X  RAY TUBES

 The anode and cathode assemblies are contained in an evacuated glass tube.

The vacuum is necessary to allow the electrons free travel from the negative  cathode to the positive anode without absorption of scattering and to withstand the high voltage applied between the cathode an anode.

Electrons are boiled off a heated filament in the cathode and accelerated to the anode. When they strike the anode of them produce x-rays.

GRID CONTROLLED TUBES

Some tubes ,focusing cathode cup is insulated from the rest of the cathode.A negative voltage can be applied to this cup to prevent any electrons from leaving the cathode while the voltage is present. Voltage cup is called the GRID voltage and can either turn the tube current on or off in a short time.The advantage of grid controlled tubes is their ability to form the short x-ray pulses needed in angiography and cineradiography.

ANODE DESIGN

Some mammographic and portable units have stationary anodes but most x-ray employ rotating anode tubes.The line focus principle and the heel effect are the same for both stationary and rotating anodes.