Basic principes of Computed Tomography (CT), part 1 (Tube)

Many people ask us, what CT is, if the part is radioactive after scanning or if they can scan an automobile.

Today we will look on the

Tube

Tube is mainly a vaccum energy converter. I recieves electric energy and convert it into heat and x-rays. In fact 99% of electric energy is transformed into heat and only 1% into x-rays.
Tube contains two main parts - cathode and anode

Cathode is basically a tungsten filament and when a current flow through it, it gets heated and starts to emit electrons. This process is called thermionic emission.

Anode is tungsten (molybdenum in mammography) disc. Electrons produced on cathode filament interacts here in following ways:

• interaction with K-shell electron: causes production of characteristic radiation.
• interaction with nucleus: causes bremsstrahlung radiation.
• interaction with outer shell electrons: causes line spectrum.

There are some few additional components like rotor, induction stator, tube window, cooling dielectric oil,....

Lets focus on anode as one of the most important parts. As we mentioned before, it is metal disc, shaped in the form of a beveled disk. It is usually made by alloy of tungsten with minority of rhenium

Because of high heat production is body made from materials with high heat storage capacity (graphite, molybdeum,...). In body is cooling system filled with cooling liquid, usually dielectric oil. For better heat distribution is anode on rotor, which rotates with it and heating different spot, while the previous one is cooled down. (In industrial CT for better resolution, in medical it rotates with speeds up to 10 000 rpm). Temperatures can reach 2000°C.

X-Rays are produced on small spot on anode called Focal spot, smaller this spot is, better resuts are given when scanning parts.

Tubes are not only with this reflexive principe, but also with transmision one, where X-rays are produced in same way as electrons.

Tubes are also open or closed. Basic one are closed. These are made as one use only and when they stop working, you simply replace them. It is also good for no need to make vacuum.
Open tubes are made for easier replacing parts, especially filaments. Disadvantage is need to make vacuum before use.

RV