An X-Ray machine consists of two main components – X-Ray generator and an image detection system. An X-Ray tube essentially is a high-vacuum diode consisting of a Cathode and Anode. X-ray tube operates by generating a stream of electrons by heating up a cathode (tungsten) filament. This stream of electrons is directed at high speed at a high voltage Anode disc (usually tungsten). X-radiation is produced due to the electrons’ interaction with the atomic particles of the Anode. The X-Rays are focused by a collimator on to a photographic plate, where the image is captured. The body-part being investigated is placed in the path of the X-Rays between the tube and the photographic plate.
The main parts of an X-ray generator
- the tube,
- the high voltage generator,
- the control console, and
- the cooling system.
Functioning of an X-Ray generator
The X-Ray Tube
The tube cathode (filament) is heated with a low-voltage current of a few amps. The filament heats up and the electrons in the wire start breaking free. To kick off the flow of electrons, a strong electrical potential is created between the cathode and the anode by the generator. Electrons that break free of the cathode are strongly attracted to the anode disc. The electron flow between the cathode and the anode is called the tube current. The tube current is measured in milliamps (mA) and is controlled by regulating the low-voltage, heating current applied to the cathode. Higher the temperature of the filament, the larger the number of electrons that leave the cathode and travel to the anode. The mA or current setting on the control console regulates the filament temperature and hence the intensity of the X-ray output.
High Voltage generator
The high-voltage between the cathode and the anode affects the speed at which the electrons travel and strike the anode. The higher the voltage (kV), the higher the speed and, therefore, energy the electrons have when they strike the anode. Higher the energy of the electrons striking the anode, better the X-rays penetration. The high-voltage potential is measured in kilovolts (kV) which is controlled using the kV control on the console.
A focusing cup is used to concentrate the stream of electrons to a small area on the anode, called the focal spot. The focal spot size is an important factor in the system’s ability to produce a sharp image. Most of the energy (nearly 98%) applied to the tube is transformed into heat at the focal spot of the anode. Therefore, it is necessary to cool the anode. Tubes are cooled using a water or oil recirculating system.Vacuum is maintained inside the tube, in order to prevent the cathode from burning up and to prevent arcing between the anode and the cathode.
The other important component of an X-ray generating system is the control console. There are three main adjustable controls that regulate the 1) tube voltage in kilovolts, 2) the tube amperage in milliamps, and the 3) exposure time in minutes and seconds. Some systems also have a switch to change the focal spot size of the tube.
HF X-Ray machines today have digital console with settings for procedure being done (e.g AP Lumbar) and the patient thickness, rather than the Voltage, Current and Exposure time, making it easier for the technician.