherapy, worse with smaller angles.
Light beam diaphragm
Lead shutters control size and shape of beam, enables radiographer to visualise and control beam with light.
What is the maximum spectral energy?
The generating voltage
Why do we filter out lower energies
Contribute to skin dose but won’t penetrate enough to be useful.
