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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.
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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.
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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:
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An autotransformer. This
allows fluctuations in the mains input voltage to be corrected before the
current is fed to the high tension transformer .
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A step-down or filament
transformer. This permits the supply of a suitably reduced current to the
cathode filament.
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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
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This term is used to denote the apparatus to
support the X-ray tube during the performance of radiography.
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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
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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.
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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.
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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
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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 .
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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
Milliammeter and
milliaimperage control:
The
effect of Milliamperage
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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.
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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.
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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
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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.
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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.
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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:
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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.
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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:
Safety devices
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Certain safety devices are
incorporated in an X-ray equipment to overcome electrical hazards and are
discussed below briefly:
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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.
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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
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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.
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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.
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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
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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).
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The purpose of fluoroscopy is
the immediate visualization of radiographic image by allowing the X-ray beam to
impinge on a suitable fluorescent screen.
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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
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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.
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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.
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