The X-ray Tube

In essence, the X-ray tube turns electricity into X-rays by firing electrons across a vacuum which interact with a Tungsten target based within a Copper anode at the other end. The interaction with the Tungsten target converts the energy in the electrons into X-ray photons (this is discussed in detail in the Making X-rays section). This section will look at each element of this description in turn in order to make clear the process that happens within the X-ray tube.

Electricity

Current is the flow of charge and is expressed in amperes (A) though more generally in radiography as milliamperes or milliamps (mA). Charge is carried along an electric circuit by the movement of electrons. Electrons are negatively charged and so will flow towards the positively charged end of a circuit and away from the negatively charged end. The difference in charge between the two ends of a circuit is called the potential difference, but is better known as voltage. Therefore, increasing the voltage will increase the speed of flow of the electrons through the circuit.

The power from the national grid is runs as an alternating current (AC) where the direction of the flow of charge changes back and forth 50 times a second. The X-ray tube requires a direct current (DC) as the charge must travel in one direction only in order to ensure the electrons are continuously fired at the target for the length of the intended exposure. Converting AC from the mains into DC requires the use of a generator which takes various steps in order to achieve a consistent one-direction output.

Firing Electrons

The X-ray tube comprises the positively charged Tungsten target within a Copper anode and the negatively charged Tungsten filament cathode. The Copper in the anode works to dissipate the heat from the Tungsten target. The electrons in the circuit reach the Tungsten filament which, due to the high melting point of Tungsten, is able to become so hot due to the resistance in the wire that the electrons are released from the filament by thermionic emission. They are repelled from the negatively charged filament and the negatively charged focussing cup which uses this repulsion to narrow the beam of electrons. They are also attracted by the anode due to its positive charge.

A potential difference exists between the cathode and the anode and this voltage is controlled by selecting the kilovoltage (kV) for an exposure. Increasing the kV will increase the speed at which the electrons flow across the tube and therefore the energy with which they interact with the atoms of the Tungsten target. This results in the X-ray photons produced having a greater energy or ‘penetrating power’ (more detail in Making X-rays section) which is expressed in kiloelectron volts (keV). The increase of 1kV in the X-ray tube will result in an increase of up to 1keV of the resultant X-ray photon.

The amount of electrons moving across the tube is controlled by the current of the circuit. The higher the current, the more electrons flow through the circuit and therefore more electrons will be released by thermionic emission from the Tungsten filament. The amount of electrons that interact with the anode will directly affect the amount of X-ray photons produced. So increasing the tube current (mA) will increase the amount X-ray photons.

Vacuum

The X-ray tube is a glass tube evacuated of all the air. This prevents any oxidation of the electrical components and also means that there are no issues of increasing pressure within the tube due to the heat produced. The entire tube is suspended in oil which acts as a cooling system by dissipating the heat through convection, therefore keeping the components within a safe operating temperature.