Radiology Lecture 2.docx

Lecture 2 Chapter 2 – Radiation Physics The world is composed of matter and energy. Matter is anything that occupies space and has mass; when matter is altered, energy results. WHAT IS THE FUNDAMENTAL UNIT OF MATTER? The fundamental unit of matter is the atom. All matter is composed of atoms...

Lecture 2 Chapter 2 – Radiation Physics The world is composed of matter and energy. Matter is anything that occupies space and has mass; when matter is altered, energy results. WHAT IS THE FUNDAMENTAL UNIT OF MATTER? The fundamental unit of matter is the atom. All matter is composed of atoms, or tiny invisible particles. WHAT ARE THE COMPONENTS OF AN ATOM? The atom consists of two parts: (1) a central nucleus and (2) orbiting electrons The nucleus, or dense core of the atom, is composed of particles known as protons and neutrons (also known as nucleons). Protons carry positive electrical charges, whereas neutrons carry no electrical charge. Electrons are tiny, negatively charged particles that have very little mass; an electron weighs approximately 1/1800 as much as a proton or neutron. The arrangement of the electrons and neutrons in an atom resembles that of a miniature solar system. Just as the planets revolve around the sun, electrons travel around the nucleus in well- defined paths known as orbits or shells. The shells are designated with the letters K, L, M, N, O, P, and Q; the K shell is located closest to the nucleus and has the highest energy level Each shell has a maximum number of electrons it can hold. Electrons are maintained in their orbits by the electrostatic force, or attraction, between the positive nucleus and the negative electrons. This is known as the binding energy, or binding force, of an electron. The binding energy is determined by the distance between the nucleus and the orbiting electron and is different for each shell. The binding energy is greatest in the shell closest to the nucleus. To remove a K-shell electron from a tungsten atom, 70 keV (70,000 eV) of energy would be required, whereas only 3 keV (3000 eV) of energy would be necessary to remove an electron from the M-shell. The energy required to remove an electron from its orbital shell must exceed the binding energy of the electron in that shell. WHAT IS THE SIMPLEST ATOM? The simplest atom is hydrogen. A molecule can be defined as two or more atoms joined by chemical bonds, or the smallest amount of a substance that possesses its characteristic properties. Molecules are formed in one of two ways: (1) by the transfer (IONIC) of electrons or (2) by the sharing (COvalent) of electrons between the outermost shells of atoms. Atoms can exist in a neutral state or in an electrically unbalanced state. Normally, most atoms are neutral. A neutral atom contains an equal number of protons (positive charges) and electrons (negative charges). If the atom gains an electron, it has more electrons than protons and neutrons, and therefore it has a negative charge. Similarly, the atom that loses an electron has more protons and neutrons and thus has a positive charge. An atom that gains or loses an electron and becomes electrically unbalanced is known as an ion. Ionization is the production of ions, or the process of converting an atom into ions. When an electron is removed from an atom in the ionization process, an ion pair results. The atom becomes the positive ion, and the ejected electron becomes the negative ion. This ion pair reacts with other ions until electrically stable, neutral atoms are formed. Radiation is the emission and propagation of energy through space or a substance in the form of waves or particles. Radioactivity can be defined as the process by which certain unstable atoms or elements undergo spontaneous disintegration, or decay, in an effort to attain a more balanced nuclear state. Ionizing radiation can be defined as radiation that is capable of producing ions by removing or adding an electron to an atom. Ionizing radiation can be classified into two groups: (1) particulate radiation and (2) electromagnetic radiation. Particulate radiations are tiny particles of matter that possess mass and travel in straight lines and at high speeds. Particulate radiations transmit kinetic energy by means of their extremely fast-moving, small masses. Four types of particulate radiations are recognized Electrons can be classified as beta particles or cathode rays. They differ in origin only. Beta particles are found in radioactive atoms. WHAT ARE CATHODE RAYS? Cathode rays are streams of high-speed electrons that originate in an x-ray tube. Alpha particles are emitted from the nuclei of heavy metals and exist as two protons and neutrons, without electrons. WHAT ARE PROTONS? Protons are accelerated particles, specifically hydrogen nuclei, with a mass of 1 and a charge of +1. WHAT ARE NEUTRONS? Neutrons are accelerated particles with a mass of 1 and no electrical charge. Electromagnetic radiation can be defined as the propagation of wavelike energy (without mass) through space or matter. The energy propagated is accompanied by oscillating electric and magnetic fields positioned at right angles to one another, thus the term electromagnetic Electromagnetic radiations are man-made or occur naturally; examples include cosmic rays, gamma rays, x-rays, ultraviolet rays, visible light, infrared light, radar waves, microwaves, and radio waves. In the electromagnetic spectrum, only high-energy radiations (cosmic rays, gamma rays, and x-rays) are capable of ionization. Electromagnetic radiations are believed to move through space as both a particle and a wave; therefore two concepts, the particle concept and the wave concept, must be considered. The particle concept characterizes electromagnetic radiations as discrete bundles of energy called photons or quanta. Photons are bundles of energy with no mass or weight that travel as waves at the speed of light and move through space in a straight line, “carrying the energy” of electromagnetic radiation. The wave concept characterizes electromagnetic radiations as waves and focuses on the properties of velocity, wavelength, and frequency, as follows: Velocity refers to the speed of the wave. All electromagnetic radiations travel as waves or a continuous sequence of crests at the speed of light in a vacuum. Wavelength can be defined as the distance between the crest of one wave and the crest of the next. Wavelength determines the energy and penetrating power of the radiation; the shorter the distance between the crests, the shorter is the wavelength and the higher is the energy and ability to penetrate matter. Frequency refers to the number of wavelengths that pass a given point in a certain amount of time. Frequency and wavelength are inversely related; if the frequency of the wave is high, the wavelength will be short, and if the frequency is low, the wavelength will be long. The amount of energy an electromagnetic radiation possesses depends on the wavelength and frequency. Low-frequency electromagnetic radiations have a long wavelength and less energy. Conversely, high-frequency electromagnetic radiations have a short wavelength and more energy. For example, communications media use the low-frequency, longer waves of the electromagnetic spectrum; the wavelength of a radio wave can be as long as 100 meters, whereas the wavelength of a television wave is approximately 1 meter. In contrast, diagnostic radiography uses the high-frequency, shorter waves in the electromagnetic spectrum; x-rays used in dentistry have a wavelength of 0.1 nanometer, or 0.0000000001 meter. Properties of x-rays Box 2-1 X-rays can be defined as weightless bundles of energy (photons) without an electrical charge that travel in waves with a specific frequency at the speed of light. X-ray photons interact with the materials they penetrate and cause ionization. STUDENTS ABOUT: Control panel and extension arm. The x-ray tubehead is a tightly sealed, heavy metal housing that contains the x-ray tube that produces dental x-rays. The component parts of the tubehead include: Metal housing, or the metal body of the tubehead that surrounds the x-ray tube and transformers and is filled with oil—protects the x-ray tube and grounds the high-voltage components. Insulating oil, or the oil that surrounds the x-ray tube and transformers inside the tubehead—prevents overheating by absorbing the heat created by the production of x- rays. Tubehead seal, or the aluminum or leaded-glass covering of the tubehead that permits the exit of x-rays from the tubehead—seals the oil in the tubehead and acts as a filter to the x-ray beam. X-ray tube, or the heart of the x-ray generating system. Transformers are devices that alter the voltage of incoming electricity. Aluminum disks, or sheets of 0.5mm-thick aluminum placed in the path of the x-ray beam, filter out the nonpenetrating, longer-wavelength x-rays. Lead collimator, or a lead plate with a central hole that fits directly over the opening of the metal housing, where the x-rays exit—restricts the size of the x-ray beam. The PID is a cylinder that extends from the opening of the metal housing of the tubehead—aims and shapes the x-ray beam. The PID is sometimes referred to as the cone. The x-ray tube is the heart of the x-ray generating system. The x-ray tube is a glass vacuum tube from which all the air has been removed. The x-ray tube used in dentistry measures several inches long by 1 inch in diameter. The component parts of the x-ray tube include a leaded-glass housing, negative cathode, and positive anode The leaded-glass housing is a leaded-glass vacuum tube that prevents x-rays from escaping in all directions. One central area of the leaded-glass tube has a “window” that permits the x-ray beam to exit the tube and directs the x-ray beam toward the aluminum disks, lead collimator, and PID. The cathode, or negative electrode, consists of a tungsten wire filament in a cup-shaped holder made of molybdenum. The purpose of the cathode is to supply the electrons necessary to generate x-rays. In the x-ray tube, the electrons produced in the negative cathode are accelerated toward the positive anode. The cathode includes: The tungsten filament, or coiled wire made of tungsten, which produces electrons when heated. The molybdenum cup, which focuses the electrons into a narrow beam and directs the beam across the tube toward the tungsten target of the anode. The anode, or positive electrode, consists of a wafer-thin tungsten plate embedded in a solid copper rod. The purpose of the anode is to convert electrons into x-ray photons. The anode includes: A tungsten target, or plate of tungsten, which serves as a focal spot and converts bombarding electrons into x-ray photons. The copper stem, which functions to dissipate the heat away from the tungsten target. Electricity is the energy that is used to make x-rays. Electricity is necessary to produce x-rays. Electrical energy consists of a flow of electrons through a conductor; this flow is known as the electrical current. The electrical current is termed direct current (DC) when the electrons flow in one direction through the conductor. The current is a steady constant electrical charge. The term alternating current (AC) describes an electrical current in which the electrons flow in two, opposite directions. The current alternates between positive and negative, resulting in a voltage waveform shaped like a sine wave. Rectification is the conversion of AC to DC. The dental x-ray tube acts as a self-rectifier in that it changes AC into DC while producing x-rays. This ensures that the current is always flowing in the same direction, more specifically, from cathode to anode. Amperage is the measurement of the number of electrons moving through a conductor. Current is measured in amperes (A) or milliamperes (mA). Voltage is the measurement of electrical force that causes electrons to move from a negative pole to a positive one. Voltage is measured in volts (V) or kilovolts (kV). In the x-ray tube, the amperage, or number of electrons passing through the cathode filament, can be increased or decreased by the milliamperage (mA) adjustment on the control panel of the x-ray machine. The voltage of the x-ray tube current, or the current passing from the cathode to the anode, is controlled by the kilovoltage (kV) adjustment on the control panel. A circuit is a path of electrical current. Two electrical circuits are used in the production of x- rays: (1) a low-voltage, or filament, circuit and (2) a high-voltage circuit. The filament circuit uses 3 to 5 V, regulates the flow of electrical current to the filament of the x-ray tube, and is controlled by the milliampere settings. The high-voltage circuit uses 65,000 to 100,000 V, provides the high voltage required to accelerate electrons and to generate x-rays in the x-ray tube, and is controlled by the kilovoltage settings. A transformer is a device that is used to either increase or decrease the voltage in an electrical circuit. Transformers alter the voltage of the incoming electrical current and then route the electrical energy to the x-ray tube. In the production of dental x-rays, three transformers are used to adjust the electrical circuits: (1) the step-down transformer, (2) the step-up transformer, and (3) the autotransformer. A step-down transformer is used to decrease the voltage from the incoming 110- or 220-line voltage to the 3 to 5 V used by the filament circuit. A step-up transformer is used to increase the voltage from the incoming 110- or 220- line voltage to the 65,000 to 100,000 volts used by the high-voltage circuit. A step-up transformer has more wire coils in the secondary coil than in the primary coil. An autotransformer serves as a voltage compensator that corrects for minor fluctuations in the current. Following is a step-by-step explanation of x-ray production: Electricity from the wall outlet supplies the power to generate x-rays. When the x-ray machine is turned on, the electrical current enters the control panel through the cord plugged into the wall outlet. The current travels from the control panel to the tubehead through the electrical wires in the extension arm. The current is directed to the filament circuit and step-down transformer in the tubehead. The transformer reduces the 110 or 220 entering-line voltage to 3 to 5 V. The filament circuit uses the 3 to 5 V to heat the tungsten filament in the cathode portion of the x-ray tube. Thermionic emission occurs, defined as the release of electrons from the tungsten filament when the electrical current passes through it and heats the filament. The outer-shell electrons of the tungsten atom acquire enough energy to move away from the filament surface, and an electron cloud forms around the filament. The electrons stay in an electron cloud until the high-voltage circuit is activated. When the exposure button is pushed, the high-voltage circuit is activated. The electrons produced at the cathode are accelerated across the x-ray tube to the anode. The distance between the cathode and anode is very short, less than ½ inch. The molybdenum cup in the cathode directs the electrons to the tungsten target in the anode. The electrons travel from the cathode to the anode. When the electrons strike the tungsten target, their energy of motion (kinetic energy) is converted to x-ray energy and heat. Less than 1% of the energy is converted to x-rays; the remaining 99% is lost as heat. The heat produced during the production of x-rays is carried away from the copper stem and absorbed by the insulating oil in the tubehead. The x-rays produced are emitted from the target in all directions; however, the leaded-glass housing prevents the x-rays from escaping from the x-ray tube. A small number of x-rays are able to exit from the x-ray tube through the unleaded glass window portion of the tube. The x-rays travel through the unleaded glass window, the tubehead seal, and the aluminum disks. The aluminum disks remove or filter the longer wavelength x-rays from the beam. Next, the lead collimator restricts the size of the x-ray beam. The x-ray beam then travels down the lead-lined PID and exits the tubehead at the opening of the PID. The kinetic energy of the electrons is converted to x-ray photons through one of two mechanisms: (1) general (braking or bremsstrahlung) radiation and (2) characteristic radiation. General radiation, or braking radiation (bremsstrahlung). Speeding electrons slow down because of their interactions with the tungsten target in the anode. Many electrons that interact with the tungsten atoms undergo not one but many interactions within the target. The term braking refers to the sudden stopping of high-speed electrons when they hit the tungsten target in the anode. Most x-rays are produced in this manner; approximately 70% of the x-ray energy produced at the anode can be classified as general radiation. An electron rarely hits the nucleus of the tungsten atom. When it does, however, all its kinetic energy is converted into a high-energy x-ray photon. Instead of hitting the nucleus, most electrons just miss the nucleus of the tungsten atom. Consequently, an x-ray photon of lower energy results. As a result, general radiation consists of x-rays of many different energies and wavelengths. Characteristic radiation Produced when a high-speed electron dislodges an inner-shell electron from the tungsten atom and causes ionization of that atom. Once the electron is dislodged, the remaining orbiting electrons are rearranged to fill the vacancy. This rearrangement produces a loss of energy that results in the production of an x-ray photon. The x-rays produced by this interaction are known as characteristic x-rays. Characteristic radiation accounts for a very small part of x-rays produced in the dental x- ray machine. It occurs only at 70 kV and above because the binding energy of the K-shell electron is approximately 70 keV. Terms such as primary, secondary, and scatter are often used to describe x-radiation. Primary radiation refers to the penetrating x-ray beam that is produced at the target of the anode and that exits the tube head. This x-ray beam is often referred to as the primary beam, or useful beam. Secondary radiation refers to x-radiation that is created when the primary beam interacts with matter. (In dental radiography, “matter” includes the soft tissues of the head, the bones of the skull, and the teeth.) Secondary radiation is less penetrating than primary radiation. Scatter radiation is a form of secondary radiation and is the result of an x-ray that has been deflected from its path by the interaction with matter. Scatter radiation is deflected in all directions by the patient’s tissues and travels to all parts of the patient’s body and to all areas of the dental operatory. Scatter radiation is detrimental to both the patient and the radiographer. At the atomic level, four possibilities can occur when an x-ray photon interacts with matter: (1) no interaction, (2) absorption or photoelectric effect, (3) Compton scatter, and (4) coherent scatter. It is possible for an x-ray photon to pass through matter or the tissues of a patient without any interaction The x-ray photons that pass through a patient without interaction are responsible for producing densities and make dental radiography possible. Absorption refers to the total transfer of energy from the x-ray photon to the atoms of matter through which the x-ray beam passes. Absorption depends on the energy of the x-ray beam and the composition of the absorbing matter or tissues. At the atomic level, absorption occurs as a result of the photoelectric effect. In the photoelectric effect, ionization takes place. An x-ray photon collides with a tightly bound, inner-shell electron and gives up all its energy to eject the electron from its orbit The photoelectric effect accounts for 30% of the interactions of the dental x-ray beam with matter. Compton scatter or simply scatter. At the atomic level, the Compton effect accounts for most of the scatter radiation. The new, weaker x-ray photon interacts with other atoms until all its energy is gone. Compton scatter accounts for 62% of the scatter that occurs in diagnostic radiography. Coherent scatter occurs when a low-energy x-ray photon interacts with an outer-shell electron. Essentially, the x-ray photon is “unmodified” and simply undergoes a change in direction without a change in energy. Coherent scatter accounts for 8% of the interactions of the dental x-ray beam with matter.