The X-ray Circuit

The X-ray Circuit


This chapter provides a concise overview of the nature of electricity, electrical devices, and the basics of x-ray circuitry and principles of operation. It is true that many types of x-ray equipment are automated (Figure 4-1). However, a radiographer is not someone who merely “pushes buttons.” Rather, he or she has an understanding of the principles of x-ray production and has mastered the art of producing quality images with minimal radiation exposure to the patient. To reach this level of mastery, the radiographer must understand the basic elements of the x-ray machine and the steps in the process. Consider a pilot who flies a modern jet. A pilot untrained for that aircraft may be able to get it off the ground and flying, but without some understanding of the jet’s instrumentation he or she is not likely to stay in the air very long. The safety of the passengers aboard that aircraft rests with the training and knowledge of that pilot. Similarly, the radiographer is responsible for the safety of the patient; the radiation dose that patient receives depends on the radiographer’s understanding and safe operation of the x-ray machine. The concepts presented here are important to the radiographer in that they ground his or her practice in a fundamental understanding of what is happening each time he or she operates the x-ray machine. By understanding what happens within the x-ray machine with each selection made at the operating console, the radiographer is able to use the machine with maximum efficiency and minimal radiation exposure to the patient. The knowledgeable radiographer is also able to make adjustments in exposure technique with variations in machines and daily operation.

Nature of Electricity

The nature of electricity may be understood through a discussion of electrostatics and electrodynamics. Electrostatics is the study of stationary electric charges and electrodynamics is the study of electric charges in motion. The latter is most often considered as “electricity.” A few fundamental concepts must first be discussed.

Electric charge is a property of matter. The smallest units of charge exist with the electron and the proton. Electrons have one unit of negative charge and protons have one unit of positive charge. Electrical charges are measured in the systeme internationale (SI) unit “coulomb.” One coulomb is equal to the electrical charge of 6.25 × 1018 electrons. A measure of electrons is used because electricity most often results from their movement. Except in decaying radioactive elements, protons are generally fixed in their position inside the nucleus of the atom. Electrons, on the other hand, are relatively free to move about, depending on the material. Some materials, such as copper and gold, have a very large number of free electrons, making them good conductors of electricity. Glass and plastic, on the other hand, have very few free electrons, making them good insulators. This is discussed in greater detail later in this chapter.

Although an understanding of the laws of electrostatics is not the primary focus of this chapter, it is helpful in understanding the nature of electricity. There are five general principles of electrostatics. They are as follows:

In electrostatics, electrification of objects occurs when they gain either a net positive or a net negative charge. An object may be electrified in three ways: by friction, by contact, or by induction. The classic physics experiment involving rubbing a rubber rod with fur is an example of electrification by friction. Once charged, the rod can be discharged by placing it in contact with a conductor. This is an example of electrification by contact. Electrification by induction is the process by which an uncharged metallic object experiences a shift of electrons when brought into the electric field of a charged object. Induction occurs as a result of the interaction of the electric fields around two objects that are not in contact with each other. This is very useful in the design of the x-ray tube, as is discussed in Chapter 5.

Electrodynamics describes electrical charges in motion. This movement is associated with “electricity,” and it is the intended meaning for all further discussions of electricity in this text. For electric current to move, an electric potential must exist. Electric potential is the ability to do work because of a separation of charges. If one has an abundance of electrons at one end of a wire and an abundance of positive charges at the other end (separation of charges), electrons will flow from abundance to deficiency.

Electric Potential, Current, and Resistance

Electric potential, current, and resistance are expressions of different phenomena surrounding electricity. Electric potential is the ability to do work because of a separation of charges. Current is an expression of the flow of electrons in a conductor. Finally, resistance is that property of an element in a circuit that resists or impedes the flow of electricity. It should be noted that there is nothing magical about the production of x-rays; it is simply the manipulation of electricity. Of course significant engineering and technological knowledge is required to design and manufacture the equipment, but, when viewed at its most basic level, x-ray production is simply the manipulation of electricity. In fact, the units of measure for electric potential (the volt) and current (the ampere) are the factors selected on the operating console of the x-ray machine to produce x-rays. They are expressed in thousands and thousandths, respectively, but they are electrical terms and are not exclusive to radiology.

Electric potential is measured in volts, named for the Italian physicist Volta who invented the battery. A volt may be defined as “the potential difference that will maintain a current of 1 ampere in a circuit with a resistance of 1 ohm” (amperes and ohms are discussed next). It is the expression of the difference in electric potential between two points. The volt is also equal to the amount of work in joules that can be done per unit of charge. (Refer to Chapter 1 for a review of the definition and calculation of work.) A volt is the ratio of joules to coulombs (volt = joules/coulombs). For example, a battery that uses 6 joules of energy to move 1 coulomb of charge is a 6-volt battery.

Again, one of the exposure factors selected on the control panel of the x-ray machine is kilovoltage peak (kVp). The role of kVp within the machine and in image production is discussed later in this text. For now, note that the radiographer is literally selecting the thousands

of volts that will be applied to the x-ray tube to produce x-rays. An understanding of this unit of measure and the concepts presented here are vital to the competent and safe operation of the x-ray machine.

Current is measured in amperes, named for André-Marie Ampere, a French physicist who made significant contributions to the study of electrodynamics. The ampere may be defined as “1 coulomb flowing by a given point in 1 second.” Reflecting its relationship to the definition of volt (discussed previously), it may also be defined as “the amount of current flowing with an electric potential of 1 volt in a circuit with a resistance of 1 ohm.” For electric current to flow, there must be a potential difference between two electrodes and a suitable medium through which it can travel. With regard to potential difference, electrons flow from abundance to deficiency and will continue to do so as long as that difference exists. Electricity behaves differently depending on the medium through which it travels. Suitable media are conductors and those resisting electric current flow are insulators. Both types of media are important to the production of x-rays. Two in particular, vacuums and metallic conductors, are of particular usefulness in x-ray production. In a vacuum tube, electrons tend to jump the gap between oppositely charged electrodes. This is part of the environment that exists inside an x-ray tube. With metallic conductors, electrons from the conductor’s atoms will move out of the valence shell to a higher energy level just beyond, called the conduction band, where they are free to drift along the external surface of the conductor (refer to Chapter 2 for a discussion of atomic structure). Copper is particularly useful as a conductor and is commonly used as such in electronic devices. Other metals with this characteristic are used extensively in x-ray machine and x-ray tube design.

The two types of current, direct current (DC) and alternating current (AC), are also important to x-ray production and should be understood before moving on. DC is a type of current that flows in only one direction. A battery is a good example: It has a positive and a negative electrode, and, when placed in an electrical circuit, electrons flow from the negative terminal to the positive terminal (current flows in the opposite direction, a topic clarified later). AC is current that changes direction in cycles as the electric potential of the source changes (the negative and positive “terminals,” if you will, alternate). In the United

States the electricity that flows into homes alternates at 60 cycles per second. This is expressed as a frequency of 60 Hz (see Chapter 3 for a definition and discussion of hertz). Both AC and DC are used in basic x-ray production.

Resistance is measured in ohms, named for the physicist Georg Simon Ohm who discovered the inverse relationship between current and resistance. The ohm may be defined as “the electrical resistance equal to the resistance between two points along a conductor that produces a current of 1 ampere when a potential difference of 1 volt is applied.” Ohm’s law states that the potential difference (voltage) across the total circuit or any part of that circuit is equal to the current (amperes) multiplied by the resistance. It is expressed by the formula V = IR, in which V is voltage, I is current, and R is resistance.

Resistance is that property of a circuit element that impedes the flow of electricity. The amount of resistance of a particular conductor depends on four things: material, length, cross-sectional area, and temperature.

Although resistance may sound like a hindrance to the x-ray production process, it is quite useful and is an important part of the process of x-ray production.

Conductors, Insulators, and Electronic Devices

Conduction and insulation are properties of elements and materials used in daily life. As previously stated, conductors are those materials with an abundance of free electrons that allow a relatively free flow of electricity. Although any such material conducts electricity, metals are typically used to serve this purpose. Copper typifies a conductive material. Its valence electrons are relatively free and will readily move to the conduction band, allowing a free flow of electricity. Gold is also a good conductor, but is considerably more expensive because it is a precious metal and is not widely used for this purpose. Water is also a good conductor of electricity because of the mineral impurities it often contains.

In contrast, most nonmetallic elements are made up of atoms with tightly bound electrons and do not conduct electricity well even when attracted by a potential difference. Such materials are insulators. Insulators have virtually no free electrons and, as such, are very poor conductors of electricity. But it is this very property that makes them particularly useful in containing the flow of electricity. Covering a copper wire with rubber or plastic “insulates” the wire and restricts the flow of electricity to the copper wire; such is the case with an electric cord (extension cord). Glass, ceramic, and wood are also good insulators. This combination of conductors and insulators is prevalent in daily life.

An electric circuit is a closed pathway composed of wires and circuit elements through which electricity may flow. This pathway for electricity must be closed (complete) for electricity to flow. This is what is meant by a closed circuit. In contrast, an open circuit is one in which the pathway is broken, such as when a switch is turned off. Turning off a switch opens the pathway and turning on a switch closes the pathway. An x-ray circuit is a complex version that has different voltages and currents flowing through different sections.

The term electronic devices may mean a number of things depending on context. Music players, cell phones, video gaming systems, televisions, and so on are all referred to as electronic devices. This same definition can be applied to many devices used in health care. An understanding of seven electronic devices aid an understanding of the x-ray circuit: battery, capacitor, diode, protective devices (fuses and circuit breakers), resistor or rheostat, switch, and transformer.

A battery is a device that produces electrons through a chemical reaction, stores an electric charge for the long term, and provides an electric potential. A capacitor is like a battery in that it stores an electric charge, but it works very differently in that it cannot produce new electrons and stores the charge only temporarily. A diode (e.g., solid-state rectifier) is a “one-way valve” device that allows electrons to flow in one direction only. Protective devices, such as fuses and circuit breakers, act as emergency devices that “break” or open the circuit if there is a sudden surge of electricity to the circuit or device. This act of opening the circuit protects the other circuit elements and the device as a whole. A fuse is simply a section of special wire, usually encased in glass that quickly melts if the current flow rises excessively, thus opening the circuit. A circuit breaker acts in the same manner as a fuse. If the current flow rises excessively, the circuit breaker’s internal switch is tripped (opened), stopping the flow of electricity. A resistor is a device designed to inhibit the flow of electrons, thereby precisely regulating the flow of electricity through that part of the circuit where it is placed. A rheostat is simply an adjustable or variable form of resistor. A switch is a device that opens a circuit (breaks the pathway). Finally, a transformer is a device that can increase or decrease voltage by a predetermined amount.

Table 4-1 provides a summary of these devices and the symbols of each.

Feb 27, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on The X-ray Circuit
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