 |
| The Cathode Side of X-ray Tube |
Filament
The filament is a coil of wire similar to that in a kitchen toaster except much smaller. The filament is usually approximately 2 mm in diameter and 1 to 2 cm long. In a kitchen toaster, an electric current is conducted through the coil, causing it to glow and emit a large quantity of heat. An x-ray tube filament emits electrons when it is heated. When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are “boiled off” and ejected from the filament. This phenomenon is known as thermionic emission. Filaments are usually made of thoriated tungsten. Because tungsten provides for higher thermionic emission than other metals. Its melting point is 3410° C, and therefore it is not likely to burn out like the filament of a light bulb.
Focusing Cup
The filament is embedded in a metal cup called the focusing cup. Because all the electrons accelerated from cathode to anode are electrically negative, the electron beam tends to spread out owing to electrostatic repulsion. Some electri=ons can even miss the anode completely. The focusing cup is negatively charged so that it electrostatically confines the electron beam to a small area of the anode.
Effectiveness of Focusing Cup is determined by:
- Size
- Shape
- Charge
- Filament size and shape
- Position of the Filament in the focusing cup
Filament Current
When the x-ray imaging system is first turned on, a low current passes through the filament to warm it and prepare it for the thermal jolt necessary for x-ray production. At low filament current, there is no tube current because the filament does not get hot enough for thermionic emission. Once the filament current is high enough for thermionic emission, a small rise in filament current results in a large rise in tube current.
Dual Focus
Most diagnostic x-ray tube have two focal spot, one large and the other is small. The small focal spot is used when better spatial resolution is required. The large focal spot is used when large body parts are imaged and when other techniques that produced high heat are required. Normally, either filament can be used with the lower mA station, approximately 300 mA or less. At approximately 400 mA and up, only the larger focal spot is allowed because the heat capacity of anode could be exceeded if the small focal spot were used. The size of small focal spot ranges from 0.1 to 1 mm; large focal spots usually ranges from 0.3 to 2 mm. The small focal size is associated with the small filament and the large focal spot size with the large filament.
