The X-ray Machine
-✔ The general purpose x-ray examination room contains a radiographic
imaging system and a fluoroscopic imaging system.
- ✔The fluoroscopic x-ray tube is usually located under the examining table; the
radiographic x-ray tube is attached to an overhead movable crane assembly
that permits easy positioning of the tube and aiming of the x-ray beam.
-✔ Regardless of the type of x-ray imaging system used, a patient-supporting
examination couch is required.
- This examination couch may be flat or curved but must be uniform in
thickness and as transparent to x-rays as possible.
- Just under the couch is an opening to hold a thin tray for a cassette and grid.
✔Regardless of its design, every x-ray imaging system has three principal
parts: the x-ray tube, the operating console, and the high-voltage
generator.
- In some types of x-ray imaging systems, such as dental and portable
machines, these three components are housed compactly.
- With most systems, the x-ray tube is located in the examination room, and
the operating console is located in an adjoining room with a protective
barrier separating the two.
- The protective barrier must have a window for viewing the patient during
the examination. Ideally, the room should be designed so that it is possible
to reach the operating console without having to enter the “radiation area” of
the examination room.
1. OPERATING CONSOLE
- The operating console allows radiologic technologists to control the x-ray
tube current and voltage so that the useful x-ray beam is of proper quantity
and quality.
- Radiation quantity refers to the number of x-rays or the intensity of the x-
ray beam. Radiation quantity is usually expressed in milligray (mGya) or
milligray/milliampere- second (mGya/mAs).
- Radiation quality refers to the penetrability of the x-ray beam and is
expressed in kilovolt peak (kVp).
- Most x-ray imaging systems are designed to operate on 220 V power, although some can operate on 110 V or 440 V.
1.1 AUTOTRANSFORMER
- The power supplied to the x-ray imaging system is delivered first to the
autotransformer. The voltage supplied from the autotransformer to the high-
voltage transformer is controlled but variable.
- The autotransformer works on the principle of electromagnetic induction. It
has only one winding and one core with primary and secondary coils.
- Autotransformer Law:
where
Vp = the primary voltage, VS = the secondary voltage
NP = the number of windings enclosed by primary connections
NS = the number of windings enclosed by secondary connections.
Ip = the primary current.
IS = the secondary current.
Question1: An autotransformer connected to a 440-V supply contains 4000 turns, all
of which are enclosed by the primary connections. If 2300 turns are enclosed by
secondary connections, what voltage is supplied to the high-voltage generator?
Answer: 253V
Question2: A filament transformer with a turns ratio of 1/10 provides 6.2 A to the
filament. What is the current through the primary coil of the filament transformer?
Answer: Ip = 0.62A
1.2. HIGH-VOLTAGE GENERATOR
- The high-voltage generator of an x-ray imaging system is responsible for
increasing the output voltage from the autotransformer to the kVp necessary
for x-ray production.
- The high-voltage generator contains three primary parts: the high-voltage
transformer, the filament transformer, and rectifiers.
- The high voltage transformer is a step-up transformer, that is, the secondary
voltage is higher than the primary voltage because the number of secondary
windings is greater than the number of primary windings.
- The ratio of the number of secondary windings to the number of primary
windings is called the turns ratio.
- The turns ratio of a high-voltage transformer is usually between 500 : 1 and
1000 : 1. Because transformers operate only on alternating current, the
voltage waveform on both sides of a high-voltage transformer is sinusoidal.
- The primary voltage is measured in volts (V), and the secondary voltage is
measured in kilovolts (kV). The primary current is measured in amperes (A),
and the secondary current is measured in milliamperes (mA).
Question3: The turns ratio of a high-voltage transformer is 700 : 1, and the
supply voltage is peaked at 120 V. What is the secondary voltage supplied to
the x-ray tube?
Answer: 84 kVp
The current from a common wall plug is 60 Hz alternating current (AC). The
current changes direction 120 times each second. However, an x-ray tube
requires a direct current (DC).
- Rectification is the process of converting AC to DC.
- The electronic device that allows current flow in only one direction is a
rectifier. Although transformers operate with alternating current, x-ray tubes
must be provided with direct current.
- Rectification is accomplished with diodes.
- A diode is an electronic device that
contains two
electrodes of semiconductors (n-type and p-
type).
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