White and Pharoh ORAL RADIOLOGY- 7E (1)_240526_085644-1-15.pdf

PART I Foundations CHAPTER Physics...

PART I Foundations CHAPTER Physics 1 o m t.c OUTLINE Composition of Matter Production of X Rays Coherent Scattering po Atomic Structure Bremsstrahlung Radiation Photoelectric Absorption Ionization Characteristic Radiation Compton Scattering Nature of Radiation Factors Controlling the X-Ray Beam Beam Attenuation Particulate Radiation Exposure Time (s) Dosimetry gs Electromagnetic Radiation Tube Current (mA) Exposure X-Ray Machine Tube Voltage Peak (kVp) Air Kerma X-Ray Tube Filtration Absorbed Dose Power Supply Timer lo Collimation Inverse Square Law Equivalent (Radiation-Weighted) Dose Effective Dose Tube Rating and Duty Cycle Interactions of X Rays with Matter Radioactivity r y.b ra One atom says to a friend, “I think I lost an electron.” The friend classic view of the atom has been replaced by the Standard Model, replies, “Are you sure?” “Yes,” says the first atom, “I’m which describes the subatomic particles (Table 1-1), and the positive.” Quantum Mechanical Model, which describes the arrangement llib of electrons in an atom. In addition to matter particles, the Stan- dard Model also describes force carrier particles—particles that D entists make radiographic images of patients when they mediate interactions between matter particles (Table 1-2). seek additional information beyond that available from a clinical examination or their patient’s history. Dentists ATOMIC STRUCTURE a combine the information from these images with their findings Nucleus from the clinical examination and history to form a diagnosis. In all atoms except hydrogen, the nucleus consists of positively nt When a diagnosis is established, treatment can be provided. This charged protons and neutral neutrons. A hydrogen nucleus con- chapter considers the initial steps in making radiographic images, tains a single proton. Protons and neutrons are made of quarks including the operation of an x-ray machine and the interactions (Fig. 1-1). Protons consist of two up quarks and one down quark of radiation with matter. and thus have a charge of +1. Neutrons are made of one up de quark and two down quarks and thus are neutral. Although the COMPOSITION OF MATTER positively charged protons repel each other, the nucleus is held together by the strong nuclear force, the rapid exchange of gluons. Matter is anything that has mass and occupies space. All visible The strong nuclear force overwhelms the repulsive electromagnetic matter in the universe (all stable matter) is made of up quarks, effect at the incredibly short distances inside an atomic nucleus. down quarks, and electrons. These particles are fundamental The number of protons in the nucleus determines the identity because they have no inner structure and cannot be divided. Up of an element. This is its atomic number (Z), the nuclear charge. quarks and down quarks combine to form neutrons and protons A change in the number of protons in an atom changes it to in atomic nuclei. Electrons are located in orbitals outside the another element. Each of the more than 100 elements has a unique nuclei. Historically, the atom has been viewed as a miniature solar atomic number, a corresponding number of orbital electrons in system with a nucleus at the center and revolving electrons. This the ground state, and unique chemical and physical properties. The 1 2 PART I Foundations TABLE 1-1 Fundamental Particles FAMILIES OF PARTICLES Charge I II III Quarks + 23 Up u Charm c Top t − 13 Down d Strange s Bottom b Leptons −1 Electron e Muon µ Tau τ m 0 Electron neutrino νe Muon neutrino νµ Tau neutrino ντ Stable particles o t.c TABLE 1-2 Force-Carrier Particles Particle Symbol Action po Photon γ Make up x-ray beams and mediate electromagnetic interactions Gluon g Mediate strong nuclear force that binds quarks into s protons and neutrons and bind nuclei together gs d W boson W Mediate weak interactions; associated with beta decay p Z boson Z Mediate weak interactions; associated with neutrino FIGURE 1-2 Electron orbitals are clouds of varying density, probability plots of the location scatter of the electron. The s-type electron orbital is spherical and centered around the nucleus. The lo p-type electron orbitals are bilobed and centered around the nucleus. Four of the five d-type electron orbitals are made up of four lobes, centered on the nucleus. The fifth d-type orbital is bilobed with an encircling ring (not shown). y.b orbitals are the first to be filled in every element. Next are the p-type orbitals, which are bilobed and centered on the nucleus. Next are the d-type orbitals, which consist of four lobes arranged r around the nucleus—they are bilobed with a ring. In an atom with many electrons, the electron clouds of one orbital are superim- ra posed with the electron clouds of other orbitals. No known atom has more than seven orbitals. Electrons occupy the lowest energy available orbitals—those not already occupied by other electrons. llib FIGURE 1-1 Schematic view of a hydrogen atom showing a nucleus with one proton, A change in the number of electrons of an atom changes the charge composed of two up quarks (U) and one down quark (D) and two surrounding electrons (e−) of an atom. within a 1s spherical orbital. Compared with the scale of the 1s orbital, the nucleus and electrons are much smaller than shown. IONIZATION When the number of electrons in an atom is equal to the number a of protons in its nucleus, the atom is electrically neutral. If a neutral atom loses an electron, it becomes a positive ion, and the nt total number of protons and neutrons in the nucleus of an atom free electron becomes a negative ion. This process of forming an is its atomic mass (A). A change in the number of neutrons in an ion pair is termed ionization. Ionizing an atom requires sufficient atom changes the stability of the element. Nearly the entire mass energy to overcome the electron binding energy, the electrostatic of the atom consists of the protons and neutrons in the nucleus. force binding the electrons to the nucleus. The binding energy of de an electron is related to the atomic number of the atom and the Electron Orbitals orbital type. Elements with a large atomic number (high Z) have Electrons exhibit both particle-like properties (e.g., they have mass) more protons in their nucleus and thus bind electrons in any given and wavelike properties (e.g., they generate interference patterns). orbital more tightly than smaller Z elements. Within a given atom, Electrons exist within three-dimensional volumes called orbitals. electrons in the inner orbitals are more tightly bound than the Orbitals represent the probability locations of the electron in space more distant outer orbitals. Tightly bound electrons require the at any instant in time—the regions in which the electron is most energy of x rays or high-energy particles to remove them, whereas likely to exist. The letters s, p, d, f, g, and h are used to describe loosely bound outer electrons can be displaced by ultraviolet radia- orbital shapes (Fig. 1-2). These letters replace the K, L, M, N, O, tion. However, nonionizing types of radiation, such as visible light, and P designations previously used. Only two electrons may infrared, and microwave radiation, and radio waves do not have occupy an orbital. The s-type orbital is spherical. The s-type sufficient energy to remove bound electrons from their orbitals.  C H A P T E R 1 Physics 3 NATURE OF RADIATION converting a proton into a neutron, a β+ particle (positron), and a neutrino. Positrons quickly annihilate with electrons to form two Radiation is the transmission of energy through space and matter. γ rays. This reaction is the basis for positron emission tomography It may occur in two forms: (1) particulate (Table 1-3) and (2) elec- scanning (see Chapter 14). tromagnetic. Natural radioactivity and radiation therapy may The capacity of particulate radiation to ionize atoms depends involve both particulate and electromagnetic radiation. Oral and on its mass, velocity, and charge. The rate of loss of energy from maxillofacial radiology involves only electromagnetic radiation. a particle as it moves along its track through matter (tissue) is its linear energy transfer (LET). A particle loses kinetic energy each PARTICULATE RADIATION time it ionizes adjacent matter. The greater the physical size of the m Small atoms have roughly equal numbers of protons and neutrons, particle, the higher its charge, and the lower its velocity, the greater whereas larger atoms tend to have more neutrons than protons. its LET. For example, α particles, with their high mass compared Larger atoms are unstable because of the unequal distribution of with an electron, high charge, and low velocity, are densely ion- protons and neutrons, and they may break up, releasing α (alpha) izing, lose their kinetic energy rapidly, and have a high LET. β− o or β (beta) particles or γ (gamma) rays. This process is called radio- particles are much less densely ionizing because of their lighter activity. When a radioactive atom releases an α or a β particle, the mass and lower charge; they have a lower LET. High LET radiations t.c atom is transmuted into another element. α particles are helium concentrate their ionization along a short path, whereas low LET nuclei consisting of two protons and two neutrons. They result radiations produce ion pairs much more sparsely over a longer path from the radioactive decay of many large atomic number elements. length. Because of their double positive charge and heavy mass, α particles Another type of radioactivity is γ decay. γ rays are photons, a po densely ionize matter through which they pass. They quickly give form of electromagnetic radiation (see next section). They result as up their energy and penetrate only a few micrometers of body part of a decay chain where a nucleus converts from an excited tissue. (An ordinary sheet of paper absorbs them.) After stopping, state to a lower level ground state; this often happens after a α particles acquire two electrons and become neutral helium nucleus emits an α or β particle or after nuclear fission or fusion. gs atoms. An unstable atom with an excess of neutrons may decay by ELECTROMAGNETIC RADIATION converting a neutron into a proton, a β− particle, and a neutrino. Electromagnetic radiation is the movement of energy through β− particles are identical to electrons. High-speed β− particles are space as a combination of electric and magnetic fields. It is gener- not densely ionizing; they are able to penetrate matter to a greater lo ated when the velocity of an electrically charged particle is altered. depth than α particles can—up to 1.5 cm in tissue. This deeper γ rays, x rays, ultraviolet rays, visible light, infrared radiation (heat), penetration occurs because β− particles are smaller and lighter and microwaves, and radio waves all are examples of electromagnetic y.b carry a single negative charge; they have a much lower probability radiation (Fig. 1-3). γ rays originate in the nuclei of radioactive of interacting with matter than α particles. β− particles from radio- atoms. They typically have greater energy than x rays. In contrast, active iodine-131 are used for treatment of some thyroid cancers. x rays are produced outside the nucleus and result from the interac- An unstable atom with an excess of protons may decay by tion of electrons with large atomic nuclei in x-ray machines. The higher energy types of radiation in the electromagnetic spectrum— r ultraviolet rays, x rays, and γ rays—are capable of ionizing matter. Some properties of electromagnetic radiation are best explained by ra TABLE 1-3 Particulate Radiation quantum theory, whereas others are most successfully described by wave theory. Particle Symbol Elementary Charge* Rest Mass (amu) Quantum theory considers electromagnetic radiation as small llib Alpha α +2 4.00154 discrete bundles of energy called photons. Each photon travels at the speed of light and contains a specific amount of energy. The + Beta (positron) β + +1 0.000549 unit of photon energy is the electron volt (eV), the amount of − Beta (electron) β − −1 0.000549 energy acquired by one electron accelerating through a potential difference of one volt. The relationship between wavelength and a Electron e − −1 0.000549 photon energy is as follows: 0 Neutron n 0 1.008665 nt E = h × c/λ Proton p +1 1.007276 amu, Atomic mass units, where 1 amu = 112 the mass of a neutral carbon-12 atom. where E is energy in kiloelectron volts (keV), h is Planck’s constant (6.626 × 10−34 joule-seconds or 4.13 × 10−15 eV-seconds), c is the de *Elementary charge of 1 equals that the charge of a proton or the opposite of an electron. MR imaging Wavelength (nm) X-ray imaging 1013 1011 109 107 105 103 10 0.1 10-3 FIGURE 1-3 Electromagnetic spectrum showing the relationship between photon wavelength and energy and the physical properties of various portions of the spectrum. Photons with shorter wavelengths have higher energy. Photons 10-10 10-8 10-6 10-4 10-2 1 102 104 106 108 used in dental radiography (blue) have energies of 10 to 120 keV. Magnetic resonance (MR) imaging uses radio waves (orange). Radio Microwave IR Visible UV X-rays Gamma rays Photon energy (eV) 4 PART I Foundations Direction of photon propagation Magnetic field FIGURE 1-4 Electric and magnetic fields associated with electromagnetic radiation. Electric field o m velocity of light, and λ is wavelength in nanometers. This expres- t.c sion may be simplified to: Power supply E = 1.24 /λ X-ray tube Aluminum filter po Quantum theory has been successful in correlating experimen- X-ray beam tal data on the interaction of radiation with atoms, the photoelec- tric effect, and the production of x rays. The wave theory of Aiming cylinder electromagnetic radiation maintains that radiation is propagated Collimator Yoke gs in the form of waves, similar to the waves resulting from a distur- Power supply bance in water. Such waves consist of electric and magnetic fields Oil oriented in planes at right angles to one another that oscillate FIGURE 1-5 Tube head showing a recessed x-ray tube, components of the power supply, perpendicular to the direction of motion (Fig. 1-4). All electromag- and oil that conducts heat away from the x-ray tube. Path of useful x-ray beam (blue) from lo netic waves travel at the velocity of light (c = 3.0 × 108 m/s) in a the anode, through the glass wall of the x-ray tube, oil, and finally an aluminum filter. vacuum. Waves of all kinds exhibit the properties of wavelength The beam size is restricted by the metal tube housing and collimator. Low-energy photons (λ) and frequency (ν) and are related as follows: are preferentially removed by the filter. y.b λ × ν = c = 3 × 108 m/s Focal spot on where λ is in meters and ν is in cycles per second (hertz). Wave Filament and tungsten target Glass envelope theory is more useful for considering radiation in bulk when mil- electron cloud Vacuum r lions of quanta are being examined, as in experiments dealing with Copper refraction, reflection, diffraction, interference, and polarization. ra stem High-energy photons such as x rays and γ rays are typically characterized by their energy (electron volts), medium-energy e + photons (e.g., visible light and ultraviolet waves) are typically llib characterized by their wavelength (nanometers), and low-energy Electronic photons (e.g., AM and FM radio waves) are typically characterized focusing by their frequency (KHz and MHz). cup Cathode (-) Tube Anode (+) window Useful x-ray beam X-RAY MACHINE a FIGURE 1-6 X-ray tube with the major components labeled. The path of the electron X-ray machines produce x rays that pass through a patient’s tissues beam is shown in yellow. X rays produced at the target travel in all directions. The useful x-ray nt and strike a digital receptor or film to make a radiographic image. beam is shown in blue. The primary components of an x-ray machine are the x-ray tube and its power supply. The x-ray tube is positioned within the tube head, along with some components of the power supply (Fig. 1-5). stream from the filament in the cathode to the target in the de An electrical insulating material, usually oil, surrounds the tube anode, where the energy from some of the electrons is converted and transformers. Often, the tube is recessed within the tube head into x rays. For the x-ray tube to function, a power supply is to improve the quality of the radiographic image (see Chapter 6). necessary to: The tube head is typically supported by an arm that is usually Heat the cathode filament to generate electrons. mounted on a wall. A control panel allows the operator to adjust Establish a high-voltage potential between the anode and the duration of the exposure, and often the energy and exposure cathode to accelerate the electrons toward the anode. rate, of the x-ray beam. Cathode X-RAY TUBE The cathode (Fig. 1-7, B; see also Fig. 1-6) in an x-ray tube consists An x-ray tube is composed of a cathode and an anode situated of a filament and a focusing cup. The filament is the source of within an evacuated glass envelope or tube (Fig. 1-6). Electrons electrons within the x-ray tube. It is a coil of tungsten wire about  C H A P T E R 1 Physics 5 A o m t.c po B C gs lo y.b FIGURE 1-7 A, Dental stationary x-ray tube with cathode on left and copper anode on right. B, Focusing cup containing a filament (arrow) in the cathode. C, Copper anode with tungsten inset. Note the elongated actual focal spot area (arrow) on the tungsten target of the anode. (B and C, Courtesy John DeArmond, Tellico Plains, TN.) r ra 2 mm in diameter and 1 cm or less in length. Filaments typically Focusing cup contain about 1% thorium, which greatly increases the release of and filament Target llib electrons from the heated wire. The filament is mounted between two stiff support wires that carry an electrical current. These two mounting wires lead through the glass envelope and connect to both the high-voltage and the low-voltage electrical sources. The Cathode (-) e Anode (+) filament is heated to incandescence by the flow of current from a the low-voltage source and emits electrons at a rate proportional Actual focal spot size to the temperature of the filament. nt The filament lies in a focusing cup (Fig. 1-7, B; see also Fig. 3 mm 1-6), a negatively charged concave reflector made of molybdenum. Central ray The parabolic shape of the focusing cup electrostatically focuses 1 mm 20° the electrons emitted by the filament into a narrow beam directed de 1 mm Effective focal at a small rectangular area on the anode called the focal spot (Fig. spot size 1-7, C, and Fig. 1-8). The electrons move to the focal spot because they are both repelled by the negatively charged cathode and 1 mm attracted to the positively charged anode. The x-ray tube is evacu- FIGURE 1-8 The angle of the target to the central ray of the x-ray beam has a strong ated to prevent collision of the fast-moving electrons with gas influence on the apparent size of the focal spot. The projected effective focal spot (seen below molecules, which would significantly reduce their speed. The the target) is much smaller than the actual focal spot size (projected to the left). This provides vacuum also prevents oxidation, or “burnout,” of the filament. a beam that has a small effective focal spot size to produce images with high resolution, while Anode allowing for heat generated at the anode to be dissipated over the larger area. The anode in an x-ray tube consists of a tungsten target embedded in a copper stem (see Figs. 1-6 and 1-7, C). The purpose of the 6 PART I Foundations target in an x-ray tube is to convert the kinetic energy of the col- Provide a low-voltage current to heat the x-ray tube filament. liding electrons into x-ray photons. The target is made of tungsten, Generate a high potential difference to accelerate electrons an element that has several characteristics of an ideal target mate- from the cathode to the focal spot on the anode. The x-ray tube rial, including the following: and two transformers lie within an electrically grounded metal High atomic number (74) housing called the head of the x-ray machine. High melting point (3422°C) High thermal conductivity (173 W · m−1 · K−1) Tube Current Low vapor pressure at the working temperatures of an x-ray tube The filament transformer (Fig. 1-10) reduces the voltage of the The conversion of the kinetic energy of the electrons into x-ray incoming alternating current (AC) to about 10 volts in the filament m photons is an inefficient process with more than 99% of the elec- circuit. This voltage is regulated by the filament current control tron kinetic energy converted to heat. A target made of a high (mA selector), which adjusts the resistance and the current flow atomic number material is most efficient in producing x rays. through the filament; this regulates the filament temperature and Because heat is generated at the anode, the requirement for a target the number of electrons emitted by the cathode. The tube current o with a high melting point is clear. Tungsten also has high thermal is the flow of electrons through the tube—that is, from the cathode conductivity, readily dissipating its heat into the copper stem. filament across the tube to the anode. Beyond the anode, this t.c Finally, the low vapor pressure of tungsten at high temperatures current is carried through the power supply back to the cathode. helps maintain the vacuum in the tube at high operating tempera- The numerical mA setting on the filament current control refers tures. The tungsten target is typically embedded in a large block of copper. Copper, also a good thermal conductor, removes heat po from the tungsten, reducing the risk of the target melting. Addi- Glass tube Anode Stator tionally, the insulating oil between the glass envelope and the (sectioned) housing of the tube head carries heat away from the copper stem. The focal spot is the area on the target to which the focusing gs cup directs the electrons and from which x rays are produced. The sharpness of a radiographic image increases as the size of the focal Rotor spot decreases (see Chapter 6). However, the heat generated per unit target area becomes greater as the focal spot decreases in size. lo To take advantage of a small focal spot while distributing the electrons over a larger area of the target, the target is placed at an Focusing cup angle to the electron beam (see Fig. 1-8). The apparent size of the Filament Focal spot y.b focal spot seen from a position perpendicular to the electron beam Electron stream (the effective focal spot) is smaller than the actual focal spot size. Typically, the target is inclined about 20 degrees to the central ray X-ray beam of the x-ray beam; this causes the effective focal spot to be approxi- mately 1 mm × 1 mm, as opposed to the actual focal spot, which FIGURE 1-9 X-ray tube with a rotating anode allows heat at the focal spot to spread out r is about 1 mm × 3 mm. This smaller effective focal spot results in over a large surface area (dark band). Current applied to the stator induces rapid rotation of a small apparent source of x rays and an increase in the sharpness the rotor and the anode. The path of the electron beam is shown in yellow, and the useful x-ray ra of the image (see Fig. 5-2), with a larger actual focal spot size to beam is shown in blue. improve heat dissipation. This type of anode is a stationary anode because it has no moving parts. llib Another method of dissipating the heat from a small focal spot kVp selector is to use a rotating anode. In this design, the tungsten target is in the form of a beveled disk that rotates when the tube is in opera- Timer tion (Fig. 1-9). As a result, the electrons strike successive areas of X-ray tube the target, widening the focal spot by an amount corresponding AC power kVp mA a to the circumference of the beveled disk, distributing the heat over supply this extended area. The focal spot of a stationary tube is now a nt focal track in rotating anode machines. Narrow focal tracks in rotating anode tubes can be used with tube currents of 100 to 500 High-voltage Filament milliamperes (mA), which is 10 to 50 times that possible with transformer transformer stationary targets. The target and rotor (armature) of the motor lie de within the x-ray tube, and the stator coils (which drive the rotor at about 3000 revolutions per minute) lie outside the tube. Such rotating anodes are not used in intraoral dental x-ray machines but Autotransformer mA selector are occasionally used in cephalometric units; are usually used in cone-beam machines; and are always used in medical computed FIGURE 1-10 Schematic of dental x-ray machine circuitry and x-ray tube with the major tomography x-ray machines, which require high radiation output components labeled. The operator selects the desired kVp from the autotransformer. The voltage for longer, sustained exposures. is greatly increased by the high-voltage step-up transformer and applied to the x-ray tube. The kVp dial measures the voltage on the low-voltage side of the transformer but is scaled to display POWER SUPPLY the corresponding voltage in the tube circuit. The timer closes the tube circuit for the desired The primary functions of the power supply of an x-ray machine exposure time interval. The mA dial measures the current flowing through the tube circuit. The are to: filament circuit heats the cathode filament and is regulated by the mA selector.  C H A P T E R 1 Physics 7 to this tube current, typically about 10 mA, which is measured by cathode. When the polarity of the voltage applied across the tube the milliammeter. This current is not the same as the current causes the target anode to be positive and the filament to be flowing through the filament to heat it. negative, the electrons around the filament accelerate toward the positive target, and current flows through the tube (Fig. 1-11, B). Tube Voltage As the tube voltage is increased, the speed of the electrons A high voltage is required between the anode and cathode to give moving toward the anode increases. When the electrons strike the electrons sufficient energy to generate x rays. The actual voltage focal spot of the target, some of their energy converts to x-ray used on an x-ray machine is adjusted with the autotransformer photons. X rays are produced at the target with greatest efficiency (see Fig. 1-10). By using the kilovolt peak (kVp) selector, the opera- when the voltage applied across the tube is high. Therefore the m tor adjusts the autotransformer and converts the primary voltage intensity of x-ray pulses tends to be sharply peaked at the center from the input source into the desired secondary voltage. The of each cycle (Fig. 1-11, C). During the following half (or negative selected secondary voltage is applied to the primary winding of half) of each cycle, the filament becomes positive, and the target the high-voltage transformer, which boosts the peak voltage of becomes negative (see Fig. 1-11, B). At these times, the electrons o the incoming line current (110 V) up to 60,000 to 120,000 V (60 do not flow across the gap between the two elements of the tube. to 120 kV); this boosts the peak energy of the electrons passing This half of the cycle is called inverse voltage or reverse bias (see t.c through the tube to 60 to 120 keV and provides them sufficient Fig. 1-11, B). No x rays are generated during this half of the voltage energy to generate x rays. The kVp dial selects the peak operating cycle (see Fig. 1-11, C). When an x-ray tube is powered with voltage between the anode and cathode. Typically, intraoral, 60-cycle AC, 60 pulses of x rays are generated each second, each panoramic, and cephalometric machines (see Chapter 10) operate having a duration of 1120 second. This type of power supply cir- po between 60 and 90 kVp, whereas cone-beam computed tomo- cuitry, in which the alternating high voltage is applied directly graphic machines (see Chapter 11) operate at 90 to 120 kVp. across the x-ray tube, limits x-ray production to half the AC cycle Because the polarity of the line current alternates (60 cycles per and is called self-rectified or half-wave rectified. Almost all second), the polarity of the x-ray tube alternates at the same fre- conventional dental x-ray machines are self-rectified. gs quency (Fig. 1-11, A). Additionally, because the line voltage varies Some dental x-ray manufacturers produce machines that replace continuously, so does the voltage potential between the anode and the conventional 60-cycle AC, half-wave rectified power supply with a full-wave rectified, high-frequency power supply. This results in an essentially constant potential between the anode and cathode. lo The result is that the mean energy of the x-ray beam produced by +110V these x-ray machines is higher than the mean energy from a con- ventional half-wave rectified machine operated at the same voltage. Line voltage (V) y.b For a given voltage setting and radiographic density, the images resulting from these constant-potential machines have a longer A contrast scale, and the patient receives a lower dose compared with conventional x-ray machines. Figure 1-11, C, also shows that the tube current is dependent r -110V on the tube voltage; as the voltage increases between the anode and cathode, so does the current flow. The reason for this is subtle. ra 1/ 1/ 3/ When the hot filament releases electrons, it creates a cloud of 120 sec 60 sec 120 sec electrons around the filament, a negative space charge. This nega- Anode voltage (kV) +70kV tive space charge impedes the further release of electrons. The llib higher the voltage, the greater the removal of the electrons from B the space charge, and the greater the tube current. TIMER Inverse voltage (reverse bias) A timer is built into the high-voltage circuit to control the duration a -70kV of the x-ray exposure (see Fig. 1-10). The electronic timer controls the length of time that high voltage is applied to the tube and the nt time during which tube current flows and x rays are produced. X-radiation intensity However, before the high voltage is applied across the tube, the Tube current (mA) 10 X-ray intensity Tube current filament must be brought to operating temperature to ensure an adequate rate of electron emission. Subjecting the filament to de C continuous heating at normal operating current shortens its life. To minimize filament damage, the timing circuit first sends a Time current through the filament for about half a second to bring it to One impulse the proper operating temperature and then applies power to the 1/ 1/ 3/ high-voltage circuit. In some circuit designs, a continuous low-level 120 sec 60 sec 120 sec current passing through the filament maintains it at a safe low FIGURE 1-11 A, A 60-cycle AC line voltage at autotransformer. B, Voltage at the temperature, further shortening the delay to preheat the filament. anode varies from zero up to the kVp setting (70 kVp in this case). C, The intensity of radiation For these reasons, an x-ray machine may be left on continuously produced at the anode (blue) is strongly dependent on the anode voltage and is highest when during working hours. the tube voltage is at its peak. (Modified from Johns HE, Cunningham JR: The physics of radiol- Some x-ray machine timers are calibrated in fractions of a ogy, ed 3, Springfield, IL, 1974, Charles C Thomas.) second, whereas others are expressed as number of pulses in an 8 PART I Foundations Bremsstrahlung photon of maximal energy e e Altered path of deflected decelerated electron e e e e A B e e e m e Incident high- e Incident high- e energy electron e e Bremsstrahlung photon energy electron of lower energy o Direct-hit interaction Near-miss interaction t.c FIGURE 1-12 Bremsstrahlung radiation is produced by the direct hit of an electron on a nucleus in the target (A) or, much more frequently, by the passage of an electron near a nucleus, which results in electrons being deflected and decelerated (B). For the sake of clarity, this diagram and other similar figures in this chapter show only the 1s, 2s, or 3s orbitals. po exposure (e.g., 3, 6, 9, 15). The number of pulses divided by 60 (the frequency of the power source) gives the exposure time in Relative number of photons seconds. A setting of 30 pulses means that there will be 30 pulses gs of radiation, equivalent to a 0.5-second exposure. Characteristic TUBE RATING AND DUTY CYCLE radiation X-ray tubes produce heat at the target while in operation. The heat buildup at the anode is measured in heat units (HU), where HU lo = kVp × mA × seconds. The heat storage capacity for anodes of Bremsstrahlung radiation dental diagnostic tubes is approximately 20 kHU. Heat is removed y.b from the target by conduction to the copper anode and then to the surrounding oil and tube housing and by convection to the 10 20 30 40 50 60 70 Photon energy (keV) atmosphere. Each x-ray machine comes with a tube rating chart that describes FIGURE 1-13 Spectrum of photons emitted from an x-ray machine operating at 70 kVp. the longest exposure time the tube can be energized for a range of The vast preponderance of radiation is bremsstrahlung (blue), with a minor addition of charac- r voltages (kVp) and tube current (mA) values without risk of damage teristic radiation. to the target from overheating. These tube ratings generally do not ra impose any restrictions on tube use for intraoral radiography. However, if a dental x-ray unit is used for extraoral exposures, it is this happens, all the kinetic energy of the electron is transformed wise to mount the tube-rating chart by the machine for easy refer- into a single x-ray photon (Fig. 1-12, A). The energy of the resultant llib ence. Duty cycle relates to the frequency with which successive photon (in keV) is numerically equal to the energy of the electron— exposures can be made without overheating the anode. The inter- that is, the voltage applied across the x-ray tube at that instant. val between successive exposures must be long enough for heat More frequently, high-speed electrons pass by tungsten nuclei dissipation. This characteristic is a function of the size of the with near or wide misses (Fig. 1-12, B). In these interactions, the anode, the exposure kVp and mA, and the method used to cool electron is attracted toward the positively charged nuclei, its path a the tube. A typical duty cycle is 1 : 60, meaning that one could is altered toward the nucleus, and it loses some of its velocity. This make a 0.25-second exposure every 15 seconds. deceleration causes the electron to lose kinetic energy that is given nt off in the form of many new photons. The closer the high-speed PRODUCTION OF X RAYS electron approaches the nuclei, the greater the electrostatic attrac- tion between the nucleus and the electron, the braking effect, and Most high-speed electrons traveling from the filament to the target the energy of the resulting bremsstrahlung photons. The efficiency de interact with target electrons and release their energy as heat. of this process is proportional to the square of the atomic number Occasionally, these electrons convert their kinetic energy into x-ray of the target; high Z metals are more effective in deflecting the photons by the formation of bremsstrahlung radiation and char- path of the incident electrons. acteristic radiation. Bremsstrahlung interactions generate x-ray photons with a con- tinuous spectrum of energy. The energy of an x-ray beam is usually BREMSSTRAHLUNG RADIATION described by identifying the peak operating voltage (in kVp). For The sudden stopping or slowing of high-speed electrons by tung- example, a dental x-ray machine operating at a peak voltage of sten nuclei in the target produces bremsstrahlung photons, the 70 kVp applies a fluctuating voltage of up to 70 kVp across the primary source of radiation from an x-ray tube. (Bremsstrahlung tube. This tube therefore produces a continuous spectrum of x-ray means “braking radiation” in German.) Occasionally, electrons photons with energies ranging to a maximum of 70 keV (Fig. 1-13). from the filament directly hit the nucleus of a target atom. When The reasons for this continuous spectrum are as follows:  C H A P T E R 1 Physics 9 Characteristic Higher-energy– Recoil electron radiation (photon) level electron e Photoelectron Vacancy Incident high- energy photon e e e e e e e m e e e e o e e e e t.c A B C D FIGURE 1-14 Production of characteristic radiation. An incident electron (A) ejects an electron from an inner orbital creating a po photoelectron, a recoil electron, and an electron vacancy (B). C, An electron from an outer orbital fills this vacancy, and a photon is emitted with energy equal to the difference in energy levels between the two orbitals. D, Electrons from various orbitals may be involved, giving rise to other characteristic photons. The energies of the photons released are characteristic of the target atom. gs The continuously varying voltage difference between the target and filament causes the electrons striking the target to have 100 varying levels of kinetic energy. lo The bombarding electrons pass at varying distances around Relative number of photons tungsten nuclei and are thus deflected to varying extents. As a y.b 2-second exposure result, they give up varying amounts of energy in the form of bremsstrahlung photons. Most electrons participate in multiple bremsstrahlung interac- 50 tions in the target before losing all their kinetic energy. As a 1-second consequence, an electron carries differing amounts of energy exposure r after successive interactions with tungsten nuclei. ra CHARACTERISTIC RADIATION Characteristic radiation contributes only a small fraction of the 0 10 20 30 40 50 60 70 photons in an x-ray beam. It is made when an incident electron Photon energy (keV) llib ejects an inner electron from the tungsten target. When this happens, an electron from an outer orbital is quickly attracted to FIGURE 1-15 Spectrum of photon energies generated in an x-ray machine showing that the void in the deficient inner orbital (Fig. 1-14). When the outer as exposure time increases (kVp and tube voltage held constant), so does the total number of orbital electron replaces the displaced electron, a photon is emitted photons. The mean energy and maximal energies of the beams are unchanged. with energy equivalent to the difference in the binding energies of a the two orbitals. The energies of characteristic photons are discrete because they represent the difference of the energy levels of specific TUBE CURRENT (mA) nt electron orbitals and are characteristic of the target atoms. The quantity of radiation produced by an x-ray tube (i.e., the number of photons that reach the patient and film) is directly FACTORS CONTROLLING THE X-RAY BEAM proportional to the tube current (mA) and the time the tube is operated (Fig. 1-16). As the mA setting is increased, more power de An x-ray beam may be modified by altering the beam exposure is applied to the filament, which heats up and releases more elec- duration (timer), exposure rate (mA), energy (kVp and filtration), trons that collide with the target to produce radiation. Thus the shape (collimation), or intensity (target-patient distance). quantity of radiation produced is proportional to the product of time and tube current. The quantity of radiation remains constant EXPOSURE TIME (s) regardless of variations in mA and time as long as their product Changing the exposure time—typically measured in fractions of a remains constant. For instance, a machine operating at 10 mA for second (s)—modifies the duration of the exposure and thus the 1 second (10 mAs) produces the same quantity of radiation when number of photons generated (Fig. 1-15). When the exposure time operated at 20 mA for 0.5 second (10 mAs). In practice, some is doubled, the number of photons generated at all energies in the dental x-ray machines fall slightly short of this ideal constancy. The x-ray emission spectrum is doubled. The range of photon energies term beam quantity or beam intensity refers to the number of is unchanged. photons in an x-ray beam. 10 PART I Foundations TUBE VOLTAGE PEAK (kVp) FILTRATION Increasing the kVp increases the potential difference between the Although an x-ray beam consists of a continuous spectrum of x-ray cathode and the anode, increasing the energy of each electron photon energies, only photons with sufficient energy to penetrate when it strikes the target. The greater the energy of an electron, through anatomic structures and reach the image receptor (digital the greater the probability it will be converted into x-ray photons. or film) are useful for diagnostic radiology. Low-energy photons Increasing the kVp of an x-ray machine increases: that cannot reach the receptor contribute to patient risk but do The number of photons generated. not offer any benefit. Consequently, it is desirable to remove these The mean energy of the photons. low-energy photons from the beam. This removal can be accom- m The maximal energy of the photons (Fig. 1-17). plished in part by placing a metallic disk (filter) in the beam path. The term beam quality refers to the mean energy of an x-ray beam. A filter preferentially removes low-energy photons from the beam, Exposure time, tube current (mA), and tube voltage are the while allowing high-energy photons that are able to contribute to three controls found on many x-ray machines. In some machines, making an image to pass through (Fig. 1-18). o the setting of the tube current, the setting of the tube voltage, or Inherent filtration consists of the materials that x-ray photons both is fixed. It is recommended that if the tube current is vari- encounter as they travel from the focal spot on the target to form t.c able, the operator select the highest mA value available and always the usable beam outside the tube enclosure. These materials operate the machine at this setting; this allows the shortest exposure include the glass wall of the x-ray tube, the insulating oil that sur- time and minimizes the chance of patient movement. Similarly, rounds many dental tubes, and the barrier material that prevents if tube voltage can be adjusted, it is recommended that the opera- the oil from escaping through the x-ray port. The inherent filtra- po tor select a desired voltage, perhaps 70 kVp, and leave the machine tion of most x-ray machines ranges from the equivalent of 0.5 to at this setting. This protocol simplifies selecting the proper patient 2 mm of aluminum. Added filtration may be supplied in the form exposure settings by using just exposure time as the means to of aluminum disks placed over the port in the head of the x-ray adjust for anatomic location within the mouth and patient size. machine. Total filtration is the sum of the inherent and added gs 100 100 lo Relative number of photons Relative number of photons y.b Nonfiltered beam 20 mA 50 Filtered beam 50 (AI filter) 10 mA r ra 0 0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Photon energy (keV) llib Photon energy (keV) FIGURE 1-18 Spectrum of filtered x-ray beam generated in an x-ray machine showing FIGURE 1-16 Spectrum of photon energies generated in an x-ray machine showing that that an aluminum filter preferentially removes low-energy photons, reducing the beam intensity, as tube current (mA) increases (kVp and exposure time held constant), so does the total number while increasing the mean energy of the residual beam. Compare with Figures 1-15, 1-16, and of photons. The mean energy and maximal energies of the beams are unchanged. 1-17. a nt 100 Relative number of photons 100 kVP de 75 FIGURE 1-17 Spectrum of photon energies generated in 90 kVP an x-ray machine showing that as the kVp is increased (tube 80 kVP current and exposure time held constant), there is a correspond- 50 ing increase in the mean energy of the beam, the total number of photons emitted, and the maximal energy of the photons. Compare with Figures 1-15 and 1-16. 25 0 10 20 30 40 50 60 70 80 90 100 Photon energy (keV)  C H A P T E R 1 Physics 11 A B m Beam restricted Beam restricted to circle to rectangle o FIGURE 1-19 Collimation of an x-ray beam (blue) is achieved by restricting its useful size. A, Circular collimator. B, Rectangular t.c collimator restricts area of exposure to just larger than the detector size and thereby reduces unnecessary patient exposure. filtration. Governmental regulations require the total filtration in po the path of a dental x-ray beam to be equal to the equivalent of 1.5 mm of aluminum for a machine operating at up to 70 kVp and 2.5 mm of aluminum for machines operating at higher voltages (see Chapter 3). gs COLLIMATION A collimator is a metallic barrier with an aperture in the middle used to restrict the size of the x-ray beam and the volume of tissue lo irradiated (Fig. 1-19). Round and rectangular collimators are most FIGURE 1-20 Intensity of an x-ray beam is inversely proportional to the square of the frequently used in dentistry. Dental x-ray beams are usually colli- distance between the source and the point of measure. When the distance from the source to mated to a circle 2 3 4 inches (7 cm) in diameter at the patient’s face. y.b a target is doubled, the intensity of the beam decreases to one quarter. A round collimator (see Fig. 1-19, A) is a thick plate of radiopaque material (usually lead) with a circular opening centered over the port in the x-ray head through which the x-ray beam emerges. Typically, round collimators are built into open-ended aiming where I is intensity and D is distance. If a dose of 1 Gy is measured cylinders. Rectangular collimators (see Fig. 1-19, B) further limit at a distance of 2 m, a dose of 4 Gy would be found at 1 m, and r the size of the beam to just larger than the x-ray film, further a dose of 0.25 Gy would be found at 4 m. reducing patient exposure. Some types of film-holding instruments Changing the distance between the x-ray tube and the patient, ra also provide rectangular collimation of the x-ray beam (see Chap- such as by switching from a machine with a short aiming tube ters 3 and 7). to one with a long aiming tube, has a marked effect on skin Collimators also improve image quality. When an x-ray beam exposure. Such a change requires a corresponding modification llib is directed at a patient, the hard and soft tissues absorb about of the kVp or mA to keep constant the exposure to the film or 90% of the photons, and about 10% pass through the patient digital sensor. to reach the film. Many of the absorbed photons generate scat- tered radiation within the exposed tissues by a process called Compton scattering (see later in chapter). These scattered photons INTERACTIONS OF X RAYS WITH MATTER a travel in all directions, and some reach the film and degrade In dental imaging, the x-ray beam enters the face of a patient, image quality. Collimating the x-ray beam thus reduces the interacts with hard and soft tissues, and strikes a digital sensor or nt exposed volume and thereby the number of scattered photons film. The incident beam contains photons of many energies but is reaching the film, resulting in reduced patient exposure and spatially homogeneous. That is, the intensity of the beam is essen- improved images. tially uniform from the center of the beam outward. As the beam goes through the patient, it is reduced in intensity (attenuated). de INVERSE SQUARE LAW This attenuation results from absorption of individual photons in The intensity of an x-ray beam (the number of photons per cross- the beam by atoms in the absorbing tissues or by photons being sectional area per unit of exposure time) depends on the distance scattered out of the beam. In absorption interactions, photons of the measuring device from the focal spot. For a given beam, the ionize absorber atoms, convert their energy into kinetic energy of intensity is inversely proportional to the square of the distance the ejected electron, and cease to exist. In scattering interactions, from the source (Fig. 1-20). The reason for this decrease in intensity photons also interact with absorber atoms but then move off in is that an x-ray beam spreads out as it moves from its source. The another direction. The frequency of these interactions depends on relationship is as follows: the type of tissue exposed (e.g., bone vs. soft tissue). Bone is more likely to absorb x-ray photons, whereas soft tissues are more likely I1 (D2 )2 to let them pass through. Although the incident beam striking = I2 (D1)2 the patient is spatially homogeneous, the remnant beam—the 12 PART I Foundations attenuated beam that exits the patient—is spatially heterogeneous because of differential absorption by the anatomic structures COHERENT SCATTERING through which it has passed. This differential exposure of the film Coherent scattering (also known as Rayleigh, classical, or elastic or digital sensor forms a radiographic image. scattering) may occur when a low-energy incident photon In a dental x-ray beam, there are three means of beam (