Special X-Ray Tube Designs
5.4 X-RAY GENERATOR FUNCTION AND COMPONENTS Electromagnetic Induction and Voltage Transformation
The principal function of the x-ray generator is to provide current at a high voltage to the x-ray tube. Electrical power available to a hospital or clinic provides up to about 480 V, much lower than the 20,000 to 150,000 V needed for x-ray produc-tion. Transformers are principal components of x-ray generators; they convert low voltage into high voltage through a process called electromagnetic induction.
Electromagnetic induction is an effect that occurs with changing magnetic fields and alternating (AC) electrical current. For example, the changing magnetic field from a moving bar magnet induces an electrical potential difference (voltage) and a current in a nearby conducting wire (Fig. 5-19A). As the magnet moves in the oppo-site direction, away from the wire, the induced current flows in the oppooppo-site direction.
The magnitude of the induced voltage is proportional to the magnetic field strength.
Electromagnetic induction is reciprocal between electric and magnetic fields.
An electrical current (e.g., electrons flowing through a wire) produces a magnetic field, whose magnitude (strength) and polarity (direction) are proportional to the magnitude and direction of the current (see Fig. 5-19B). With an alternating cur-rent, such as the standard 60 cycles per second (Hz) AC in North America and 50 Hz AC in most other areas of the world, the induced magnetic field increases and decreases with the current. For "coiled wire" geometry, superimposition of the mag-netic fields from adjacent "turns" of the wire increases the amplitude of the overall magnetic field (the magnetic fields penetrate the wire insulation), and therefore the magnetic field strength is proportional to the number of wire turns. Note that for constant-potential direct current (DC), like that produced by a battery, magnetic induction does not occur.
Transformers perform the task of "transforming" an alternating input voltage into an alternating output voltage, using the principles of electromagnetic induction.
The generic transformer has two distinct, electrically insulated wires wrapped about a common iron core (Fig. 5-20). Input AC power (voltage and current) produces oscillating electrical and magnetic fields. One insulated wire wrapping (the
"pri-Induced electron
flow in conductor Reversed electron
Reverse motion flow
B Current (electron flow) in a conductor creates a magnetic field;
amplitude and direction determines magnetic field strength and polarity
Current:
Direction:
small forward
small reverse
large reverse large
forward
FIGURE 5-19. Principles of electromagnetic induction. A: Induction of an electrical current in a wire conductor coil by a moving magnetic field. The direction of the current is dependent on the direction of the magnetic field motion. B:Creation of a magnetic field by the current in a conducting coil. The polarity and magnetic field strength are directly dependent on the ampli-tude and direction of the current.
primary winding
I( )
AC in
I( )
secondary winding
FIGURE 5-20. The basic trans-former consists of an iron core, a primary winding circuit, and a secondary winding circuit.
An alternating current flowing through the primary winding produces a changing magnetic field. The magnetic field per-meates the core and produces an alternating voltage on the secondary winding. This exchange is mediated by the permeability of the magnetic field through wire insulation and in the iron core.
Input Voltage
mary winding") carries the input load (primary voltage and current). The other insulated wire wrapping ("secondary winding") carries the induced (output) load (secondary voltage and current). The primary and secondary windings are electri-cally (but not magnetielectri-cally) isolated by insulated wires.The induced magnetic field strength changes amplitude and direction with the primary AC voltage waveform and freely passes through electrical insulation to permeate the transformer iron core, which serves as a conduit and containment for the oscillating magnetic field. The secondary windings are bathed in the oscillating magnetic field, and an alternating voltage is induced in the secondary windings as a result. The Law of Transformers states that the ratio of the number of coil turns in the primary winding to the num-ber of coil turns in the secondary winding is equal to the ratio of the primary volt-age to the secondary voltvolt-age:
Vp Np VS=Ns
where Np is the number of turns of the primary coil, Ns is the number of turns of the secondary coil, Vp is the amplitude of the alternating input voltage on the pri-mary side of the transformer, and VS is the amplitude of the alternating output volt-age on the secondary side.
A transformer can increase, decrease, or isolate voltage, depending on the ratio of the numbers of turns in the two coils. For Ns > Np, a "step-up" transformer increases the secondary voltage; for Ns <Np, a "step-down" transformer decreases the secondary voltage; and for Ns =Np, an "isolation" transformer produces a sec-ondary voltage equal to the primary voltage (see later discussion). Examples of transformers are illustrated in Fig. 5-21. A key point to remember is that an alter-nating current is needed for a transformer to function.
Output Voltage
~
FIGURE 5-21. Transformers in-crease (step up), decrease (step down), or leave unchanged (iso-late) the input voltage depending on the ratio of primary to sec-ondary turns, according to the Law of Transformers. In all cases, the input and the output circuits are electrically isolated.
Power is the rate of energy production or expenditure per unit time. The 51 unit of power is the watt (W), which is defined as 1 joule
m
of energy per second. For electrical devices, power is equal to the product of voltage and current:Because a volt is defined as 1 joule per coulomb and an ampere is 1 coulomb per second,
1 watt = 1 volt X 1 ampere
Because the power output is equal to the power input (for an ideal transformer), the product of voltage and current in the primary circuit is equal to that in the sec-ondary circuit:
where Iris the input current on the primary side and Is is the output current on the secondary side.
Therefore, a decrease in current must accompany an increase in voltage, and vice versa. Equations 5-3 and 5-5 describe ideal transformer performance. Power losses due to inefficient coupling cause both the voltage and current on the sec-ondary side of the transformer to be less than predicted by these equations.
The high-voltage section of an x-ray generator contains a step-up trans-former, typically with a primary-to-secondary turns ratio of 1:500 to 1: 1,000.
Within this range, a tube voltage of 100 kVp requires an input peak voltage of 200 V to 100 V, respectively. The center of the secondary winding is usually con-nected to ground potential ("center tapped to ground"). Ground potential is the electrical potential of the earth. Center tapping to ground does not affect the maximum potential difference applied between the anode and cathode of the x-ray tube, but it limits the maximum voltage at any point in the circuit relative to ground to one half of the peak voltage applied to the tube. Therefore, the maximum voltage at any point in the circuit for a center-tapped transformer of 150 kVp is -75 kVp or +75 kVp, relative to ground. This reduces electrical insu-lation requirements and improves electrical safety. In some x-ray tube designs (e.g., mammography), the anode is maintained at the same potential as the body of the insert, which is maintained at ground potential. Even though this places the cathode at peak negative voltage with respect to ground, the low kVp (less than 50 kVp) used in mammography does not present a big electrical insulation problem.
A simple autotransformer consists of a single coil of wire wrapped around an iron core. It has a fixed number of turns, two lines on the input side and two lines on the output side (Fig. 5-22A). When an alternating voltage is applied to the pair of input lines, an alternating voltage is produced across the pair of output lines. The Law of Transformers (Equation 5-3) applies to the autotransformer, just as it does to the standard transformer. The output voltage from the autotransformer is equal to the input voltage multiplied by the ratio of secondary to primary turns. The pri-mary and secondary turns are the number of coil turns between the taps of the input and output lines, respectively. The autotransformer operates on the principle of self-induction, whereas the standard transformer operates on the principle of mutual induction. Self and mutual induction are described in detail in Appendix A.
The standard transformer permits much larger increases or decreases in voltage, and it electrically isolates the primary from the secondary circuit, unlike the autotrans-former. A switching autotransformer has a number of taps on the input and output sides (see Fig. 5-22B), to permit small incremental increases or decreases in the out-put voltage.
The switched autotransformer is used to adjust the kVp produced by an x-ray generator. Standard alternating cutrent is provided to the input side of the auto-transformer, and the output voltage of the autotransformer is provided to the pri-mary side of the high-voltage transformer. Although variable resistor circuits can be
120 volts
FIGURE 5-22. The autotransformer (A) is an iron core wrapped with a single wire. Conducting
"taps" allow the ratio of input to output turns to vary, resulting in a small incremental change (increase or decrease) between input and output voltages. A switching autotransformer (B)allows a greater range of input to output voltages. An autotransformer does not isolate the input and out-put circuits from each other.
used to modulate voltage, autotransformers are more efficient in terms of power consumption and therefore are preferred.
Diodes (having two terminals), triodes (three terminals), tetrodes (four terminals), and pentodes (five terminals) are devices that permit control of the electrical cur-rent in a circuit. The term "diode" can refer to either a solid-state or a vacuum tube device, whereas triodes, tetrodes, and pentodes usually are vacuum tube devices.
These vacuum tube devices are also called "electron tubes" or "valves." A vacuum tube diode contains two electrodes, an electron source (cathode) and a target elec-trode (anode), within an evacuated envelope. Diodes permit electron flow from the cathode to the anode, but not in the opposite direction. A triode contains a cath-ode, an ancath-ode, and a third control electrode to adjust or switch (turn on or off) the current. Tetrodes and pentodes contain additional control electrodes and function similarly to triodes.
A pertinent example of a diode is the x-ray tube itself, in which electrons flow from the heated cathode, which releases electrons by thermionic emission, to the positive anode. If the polarity of the voltage applied between the anode and cath-ode reverses (for example, during the alternate AC half-cycle), current flow in the circuit stops, because the anode does not release electrons to flow across the vac-uum. Therefore, electrons flow only when the voltage applied between the electron source and the target electrodes has the correct polarity.
A solid-state diode contains a crystal of a semiconductor material-a material, such as silicon or germanium, whose electrical conductivity is less than that of a metallic conductor but greater than that of an insulator. The crystal in the diode is
"doped" with trace amounts of impurity elements. This serves to increase its con-ductivity when a voltage is applied in one direction but to reduce its concon-ductivity to a very low level when the voltage is applied in the opposite polarity (similar to a vacuum tube diode, described earlier). The operating principles of these solid-state devices are described in Chapter 20.
The symbols for the vacuum tube and solid-state diodes are shown in Fig. 5-23.
In each case, electrons flow through the diode from the electron source (cathode) to
-0-
Vacuum tube diode(e.g., x-ray tube)
FIGURE 5-23. Diodes are electrical devices with two terminals that allow current flow in only one direction. Note that the direc-tion of electron flow is opposite that of cur-rent flow in an electrical circuit.
the electron target (anode). The triangle in the solid-state diode symbol can be thought of as an arrow pointing in the direction oppositethat of the electron flow.
Although it is perhaps counterintuitive, electrical current is defined as the flow of pos-itive charge (opposite the direction of electron flow). Therefore, the diode symbol points in the direction of allowed current flow through the diode.
The triode is a vacuum tube diode with the addition of a third electrode placed in close proximity to the cathode (Fig. 5-24). This additional electrode, called a grid, is situated between the cathode and the anode in such a way that all electrons en route from the cathode to the anode must pass through the grid structure. In the conduction cycle, a small negative voltage applied to the grid exerts a large force on the electrons emitted from the cathode, enabling onloff switching or current con-trol. Even when a high power load is applied on the secondary side of the x-ray transformer, the current can be switched off simply by applying a relatively small negative voltage (approximately -1,000 V) to the grid. A "grid-switched" x-ray tube is a notable example of the triode: the cathode cup (acting as the grid) is electrically isolated from the filament structure and is used to turn the x-ray tube current on and off with microsecond accuracy. Gas-filled triodes with thermionic cathodes are known asthyratrons. Solid-state triodes, known asthyristors,operate with the use of a "gate" electrode that can control the flow of electrons through the device. Tetrodes and pentodes also control x-ray generator circuits; they have multiple grid elec-trodes to provide further control of the current and voltage characteristics.
Other Components
Other components of the x-ray generator (Fig. 5-25) include the high-voltage power circuit, the stator circuit, the filament circuit, the focal spot selector, and automatic exposure control circuits. In modern systems, microprocessor control arid closed-loop feedback circuits help to ensure proper exposures.
Most modern generators used for radiography have automatic exposure control (AEC) circuits, whereby the technologist selects the kVp and mA, and the AEC sys-tem determines the correct exposure time. The AEC (also referred to as a
photo-Filament Heater
Gate
I
Electrode
Solid-state triode (thyristor)
FIGURE 5-24. A diode contain-ing a third electrode is called a triode. With a "grid" electrode, the flow of electrons between the anode and cathode can be turned on or off. The thyristor is the solid-state replacement for the tube triode.
mAand mAs control
Phototimer circuits
FIGURE 5-25. A modular schematic view shows the basic x-ray generator components. Most systems are now microprocessor controlled and include service support diagnostics.
timer) measures the exposure with the use of radiation detectors located near the image receptor, which provide feedback to the generator to stop the exposure when the proper exposure to the image receptor has been reached. AECs are discussed in more detail later in this chapter.
Many generators have circuitry that is designed to protect the x-ray tubes from potentially damaging overload conditions. Combinations of kVp, mA, and expo-sure time delivering excessive power to the anode are identified by this circuitry, and such exposures are prohibited. Heat load monitors calculate the thermal loading on the x-ray tube anode, based on kVp, mA, and exposure time, and taking into account intervals for cooling. Some x-ray systems are equipped with sensors that measure the temperature of the anode. These systems protect the x-ray tube and housing from excessive heat buildup by prohibiting exposures that would damage them. This is particularly important for CT scanners and high-powered interven-tional angiography systems.
Operator Console
At the operator console, the operator selects the kVp, the mA (proportional to the number of x-rays in the beam at a given kVp), the exposure time, and the focal spot size. The peak kilovoltage (kVp) determines the x-ray beam quality (penetrability), which plays a role in the subject contrast. The x-ray tube current (mA) determines the x-ray flux (photons per square centimeter) emitted by the x-ray tube at a given kVp. The product of tube current (mA) and exposure time (seconds) is expressed as milliampere-seconds (mAs). Some generators used in radiography allow the selec-tion of "three-knob" technique (individual selection of kVp, mA, and exposure time), whereas others only allow "two-knob" technique (individual selection ofkVp and mAs). The selection offocal spot size (i.e., large or small) is usually determined by the mA setting: low mA selections allow the small focus to be used, and higher mA settings require the use of the large focus due to anode heating concerns. On
some x-ray generators, preprogrammed techniques can be selected for various exam-inations (i.e., chest; kidneys, ureter, and bladder; cervical spine). All console circuits have relatively low voltage and current levels that minimize electrical hazards.
Several x-ray generator circuit designs are in common use, including single-phase, three-phase, constant potential, and medium/high-frequency inverter generators.
All use step-up transformers to generate high voltage, step-down transformers to energize the filament, and rectifier circuits to ensure proper electrical polarity at the x-ray tube.
A rectifier is an electrical apparatus that changes alternating current into direct cur-rent. It is composed of one or more diodes. In the x-ray generator, rectifier circuits divert the flow of electrons in the high-voltage circuit so that a direct current is established from the cathode to the anode in the x-ray tube, despite the alternating current and voltage produced by the transformer. Conversion to direct current is important. If an alternating voltage were applied directly to the x-ray tube, electron back-propagation could occur during the portion of the cycle when the cathode is positive with respect to the anode. If the anode is very hot, electrons can be released by thermionic emission, and such electron bombardment could rapidly destroy the filament of the x-ray tube.
To avoid back-propagation, the placement of a diode of correct orientation in the high-voltage circuit allows electron flow during only .one half of the AC cycle (when the anode polarity is positive and cathode polarity is negative) and halts the current when the polarity is reversed. As a result, a "single-pulse" waveform is produced from the full AC cycle (Fig. 5-26A), and this is called a half-wave rectified system.
Full-wave rectified systems use several diodes (a minimum of four in a bridge rectifier) arranged in a specific orientation to allow the flow of electrons from the cathode to the anode of the x-ray tube throughout the AC cycle (see Fig. 5-26B).
During the first half-cycle, electrons are routed by two conducting diodes through the bridge rectifier in the high-voltage circuit and from the cathode to the anode in the x-ray tube. During the second half-cycle, the voltage polarity of the circuit is reversed; electrons flow in the opposite direction and are routed by the other two diodes in the bridge rectifier, again from the cathode to the anode in the x-ray tube.
The polarity across the x-ray tube is thus maintained with the cathode negative and anode positive throughout the cycle. X-rays are produced in two pulses per cycle,
FIGURE 5-26. (a): A single-diode rectifier allows electron flow through the tube during one half of
•
the alternating current (AC) cycle but does not allow flow during the other half of the cycle; it there-fore produces x-rays with one pulse per AC cycle. (b): The bridge rectifier consists of diodes that reroute the electron flow in the x-ray circuit as the electrical polarity changes. The electrons flow from negative to positive polarity through two of the four diodes in the bridge rectifier circuit for the first half-cycle, and through the alternate diodes during the second half-cycle. This ensures electron flow
the alternating current (AC) cycle but does not allow flow during the other half of the cycle; it there-fore produces x-rays with one pulse per AC cycle. (b): The bridge rectifier consists of diodes that reroute the electron flow in the x-ray circuit as the electrical polarity changes. The electrons flow from negative to positive polarity through two of the four diodes in the bridge rectifier circuit for the first half-cycle, and through the alternate diodes during the second half-cycle. This ensures electron flow