X-ray Circuit and Tube Heat Management



X-ray Circuit and Tube Heat Management




This chapter centers on a greatly simplified diagram of an x-ray circuit and is intended to aid your understanding of the various components of the circuit and how they work together to produce and control x-rays. The various features of the x-ray circuit and the x-ray control panel are discussed. It is not necessary to memorize or understand the circuits in detail. The circuits help you to understand how three relatively complex electric circuits are integrated to produce x-rays. Because x-ray tubes may be damaged by improper use and are expensive to replace, this chapter provides guidelines for the safe operation of tubes and suggestions for prolonging tube life.



X-Ray Circuit


As indicated in Fig. 6-1, the x-ray circuit is divided into three sections or subcircuits: the low-voltage circuit, the filament circuit, and the high-voltage circuit. Each circuit contains a specialty transformer. The various components of each section are numbered so that you can easily refer to them in the discussion that follows.




Low-Voltage Circuit


The low-voltage circuit is illustrated in the upper left portion of Fig. 6-1 and is expanded in Fig. 6-2, A. It is the subcircuit between the alternating current (AC) power supply (1) and the primary (input) side of the high-voltage (step-up) transformer (7). If you trace this circuit beginning at the AC power supply, you will note that current flows through several devices before reaching the primary side of the step-up transformer. From the transformer, it returns to the power source, forming an enclosed loop. With the exception of the step-up transformer, all of the devices in this subcircuit are actually located within the control console. The control console is the unit where the operator sets all of the exposure techniques, such as kilovolts peak (kVp), milliamperes (mA), and exposure time. They include the main switch (2), autotransformer (3), kVp selectors (4), exposure switch (5), and exposure timer (6).



The AC power supply (1) is wired into the building, providing electric power from the local power company. Most outpatient facilities have a 220-V power supply going into the x-ray room. Hospitals with more powerful equipment may have a larger supply. The main switch (2) controls the power to the control console. Many of the components in this circuit operate at the standard 120 volts.


Although the power supply may be rated at 220 V, the actual voltage can vary as much as ±5%, depending on the demand for power in the building or the neighborhood. Small variations in the incoming line voltage may cause large variations in the kVp to the x-ray tube. For this reason, the incoming voltage is monitored and stabilized by a voltage compensator.


The autotransformer (3) is a single-coil transformer that serves three functions: it provides the means for kVp selection, it provides compensation for fluctuations in the incoming line voltage, and it supplies power to other parts of the x-ray circuit.


The autotransformer’s primary purpose is to vary the voltage to the primary side of the step-up transformer. This is accomplished by the kVp selector (4), which is on the secondary (output) side of the autotransformer. The autotransformer varies the kVp to the tube by controlling the input to the step-up transformer.


The exposure switch (5) closes the circuit, allowing electric current to flow through the primary side of the step-up transformer. When this occurs, current is induced to flow through the secondary side of the transformer, creating voltage across the x-ray tube. As discussed earlier, this voltage causes the electron stream to flow across the tube, producing x-rays. The exposure timer (6) is a device that terminates the exposure and is set by the operator on the control console.



Filament Circuit


The filament circuit is the subcircuit of the main x-ray circuit shown as the lower portion of Fig. 6-1. It is expanded in Fig. 6-2, B. This circuit is divided into two parts by the step-down transformer (11 and 12). The primary purpose of the filament circuit is to supply a low current to heat the x-ray tube filament for thermionic emission of electrons. The filament circuit is activated any time the operator adjusts the mA on the generator.


The primary side of this circuit begins and ends with the contacts on the autotransformer (9). Current in this circuit flows from the autotransformer, through the mA selector (10) and the primary side of the step-down transformer (11), and back to the autotransformer. The secondary side begins and ends with the secondary side of the step-down transformer (12) conducting current through the x-ray tube filament (13). The step-down transformer reduces the voltage on the secondary side, providing an appropriate current to heat the filament.


The mA selector (10) controls amperage in the filament circuit. Since the current through this circuit controls filament heat, this setting determines the number of available electrons at the x-ray tube filament and thus determines the mA in the high-voltage circuit that includes the x-ray tube.



High-Voltage Circuit


The high-voltage circuit is the subcircuit shown in the upper right portion of Fig. 6-1. It is expanded in Fig. 6-3. This circuit begins and ends with the secondary side of the step-up transformer (8). It includes the x-ray tube (14) and the rectifier unit (15). Current flows in this circuit only during an exposure. This is a dangerous circuit due to the very high voltage. The high-voltage cables going to the x-ray tube are very thick due to their high insulation requirement (Fig. 6-4).




The step-up transformer is also referred to as the high-voltage or high-tension transformer. As explained in Chapter 4, it increases the incoming voltage by the value of the transformer ratio. This transformer has a very high ratio of at least 500:1. For example, if the primary side of the step-up transformer receives 180 V from the autotransformer, and the ratio is 500 : 1, the voltage induced on the secondary side will be 90,000 V, or 90 kVp.


The primary purpose of the high-voltage circuit is to supply the x-ray tube with voltage high enough to create x-rays.


The autotransformer, step-down transformer, and high-voltage transformer are all located in a tank near the x-ray machine (Fig. 6-5). Oil surrounds the transformers inside the tank for heat dissipation.





Rectification


The primary purpose of the rectifier unit (15) is to change the alternating current (AC) into direct current (DC). The process of rectification prepares the current for x-ray production by ensuring that it flows in the right direction, in this case from the filament to target. There are three ways in which current is rectified: self-rectification, half-wave rectification, and full-wave rectification. Self-rectification was an inefficient form of rectification and is no longer used. Half-wave rectification and full-wave rectification are described next.



Half-Wave Rectification


AC electrical current travels in the copper wire as a sine wave. It moves in a pulsating manner from positive to negative at a rate of 60 pulses, or waves, per second and is stated as 60 Hz (Fig. 6-6). Rectifiers use diodes to convert the circuit from AC to DC. A diode is an electronic device that permits current to flow in one direction only. Two diodes are used in half-wave rectification (Fig. 6-7). Diodes prevent “backflow” of current during the negative half of the electric cycle. This causes the negative half of the cycle to be eliminated. Note that the arrow direction of the diode symbol indicates the direction of current flow permitted by the diode.




In half-wave rectification the negative phase of the electric cycle is totally eliminated and a gap remains. The x-rays are turned off during the eliminated (negative) phase. With only the positive phase remaining, the electric current is “direct” only, or DC. The x-rays are pulsating on, off, on, off, and so forth at a rate of 60 pulses per second.



Full-Wave Rectification


By employing four diodes in the circuit, the current can be “redirected” during the negative half of the electric cycle so that current will flow in the same direction during both the positive and negative halves of the cycle. This process is called full-wave rectification because it utilizes the entire electric cycle for x-ray production. The negative impulses are made positive during full-wave rectification rather than being eliminated (Fig. 6-8). The pulsed x-ray output of a full-wave rectified machine occurs 120 times each second compared to 60 times a second for half-wave rectification. This results in a doubling of the x-ray output. The waveform of full-wave rectified current is shown in Fig. 6-9. Note there are twice as many impulses in the cycle compared with Fig. 6-7. All modern general-purpose x-ray machines are full-wave rectified. The main advantage of full-wave rectification is that the exposure time can be cut in half due to the doubling in x-ray output compared to half-wave rectification.




Mar 7, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on X-ray Circuit and Tube Heat Management
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