Production of X-ray
X-rays are produced by energy conversion between the anode and cathode when a fast-moving stream of electrons is suddenly accelerated from the cathode by applying a voltage and hit with the target anode of an x-ray tube. The x-ray tube is a vacuum containing two electrodes enclosed with a special type of glass known as ―Pyrex glass. The two electrodes are designed in a way that electrons are produced at the cathode end (negative terminal) and can be accelerated by a high potential difference towards the anode (positive terminal). The basic elements of the x-ray tube are shown below.
Cathode
The cathode is the negative terminal of the x-ray tube. In referring to an x-ray tube, the terms cathode and filament are used interchangeably. The filament is the source of electrons for the x-ray tube. The cathode has also two parts except for the filament they are connecting wires, which supply both the voltage and the amperage that heat the filament and metallic focusing cup.
Anode
An anode is the positive terminal of
the x-ray tube. The x-ray tube is of two types, stationary or rotating.
Stationary anode: The anode of the
stationary x-ray tube consists of a small plate of tungsten, 2-3mm thick
that is embedded in a large mass of copper.
Rotating anode: The anode of the
rotating x-ray tube consists of a large disc of tungsten, or an alloy of
tungsten, which rotates at the speed of about 3600 revolutions per minute (rpm)
when exposure is being made.
Glass Enclosure
It is an important part of the x-ray
tube that seals the two electrodes of the x-ray tube in a vacuum. The x-ray tube mustn't have gas inside
it, the electrons that were accelerated towards
the anode would collide with the gas molecules, lose energy and cause secondary
electrons to be ejected from the gas molecules. By this process, additional
electrons would be available for acceleration towards the anode. The purpose
of the vacuum in the x-ray tube
is to allow the number
and speed of the
accelerated electrons to be controlled independently. The x-ray tube is
specifically designed to prevent electric discharge between the electrodes.
The glass enclosure is a special type
of alloy that can bear a large amount of heat without breaking itself during
the operation
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 electrons can 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. The effectiveness of the focusing cup is determined by its size and shape, its charge, and the filament position of the filament in the focusing cup.
Line focus principle
The focal spot is the area of the
target from which the x-ray is emitted. Radiology requires small focal spots
because the smaller the focal spot, the better the spatial resolution of the image. Unfortunately, as the size of the focal spot decreases
the heating of the target
is concentrated onto a smaller
area. Before the rotating anode was
developed another design was incorporated into the x-ray tube to allow a large area
for heating while maintaining a small focal spot. This design is known as the line
focus principle. By angling, the target one makes the effective area of the
target much smaller than the actual area of the electron interaction. The focal spot is the actual x-ray source.
Protective housing
The protective housing incorporates
specially designed high-voltage receptacles to protect against accidental
electric shock. The protective housing also provides mechanical support for the
x-ray tube and protects the tube from damage caused by rough handling. The
protective housing around some x-ray tubes contains oil that serves as b both
an insulator against electric shock and as a thermal cushion to dissipate heat. Some protective housing has a cooling fan to air cool the tube or the
oil in which the x-ray tube is immersed.