Essentials of Radiographic Physics and Imaging

Chapter 4: The X-ray Circuit

Kimberly Harn RT(R)(MR)

⬤ Electrostatics is the study of stationary electric charges. ⬤ There are five general laws of electrostatics that are helpful in understanding the nature of electricity: ➢ Like charges repel and unlike charges attract each other. ➢ The electrostatic force between two charges is directly proportional to the product of their quantities and inversely proportional to the square of the distance between them (aka Coulomb’s law). ➢ Electric charges reside only on the external surface of conductors. ➢ The concentration of charges on a curved surface of a conductor is greatest where the curvature is greatest. ➢ Only negative charges (electrons) are free to move in solid conductors.

⬤ Electrodynamics is the study of electric charges in motion and what we most often consider as “electricity.” ⬤ For electric current to move, there must exist an electric potential, which 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. Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 4 Electrical Charge

⬤ 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 System International (SI) unit “coulomb,” which is equal to the electrical charge of 6.25 × 1018 electrons.

⬤ Some materials, such as copper and gold, have a very large number of electrons free to move about, making them good conductors of electricity. ⬤ Glass and plastic, on the other hand, have very few free electrons, making them good insulators.

Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 6 Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 7 Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 8 Electrical Terms

⬤ Electric potential is the ability to do work because of a separation of charges and is measured in volts. ⬤ Current is an expression of the flow of electrons in a conductor and is measured in amperes. ⬤ Resistance is that property of an element in a circuit that resists or impedes the flow of electricity and is measured in ohms.

⬤ Volt is 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.

⬤ The ampere is defined as 1 coulomb flowing by a given point in 1 second. ⬤ Reflecting its relationship to the definition of volt, ampere 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. Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 11 Units of Measure (cont’d)

⬤ The ohm is defined as the electrical resistance equal to the resistance between two points along a conductor that, when a potential difference of 1 volt is applied, produces a current of 1 ampere. ⬤ 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. ⬤ Resistance is that property of that element in a circuit that impedes the flow of electricity.

⬤ The amount of resistance of a particular conductor depends on four things: ➢ Material: Some materials allow a free flow of current because they have an abundance of free electrons, whereas other materials have tremendous resistance because they have virtually no free electrons. ➢ Length: Resistance is directly proportional to the length of the conductor; that is, a long conductor has more resistance than a short one. ➢ Cross-sectional area: A conductor with a large cross-sectional area has a lower resistance than one with a small cross-sectional area because there is a greater external surface area on which electrons can travel. ➢ Temperature: With metallic conductors, the resistance becomes greater as the temperature of the conductor rises.

⬤ 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. ➢ An open circuit is one in which the pathway is broken, such as occurs when a switch is turned off. ⬤ An x-ray circuit is a complex circuit that has different voltages and current flowing through different sections.

⬤ 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 a device that is like a battery in that it stores an electric charge, but works very differently in that it cannot produce new electrons and stores the charge only temporarily. ⬤ A diode (e.g., a solid-state rectifier) is a “one-way valve” device and allows electrons to flow in only one direction.

Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 17 Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 18 Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 19 Electrical Devices (cont’d)

⬤ 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. ⬤ 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). ⬤ A transformer is a device that can increase or decrease voltage by a predetermined amount.

⬤ Electricity and magnetism are two different parts of the same phenomenon known as electromagnetism. ⬤ Magnetism may be defined as the ability of a material to attract iron, cobalt, or nickel.

⬤ The nature of magnetic materials is that the orbital electrons of their atoms spin in predominately one direction. ➢ Such atoms create tiny magnetic dipoles. ➢ When these dipoles or “atomic magnets” form groups of similarly aligned atoms, they create magnetic domains.

⬤ A magnetic field consists of lines of force in space called flux and has three basic characteristics. ➢ The lines of flux travel from south pole to north pole INSIDE the magnet and from north pole to south pole OUTSIDE the magnet, FIGURE 4-2 Magnetic Flux. A magnetic field consists creating elliptical loops. of lines of force in space called flux ➢ Lines of flux in the same direction repel each other and lines of flux in the opposite direction attract each other.

⬤ Magnetic fields are distorted by magnetic materials and are unaffected by nonmagnetic materials. ⬤ There are three laws of magnetism that may help understand electromagnetism: ➢ The first law is that every magnet has a north and south pole. ➢ The second law states that like poles repel each other and opposite poles attract each other. ➢ The third law states that the force of attraction or repulsion varies directly with the strength of the poles and inversely with the square of the distance between them.

⬤ The strength of the magnetic field is measured in the SI unit Tesla (T). ⬤ Magnetic resonance imaging (MRI) units used for medical imaging are referred to by their magnetic field strength and operate with fields from 0.5 to 5 T.

⬤ Materials can be classified by their magnetic properties. ⬤ There are four categories: ➢ Nonmagnetic (e.g., glass, wood, plastic) are not attracted to magnetic fields at all. ➢ Diamagnetic (e.g., water, mercury, gold) are weakly repelled by magnetic fields. ➢ Paramagnetic materials (e.g., platinum, gadolinium, aluminum) are weakly attracted to magnetic fields. ➢ Ferromagnetic materials (e.g., iron, cobalt, nickel) are strongly attracted to magnetic materials.

⬤ Electricity and magnetism are two parts of the same basic force. ⬤ Any flow of electrons, whether in space or in a conductor, will be surrounded by a magnetic field. ⬤ Likewise, a moving magnetic field can create an electric current.

⬤ Electromagnetic induction: moving a conductor (such as copper wire) through a magnetic field induces an electric current in that conductor. ⬤ Two forms of electromagnetic induction: mutual induction and self induction

⬤ Mutual induction is the induction of electricity in a secondary coil by a moving magnetic field. ⬤ When a moving magnetic field is placed near a secondary coil, electricity is induced to flow in that coil.

FIGURE 4-3 Mutual Induction. Coil A is the primary coil connected to an AC power source. Coil B is the secondary coil and as the fluctuating magnetic field from A moves back and forth through the turns of B, a secondary current is induced.

⬤ Self-induction is a bit more complex. ⬤ To understand it, we must first understand Lenz’s law, which states that an induced current flows in a direction that opposes the action (changing magnetic field) that induced it. ⬤ A magnetic field is created in a coil carrying electrical current and expands outward from the center of the coil. ⬤ As it does so, it “cuts” through the turns of the coil and this act of “cutting” creates a current within the same conductor that opposes the original (Lenz’s law).

⬤ Electrical equipment of particular importance to understanding the x-ray circuit consists of electric generators, electric motors, and transformers.

⬤ Electric generators are devices that convert some form of mechanical energy into electrical energy.

FIGURE 4-4 Generator. As the loop is rotated in the magnetic field, a current is induced in the loop.

⬤ Electric motors are devices that convert electrical energy to mechanical energy through electromagnetic induction.

➢ Transformers are devices used to increase or decrease voltage (or current) through electromagnetic induction. ➢ A step-up transformer is one that increases voltage, and a step-down transformer is one that decreases voltage. ➢ It should be noted that the change in voltage and current is an inverse relationship.

FIGURE 4-5 Transformer Types. The basic design of a closed-core and shell-type transformer. The iron cores of each increases magnetic field strength and transformer efficiency.

⬤ One final type of transformer important to the x-ray machine is the autotransformer. ➢ Operates on the principle of self-induction. ➢ Has only one coil of wire around a central magnetic core serving as both the primary and secondary coil. ➢ The outside wires are attached at different points along the coil, and the induced voltage varies depending on where the connections are made.

FIGURE 4-6 Autotransformer. An autotransformer (orange) and its orientation to the rest of the x-ray circuit.

Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 40 Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 41 Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 42 X-ray Circuit

⬤ The x-ray circuit may be divided into three sections: the primary circuit, the secondary circuit, and the filament circuit.

FIGURE 4-7 Basic X-ray Circuit. The primary circuit is indicated in orange, the secondary circuit is in blue, and the filament circuit is in purple

⬤ The primary circuit consists of the main power switch (connected to the incoming power supply), circuit breakers, the autotransformer, the timer circuit, and the primary side of the step-up transformer. ⬤ The secondary circuit consists of the secondary side of the step-up transformer, the mA meter, a rectifier bank, and the x-ray tube (except for the filaments). ⬤ The filament circuit consists of a rheostat, a step-down transformer, and the filaments.

FIGURE 4-8 Parts of the X-ray Circuit. Labeled version of a basic x-ray circuit.

⬤ The main power switch is simply an on-off switch for the unit and is connected to the power supply of the facility. ⬤ The line compensator is a device usually wired to the autotransformer and automatically adjusts the power supplied to the x-ray machine to precisely 220 volts. ⬤ The circuit breakers are included in the primary circuit to protect against short circuits and electric shock. ⬤ The autotransformer is an adjustable transformer controlled by the kilovolt peak (kVp) selector on the operating console.

⬤ The step-up transformer is used to increase the voltage from the autotransformer to the kilovoltage necessary for x-ray production . This transformer is the dividing line between the primary and secondary circuits. The primary coil is in the primary circuit and the secondary coil is in the secondary circuit (hence the names). ⬤ The timer circuit (exposure timer) is located in this section because it is easier to control (turn on and off) a low voltage than a very high one.

Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 47 Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 48 Exposure timer Two types ● synchronous timer ● electronic timer (most used today)

Purpose is to make and break the high voltage across the tube on the primary side of the high voltage section

⬤ The secondary circuit begins with the rest of the step-up transformer. ⬤ The mA meter is simply a device placed in the secondary circuit that monitors x-ray tube current.

⬤ The rectifiers are needed to convert AC to DC. ➢ Within the x-ray tube, current must always flow from anode to cathode and electrons from cathode to anode, and rectifiers are used to achieve this. ➢ The rectifier commonly used in today’s x-ray circuit is a solid-state rectifier that is made of two semiconducting crystals. One is a p-type crystal and the other is an n-type crystal.

FIGURE 4-9 Solid-State Rectifier. Solid-state rectifier showing its conduction and nonconduction phases. Note the change in polarity in each half of the illustration.

⬤ The final section is the filament circuit.

FIGURE 4-16 Filament Circuit. Components of the filament circuit. (purple)

⬤ The filament circuit begins with the rheostat. ➢ This variable resistor is controlled by the mA selector on the operating console. ➢ The parameter “mA” is an abbreviation for “milliampere” and is called “tube current” because it reflects the rate of flow of electrons passing through the x-ray tube during an exposure. ⬤ Working in concert with the rheostat is the exposure timer. ➢ The rheostat controls filament temperature and the rate at which electrons are boiled off of the filament. The timer determines the duration of this process. ➢ Together, they determine the quantity of electrons boiled off of the filament and available for x-ray production. ⬤ A step-down transformer is used in the filament circuit to increase the current by reducing the voltage that is applied to the filament.

⬤ The final piece of the filament circuit is the filaments. ➢ A general-purpose radiographic tube typically has two filaments. ➢ They are represented on the operating console by the “large focal spot” and “small focal spot.” ⬤ The goal of the filament circuit is FIGURE 4-17 Filaments. Filaments (small wire coils) within to literally boil electrons out of the cathode focusing cup. the filament wire. ⬤ Normally, a rather large filament current of 5-7 amperes is required to produce a tube current in the range of milliamperes. Copyright © 2012 by Mosby, Inc., an affiliate of Elsevier Inc. 57 Filament Circuit (cont’d)

⬤ The thermionically emitted electrons are needed for x-ray production and represent one more step in our manipulation of electricity. ⬤ In the filament circuit, we are boiling these electrons off of a very small wire and must precisely control the current that is applied. If the current is too high, this tiny wire will be damaged or destroyed.

⬤ The process begins when the radiographer selects a technique that specifies kVp, mA, exposure time, and focal spot. ➢ From here, it is easiest to follow the sequence of events in two parts. We first follow voltage through the primary and secondary circuits, then go back and follow current through the filament circuit. ⬤ The kVp selected adjusts the autotransformer and determines the number of turns on the secondary side necessary to produce a voltage, through self-induction, that will be sent to the step-up transformer.

⬤ The step-up transformer increases this voltage by a fixed amount and through mutual induction produces the kilovoltage selected on the operating console. ⬤ Solid-state rectifiers are used to route electricity through the x-ray tube correctly. ⬤ After passing through the rectifiers, the electricity creates a large positive charge on the anode of the x-ray tube and a large negative charge on the cathode focusing cup (part of tube that surrounds the filaments).

⬤ At this point, we return to the autotransformer to fill in the other half of the process. ⬤ The filament circuit draws electricity from the autotransformer, which then travels to the rheostat. ⬤ The rheostat is a variable resistor controlled by the mA selector on the operating console and when the mA station was selected, the appropriate filament was also selected.

⬤ From the rheostat, electricity then travels to the step-down transformer. ⬤ The adjusted current from the step-down transformer then travels directly to the filament located within the focusing cup of the x-ray tube. ⬤ This current heats the filament to the point where electrons are literally boiled off.

⬤ Now we can join the two halves of the process: ⬤ We have a group, or cloud, of electrons created by the filament circuit (heating of the filament). ⬤ The kilovoltage applied to the x-ray tube created a large positive charge on the anode and a large negative charge on the cathode (focusing cup).

⬤ The large positive charge attracts the electrons boiled off the filament, giving them tremendous kinetic energy in the process. ⬤ The large negative charge on the cathode serves to keep the electrons crowded together; otherwise, they would repel each other and scatter throughout the tube. ⬤ The electrons travel across to the anode and interact there to produce x-rays until the timer circuit terminates the process.

Oct 8: Test 1