X-Ray Machine Circuits

Any X-ray apparatus, even of the simplest type, is an intricate and complicated piece of machinery consisting of four main parts: The X-ray tube, the transformer, the tube stand, and the control panel.

1. The X-ray tube.

2. The transformer.

  • A transformer is an electromagnetic device used for increasing or decreasing the voltage (potential difference) of incoming electrical energy to an appropriate level without appreciable loss of energy. It also transfers electrical energy from one circuit to another by mutual electromagnetic induction.

  • Transformers are an essential part of an x-ray machine in order that the current supplied through the mains may be converted into a suitable form for the operation of an X-ray tube . Normally they include:

  1. An autotransformer. This allows fluctuations in the mains input voltage to be corrected before  the current is fed to the high tension transformer .

  2. A step-down or filament transformer. This permits the supply of a suitably reduced current to the cathode filament.

  3. A high tension transformer.  The purpose of this produce current of a sufficiently high voltage ( from 40 k V upwards ) for the production of X- rays. Where the transformer is not located immediately adjacent to the tube the two parts have to be linked by suitably insulated high-tension cables. For reasons of insulation the high-tension transformers are usually housed in an oil bath.

3. The tube stand

  • This term is used to denote the apparatus to support the X-ray tube during the performance of radiography.

  • It can include many different forms of suspension, and vary from small table top stands or larger mobile floor stands, to overhead ceiling mountings.

4. The control panel.

Control panel

  • It is a separate unit connected electrically to the X-ray machine (Fig 3-20). It contains meters and switches to select kVp, mA and exposure time. The control panel vary with the type of X-ray machine but most often following components, or some of them exit.

  • On-off switch: It is a main switch to turn the unit 'on'. The switch permits flow of current to the tube at 'on' position and prevents the same at 'off ' position. For the safely of the X-ray tube and also to avoid an accidental exposure, the switch should remain in 'off ' position when machine is not being used.

  • Voltmeter and voltage compensator control: Most X-ray machines are designed to operate on a 220 voltage power source. A voltmeter measures the voltage of electric current and voltage compensator allows adjustment of voltage. In most machines these days such a system is automatic.

  • Kilovoltage selector: It allows precise selection of desired kV. In some machines this control is automatically linked to a certain milliamperage (mA) value. In such a case, a high kVp is available at a relatively low mA and vice versa.

The effect of kilovoltage

  • As already stated .it is the passage of a high voltage current across an x-ray tube which results in a production of X-rays . The higher the kilovoltage employed for this purpose , the more rapidly the electrons travel , the greater the amount of energy released on impact , and the shorter the wave –length of the X-rays produced .

  • x-rays of short wave – length are sometimes described as " hard X-rays " while those of longer wave –length may be spoken of as " soft " .

The kilovoltage Selector

  • In the modern type of X-ray apparatus and in those operating on a pre determined milliamperge the kilovoltage control will be directly calibrated (usually in ascending 5 k V values) so that the described value can be easily selected.

Milliammeter and milliaimperage control:

  • A milliammeter is placed within the high tension circuit. It indicates the current passing through the tube during an actual X-ray exposure.

The effect of Milliamperage

  • The amount of current which travels across an X-ray tube during an exposure depends on the number of electrons available to carry that current, which in turn depends on the number of electrons available to carry that current, which in turn varies with the current supplies to the filament in the cathode.

  • The tube turn varies with the current supplied to the filament in the cathode. The tube current (measured in milliamperes) is directly related to the amount of X-rays produced.

  • The above effect are often summarized by stating that kilovoltage affects the quality and the miiliamperge (more correctly the mAs) the quantity of the X-ray produced by a particular apparatus.

The Milliameter and the Milliamperge Selector

  • It will be found that the small and medium sized X-ray apparants cannot be set to operate at maximum milliamperge and maximum kilovoltage simultaneously.

  • The significance of the milliamperge is that it affects the amount of X-rays produced and a level has to be selected which is sufficient to produce an easily recognizable image on the X-ray film without obliterating that image by over exposure.

  • However the amount of radiation is also controlled by the length of the exposure and is best expressed in milliampere-seconds (i.e., the milliamperge multiplied by the time in seconds).

Timer and exposure button:

  • In any given radiographic examination, the quantity of X-rays reaching the film is directly related lo the X-ray tube current and the time for which the tube is energized i.e. the exposure time.

  • The range of exposure time in available machines is large with minimum setting being as short as 0,001 second. An exposure device mostly consists of a two-stage exposure button of which first half depression rotates the anode and a complete depression, after a short pause, causes actual radiographic exposure.

Fluoroscopy control:

  • Most diagnostic X-ray machines are supplied with a special radiographic-fluoroscopic change-over switch so that machine can be used either for conventional radiography or for fluoroscopy.

Safety devices

  • Certain safety devices are incorporated in an X-ray equipment to overcome electrical hazards and are discussed below briefly:

  • Switches: Three switches mainly operated by the technician are: the main switch, on-off switch and exposure switch. By operating the main switch, one can isolate whole X-ray machine from the electrical supply. The on-off switch, provided in the control panel, energizes the autotransformer and many auxiliary circuits such as of meters, filament etc. The exposure switch needs constant pressure in order to help it on 'ON' position.

  • Fuses and circuit breakers: A fuse is a thin wire encased in a glass tube with sealed metal ends which are in contact with the electrical circuit. The thin wire of the fuse melts if a current higher than that required for a part flows through. Fuse is included either in the casing of main switch or in X-ray circuit immediately before the autotransformer.

The Timer and Exposure Button

  • The exposure button should be attached to the control panel by a length of cable so that the person making the exposure may position himself at a safe distance from the primary beam.

  • It is necessary that the cathode filament should be activated and heated to produce electrons for a brief period before the exposure is actually made.

  • For this reason, the exposure device in many modern units consists of a two-stage button-depression of the first half activates the filament and, after a short pause, complete depression closes the circuit and makes the radiographic exposure.

Fluoroscopy Control

  • Most X-ray machines are fitted with a switch (often combined with the on/off switch) which permits the apparatus to be either for conventional radiography or for fluoroscopy. when switched to fluoroscopy the timer is by-passed and the machine is operated for prolonged periods by depressing the exposure button (or by substituting a foot switch).

  • The purpose of fluoroscopy is the immediate visualization of radiographic image by allowing the X-ray beam to impinge on a suitable fluorescent screen.

  • A number of auxiliary devices are necessary for the proper operation of an X-ray machine. These can be described under two main heads: sources of electricity, and main X-ray circuits. 

Sources of electricity

  • Most X-ray units work from a power source of 220 volts, 60 hertz though some units can operate on 110 volts or 440 volts.

  • An X-ray tube requires electrical energy for two purposes: to elicit electrons from the filament and to accelerate these electrons from the cathode to the anode.