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471RADIOLOGIC TECHNOLOGY, May/June 2019, Volume 90, Number 5

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Radiation Physics, Biology, and ProtectionEric P Matthews, PhD, R.T.(R)(CV)(MR), ENT

The production of x-ray photons for use in diagnostic imaging, how those photons interact with the body and environ-

ment, and the best means of protecting people from their effects are standard content areas that must be known and understood to pass the American Registry of Radiologic Technologists’ radiography certification examination.1 However, with the probable exception of general radiation protection concepts, these content areas might fade into dis-tant memory once technologists become certified. Because the content areas are still relevant, technologists should review the information periodically.

Radiation PhysicsRadiation has existed since the

coalescence of the gas cloud that would eventually result in our planet. However, the intentional use of radia-tion to produce diagnostic images for use in medicine did not begin until the end of the 19th century. After the

precipitous discovery by Wilhelm Roentgen in November 1895, medicine was quick to realize the possibilities of x-rays in patient diagnoses. After initial forays into diagnostic imaging, aware-ness for the propensity of radiation to damage human tissue began to grow.2 As knowledge regarding tissue damage grew, study became more focused on the production of those photons and the means to limit the damage they could cause.

In nature, 2 fundamental properties exist: matter and energy. The interaction of matter and energy forms the premise on which photon production is based.

Basic Principles of MatterMatter is defined as “the substance

of which a physical object is composed” or any “material substance that occu-pies space, has mass, and is composed predominantly of atoms consisting of protons, neutrons, and electrons, that constitutes the observable universe, and that is interconvertible with energy.”3

After completing this article, the reader should be able to:�� State the principles of matter and energy.�� Discuss types of ionizing radiation and explain what differentiates them.�� Describe a basic diagnostic imaging system.�� Outline the x-ray production process and how x-ray photons interact with matter.�� Explain the effects of radiation on biologic tissue.�� List radiation dose limits for radiation workers and ways to limit radiation exposure.

During initial education for primary pathway certification, radiologic science professionals learn the physics of the electromagnetic spectrum, mass, and energy. They also learn the principles of x-ray production (including requisite background information in general physics), human biology, radiation biology, and radiation protection. This information can be forgotten as time lengthens from initial learning, and reviewing these topics reminds technologists of the basic premise on which the profession is founded.

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department, machines are designed to convert electrical energy into electromagnetic energy. The electromagnetic energy produced in an x-ray tube exists at a specific fre-quency, wavelength, and energy, which differentiates it from all other electromagnetic energy. A unique property of electromagnetic energy is that it can be transmitted and transferred through space. When this happens, the energy is called radiation. Electromagnetic energy, electro-magnetic radiation, and radiation are all interchangeable terms. When matter comes into contact with radiation and subsequently absorbs all or part of the radiated ener-gy, that object is irradiated.6,7

Electromagnetic energy exists in 2 forms: ionizing radiation and nonionizing radiation. The difference between the 2 types of radiation devolves to the energy each possesses, and how that energy interacts with matter. Electromagnetic radiation is considered ionizing radia-tion when it possesses sufficient energy to eject an orbital electron from an atom in matter.8 Nonionizing radiation is common and generally is integrated into daily life: ultra-violet radiation is encountered in the sun’s rays; visible light enables people to see; microwaves and radio waves are responsible for television, heating food, radio stations, and communication systems.9

Ionizing radiation is equally common, but not so integral to the daily lives of most people. Ionizing radia-tion exists in the form of gamma rays, high energy ultraviolet radiation, and x-rays. Ultraviolet rays have a frequency of 1 PHz to 100 PHz, a wavelength of 300 nm to 3 nm, and an energy of 4 eV to 400 eV.2 Gamma rays have a frequency exceeding 100 EHz, a wavelength of 3 am to 0 am, and an energy that is at least 400 keV.2 X-rays have a frequency of 100 PHz to 100 EHz, a wavelength of 3 nm to 3 am, and an energy that is 0.4 keV to 400 keV.2 Because of the high energy these ionizing electromagnetic energy forms possess, they can be damaging to biological life forms. In addi-tion to the forms of ionizing electromagnetic radiation that exist, there are 4 types of particulate radiation that might result in ionizing radiation: alpha particles, beta particles, neutrons, and positrons.7

Gamma RaysGamma rays are a type of ionizing electromagnetic

radiation. Together with x-rays, gamma rays are photons.

As the definition implies, atoms and molecules are the fundamental building blocks of matter. Bushong noted that the distinguishing characteristic of matter is mass.4

Mass is the quantity of matter contained in an object, and weight is the force being exerted on any body that is under the influence of gravity. For the sake of simplicity, mass and weight are used interchange-ably in this article. By using the common term weight, matter can be defined more easily; matter must weigh something to exist. The common unit of measurement for mass is the kilogram (kg).4

Basic Principles of EnergyEnergy exists in many forms. Potential energy

describes the ability of an object to do work simply by virtue of its position. A ball resting at the top of a ramp has potential energy. If the ball is put into motion and begins rolling down the ramp, then it has kinetic energy. Chemical energy is the energy released as a byproduct of the reaction of chemicals; digestion is an example of chemical energy. Electrical energy is the release of energy that occurs when an electron moves through the stages of potential difference.4

Thermal energy is simply kinetic energy occur-ring at the molecular level. As molecules move, they release heat as a product of their vibration. The more they vibrate, the higher the temperature released. Electromagnetic energy is the energy released when 1 type of energy is converted to another; visible light, microwaves, and radio waves are all electro-magnetic energy. The international unit for energy is the joule (J). However, the joule is not the only way to measure energy. For example, in radiology the electron volt (eV) typically is used to measure energy.4

Ionizing RadiationIonizing radiation exists in 2 forms: natural and

man-made. These forms might be subclassified as either electromagnetic or particulate. Each type of radiation acts on biological tissue in the same ways, but they dif-fer in origin and construction.5

Electromagnetic EnergyElectromagnetic radiation is produced when energy

is transformed from 1 state to another. In the imaging


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A neutron is a high-speed particle that originates in an atom’s nucleus. Typically, neutrons are released through the process of fission. Neutrons are the only type of particulate radiation that can imbue radioactiv-ity to matter with which they interact, a process called neutron activation.11 Neutrons can travel a great distance and typically require a hydrogen shield (such as water) to contain them.

Alpha and beta particles are the most common type of particulate radiation. Both types of particles are gen-erated as an effect of the radioactive decay of atoms. As the atom decays, alpha and beta particles are ejected in the form of rays.

Alpha rays originate in the nucleus of heavy elements and contain 2 protons and 2 neutrons. Effectively, alpha rays are helium nuclei, which is to say they are a helium atom less its electrons. The mass of an alpha particle is large, roughly 4 atomic mass units. They also pos-sess a strong positive charge that comes with a kinetic energy of 4 MeV to 7 MeV. This imbues them with the potential for transferring a large amount of energy to the orbital electrons with which they come in con-tact.10,14 The net effect is that alpha particles typically are accompanied by ionization. In fact, a single alpha particle will ionize nearly 40 000 atoms per centimeter of air it travels through.14

Alpha particles do not travel far because of the massive amounts of energy they transfer. In air, alpha particles travel approximately 2 inches; however, in skin they can travel only microscopic distances. In fact, alpha particles impart so much kinetic energy to the atoms they come in contact with that they are easily shielded. These characteristics mean that alpha particles can irradiate, with dramatic effect, any tissue they might be deposited in, but they can be shielded by something as simple as a sheet of paper.11,14

Like alpha particles, beta particles originate in the nucleus of an atom. However, although alpha particles carry a strong positive charge and have large mass, beta particles generally are negatively charged and have a rel-atively low mass. In fact, they only differ from electrons in their point of origin (electrons originate in the orbital shells of atoms).10 Because beta particles are lighter than alpha particles, they do not impart as much of their kinetic energy on the atoms they encounter. They can

Photons have no mass, travel at the speed of light, are high energy, and can travel long distances. However, they dif-fer from x-rays in their source. X-rays are produced in the electron shells; gamma rays share their origin with alpha and beta particles in the nucleus of the atom.10

X-raysAt any given time, thousands of x-ray machines are in

use, producing millions of images every day. This gives medical imaging its title as the single largest source of man-made radiation on the planet.11-13 Man-made radia-tion accounts for the majority of radiation exposure absorbed by everyone worldwide; therefore, the aver-age dose of man-made radiation derived from medical imaging procedures and absorbed by people worldwide accounts for the majority of radiation exposure.7

X-rays, like all electromagnetic radiation, are photons. They exist without mass, as pure energy.10 Traveling at 3 3 108 m/s—the speed of light—photons are invisible to the naked eye; however, they do produce electric and magnetic fields as they travel. These electri-cal and magnetic fields are changing continuously in sinusoidal fashion. Because the travel of photons follows a sine wave and exists within the laws of mathematics, photons have multiple applications in the world of phys-ics, including radiography.6 If one were to define sine waves, they might simply state they are “variations of amplitude over time.”6 Therefore, the properties of the sine waves created by electromagnetic energy, and what governs the damage they elicit in biological tissue, are a function of the photon’s velocity, its frequency, and wavelength.

Particulate RadiationParticulate radiation is any subatomic particle capa-

ble of ionizing further particles. The most important factor that differentiates them from electromagnetic radiation is that they are incapable of ionizing unless they have been acted on and set in motion. Effectively, any subatomic particle might become particulate radia-tion depending on the energy imparted to it. Protons, neutrons, and alpha and beta particles typically are the most encountered types of particulate radiation.7,11,14 However, they are not the most commonly encountered type of radiation overall.


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parts of the console operate at low voltage to minimize the chance of electrical shock. Imaging systems generally operate at 220 V, but some are capable of operating at 110 V or 440 V. Most power grids can support continuous and uninterrupted power at any voltage, let alone 220 V; there-fore, a line voltage compensator is integral to the circuitry of the imaging system to provide a constant 220 V.16,17

Incoming power to the imaging system is delivered initially to the autotransformer. The autotransformer “has a single winding and is designed to supply a precise voltage to the filament circuit and to the high-voltage circuit of the x-ray imaging system.”18 The autotrans-former is an induction transformer that provides secondary voltage in direct proportion to the primary voltage it receives. The voltage is governed by the num-ber of turns that the connections of the transformer encompass. The kilovoltage peak (kVp) meter of most imaging systems is connected to the output circuit of the autotransformer and reads voltage; however, it reg-isters kVp because of the known conversion that takes place in the high-voltage generator.17

Milliamperage (mA) is governed at the control con-sole and regulates the f low of electrons from cathode to anode. The number of electrons that actually are produced and emitted by the filament corresponds to the temperature of the filament. When the technologist selects the mA for an exposure, he or she is setting the current that f lows to the filament, which determines how hot it will get when electrified. Most filaments operate at currents in the 3 A to 6 A range. The higher the amperage used, the hotter the filament wire, and the more electrons will be released through a process called thermionic emission.17,19 The current f lowing to the filament is set in a separate circuit called the filament cir-cuit. In addition, there is a secondary circuit in the mA selection process that offsets the space charge effect. The space charge effect occurs as the kVp increases and the anode becomes more attractive to the electrons in the stream that would not typically have sufficient energy to leave the filament cup. As electrons that would not normally cross the tube join the stream and produce photons, the effective mA and kVp increase, even though the actual mA and kVp do not.17

The filament heating isolation step-down transform-er (filament transformer) steps down the line voltage

travel further through air (up to 40 inches) and can penetrate biological tissue to a greater depth than alpha particles.11

Basic Imaging SystemX-ray tubes work on a relatively simple principle.

High-energy, fast-moving electrons are released from a cathode and accelerated across a vacuum tube. They collide with an anode and release their kinetic energy, which is transformed into electromagnetic energy in the form of x-rays.15 The x-rays that are produced are lim-ited only by the energy of the incident electrons, which is equal to the voltage of the tube, multiplied by the electron charge. This mathematical formula is used to define the imaging system. Diagnostic imaging systems typically operate at 25 kVp to 150 kVp, with a tube cur-rent of between 100 mA and 1200 mA.16

In addition to the functional production of x-rays, imaging systems must include mechanisms to support the patient during examinations. It is not uncommon for patient tables to be free-f loating and movable in 3 dimensions. Examination tables include an image receptor, or a tray for an image receptor, and are constructed to a uniform thickness to decrease the pos-sibility of variable attenuation of the x-ray beam.

The x-ray tubes that accompany most diagnostic imaging systems act in concert with the control con-sole and the high-voltage generator. Certain imaging systems have these components separated by some dis-tance, though other systems (such as those for mobile imaging) have them in a very compact space. The actual location and layout of components is proprietary to manufacturers, but the construction and work of the components remains constant.

The portion of the x-ray system that is most familiar to radiographers is the control console. The control console regulates the voltage and amperage of the x-ray tube to govern the production of photons. The amper-age selected by the radiologic technologist governs the radiation quantity, which is responsible for the number of x-rays produced in the beam. Radiation quality is governed by the voltage used and regulates the x-ray penetrability of the beam.17

The control console has meters or gauges in addition to the selector controls for voltage and amperage. These


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in half. Full-wave rectifiers simply take the half of the wave-cycle represented by the reversed current in alter-nating current power systems and reverse it, providing direct current (see Figure 2).17

However, even the use of full-wave rectification does not provide constant power to the x-ray tube because of the sinusoidal waveform of electron f low. To further increase the usable number of electrons being produced, x-ray imaging systems use 3-phase power to superimpose one wave-form over another (see Figure 3). Coupled with rectification, 3-phase power results in nearly constant power being supplied to the x-ray tube, with minimal voltage dip between wave peaks (see Figure 4). In countries with power grids operating at a different frequency, the process is the same, with the exception that the incoming alternat-ing current is not 60 Hz.17

X-ray TubesX-ray tubes need only 3 components to function:

evacuated glass envelope (vacuum tube), a source of electrons, and a target material.19 However, to make an efficient x-ray tube requires a slightly more complex design. Typically, diagnostic imaging machines have a

to about 5% of its original strength and provides the current necessary to heat the filament. The step-down transformer is heavily insulated. This is important because the secondary windings carry about 5 A to 8 A at 12 V of electric potential.17

A critical component of the imaging system is the exposure timer. Simply put, the timers govern the length of time that the x-ray tube is energized. In turn, this regulates the number of photons produced by limiting the length of time for the production of electrons at the cathode. It is important to remember that technologists begin the production of electrons and the exposure, but the timing circuit stops the production and exposure. The timing circuit has a primary timer and a secondary timer (sometimes called a back-up timer or a guard timer). In modern radiographic machines, most exposure timers are computer-controlled electronic timers.17

The high-voltage generator of diagnostic x-ray imag-ing systems is responsible for increasing the voltage, to a point where sufficient kinetic energy is produced to generate x-ray photons at the anode. The high-voltage generator has many parts, but the most important is the high-voltage transformer. The high-voltage transformer has more windings on the secondary side than the pri-mary side; as such it is a step-up transformer.

For an x-ray tube to function, it must be provided direct current. In the imaging system, this occurs in the voltage rectifier, housed in the high-voltage generator. The rectifier converts alternating current at 60 Hz (in the United States) to direct current. The sole purpose of the rectifier is to guarantee that the current in the x-ray tube f lows in 1 direction only, from cathode to anode. Rectifiers are constructed using diodes, which are elec-trical devices that contain 2 electrodes each. Although it is possible for the tube to serve as a rectifier in some systems, this is inefficient because a single diode results in half-wave rectification (see Figure 1).17

Half-wave rectification uses half the cycle in the line voltage and wastes half of the electrons. Therefore, to produce the amount of electrons needed (and by asso-ciation, the number of photons) for an exposure, the time must be doubled. Full-wave rectified imaging sys-tems are much more efficient because they allow for use of the entire electron stream, cutting exposure times

1/120 seconds

1/30 seconds

Figure 1. Sixty hertz half-wave rectified power waveform. Image courtesy of author.

Figure 2. Sixty hertz full-wave rectified power waveform. Image cour-tesy of author.


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The x-ray tube is contained in a vacuum tube. A vacuum is not required to produce x-rays, but electrons travel more efficiently in a vacuum; therefore, vacuum tubes allow more electrons to make it from the cathode to the anode. Consequently, more photons can be pro-duced in a vacuum tube. Further, less heat is produced in a vacuum. Overall, the vacuum tube exponentially increases the efficiency of the process.21

In the x-ray tube itself, the cathode is the negative side of the functional components. There are 2 primary parts of the cathode, the filament and a focusing cup system (see Figure 5).17 Most modern diagnostic imag-ing filaments are made of thoriated tungsten. When a current is introduced to the filament, it heats up. The heating causes electrons to boil off the filament and be ejected from the filament wire. They are contained in the focusing cup by a negative electrical charge using the basic premise of like charges repelling. The electro-statically contained electrons are held until the filament cup is saturated, at which point they are released. A by-product of the emission of electrons from the filament is tungsten vaporization. The vaporized tungsten deposits on the inside of the tube glass and is the leading cause of x-ray tube failure.17

Opposing the cathode is the positive side of the x-ray tube, or the anode. The anode might be stationary or rotating; however, rotating anodes are more common. By rotating the anode, the target area for the electron stream is distributed across a greater area of the anode face (approximately 500 times greater), increasing lon-gevity for the whole tube. The anode converts about 1% of the kinetic energy of the electron stream into photons; the remainder is converted to heat. Therefore, the anode must be able to dissipate heat, and rotating the anode also aids in this process. An electromagnetic induction motor is used to rotate the anode by sequenc-ing the stators on the outside of the glass tube, allowing interaction with the rotor on the inside of the glass tube. The magnets in the stators are energized, causing the magnetic rotor to spin in the magnetic field as the sta-tors are sequentially energized.17,19

X-ray ProductionMuch like x-ray tubes, x-ray production is rela-

tively simple; however, it is never straightforward.

tube housing and a support system, and a vacuum tube that contains a heated filament (electron source) and a rotating anode (for cooling); however, the design of these elements might vary by manufacturer.

The external components of the x-ray tube are the protective housing and the support. The support sys-tem is either ceiling mounted, a f loor-to-ceiling system, or a C-arm system. Each system exists solely to sup-port the heavy weight of the tube housing and help it align with the image receptor. The tube housing itself is heavy because of its construction and primary function as a shield to stop leakage radiation.

Leakage radiation is any radiation not directed toward the patient and emitted through the tube win-dow. In fact, the majority of the photons produced are not directed toward the patient. This is because radia-tion is emitted isotropically and travels in all directions from the face of the anode. A well-designed protective housing reduces the amount of leakage radiation to less than 1 mGya/h at 1 m, even when operated at maximum output.20

+V

V1

V1

V2

V2

V3

V3

-V

0

120º 120º 120º

120º

120º

120ºTime

output voltage

Figure 3. Three-phase power waveform. © 2019 ASRT. Abbreviation: V, voltage.

Figure 4. Three-phase full-wave rectification (3-wave 6-pulse) power waveform. © 2019 ASRT.


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nominal rotational speed of the anode is achieved, the technologist might make the exposure, which generates the photons. When the exposure is made, high voltage is applied to the cathode and anode, causing an attrac-tion between the anode and the electron cloud being held in the focusing cup.

The electrons are accelerated across the x-ray tube and strike the target on the anode. The electrons that make the journey across the tube and strike the anode are known as projectile electrons. When the electrons strike the anode, they transfer their kinetic energy to the atoms in the target and produce heat or x-ray pho-tons.17,19

X-ray EmissionThe process of generating x-rays in diagnostic imag-

ing systems requires the production of electrons with sufficient energy to produce 2 types of x-rays. The collisions that occur between projective electrons and the target material generate characteristic or bremsstrahlung radiation. Characteristic radiation is defined by the type of target material used to produce it. Bremsstrahlung (which literally means braking or

Fundamentally, the only things needed to produce an x-ray are a source of electrons, a means of accelerat-ing those electrons to sufficient speed to imbue them with high energy, and a target with which the electron stream can interact. This is an oversimplified descrip-tion; the actual process that results in the production of x-rays is more complex and outside the scope of this article.

The first step in producing x-ray photons is for the technologist to partially depress the exposure button and prespin the anode. As the rotor button is depressed (which is sometimes the first step in a multistage switch), the stators are sequentially charged, causing the rotor to spin. The rotor continually increases in speed until it is revolving at least 3000 rpm. Concurrently, the filament heats and begins to glow. As the incandescence of the cathode increases, thermionic emission begins. The electrons that are boiled off the filament during the process of thermionic emission form a cloud (or space charge) outside the filament. The negatively charged electrons are held in place adjacent to the filament by the negatively charged focusing cup. Once the appropri-ate number of electrons have been generated and the

Figure 5. X-ray tube components. © 2016 ASRT.

Rotating anodeGlass enclosure

Window

Cathode assembly

Target

Focusing cup

Filament


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modern imaging machines are produced through a dif-ferent process known as bremsstrahlung radiation.17

Bremsstrahlung radiation is the by-product of projec-tile electrons passing through an atom without coming in contact with an orbital shell electron; however, they pass sufficiently close to the nucleus to have their direction of travel altered and the speed at which they are moving slowed. This occurs because the electrons are negatively charged, and the nucleus is positively charged; therefore, the incipient electrons come into contact with the electrical field of the nucleus and it acts on them. As the electrons are turned and slowed, they might lose some of their energy, all of their energy, or none of their energy. The result is that a projectile electron with an energy of 120 kVp can produce an x-ray photon of anywhere from 0 keV to 120 keV. The photons produced by this process become higher in energy as the kVp is set higher because more electrons have sufficient energy to pass closer to the nucleus. The closer the electron passes to the nucleus before being braked and turned, the greater the energy release from that electron.17,19,22

Regardless of their source of origin, either as char-acteristic or bremsstrahlung photons, x-rays all have defining characteristics. They exist on a continuous spectrum; they have a broad range of energy from 0 kVp to the maximum kVp the technologist selected in mak-ing the exposure.16 They are penetrating, they have no charge, travel at the speed of light, and are capable of ionizing tissue. Unlike other electromagnetic radia-tion (eg, light), they cannot be focused but travel in a straight line. They interact with phosphors and cause f luorescence or luminescence. They elicit chemical and biological effects and can produce secondary or scatter radiation.19

X-ray Interaction With MatterX-rays interact with matter through coherent scatter-

ing, Compton scattering, photoelectric absorption, pair production, and photodisintegration. The interaction that occurs corresponds to the energy of the photon. Low-energy x-rays tend to interact with the whole atom. Photons with midrange energies tend to interact with the orbital electrons. High-energy photons tend to interact with the nucleus.17

braking beam) radiation is produced when projectile electrons interact with the target material and are rap-idly slowed.16,17,19

In the diagnostic imaging tube, the target is tung-sten, resulting in x-rays produced that are characteristic of that material. A tungsten atom has 6 orbital shells, which are labeled K, L, M, N, O, and P. K-shell electrons are closest to the nucleus, and P-shell electrons are fur-thest away. The attraction, or binding energy, of each electron shell decreases the further it is located from the nucleus.22 The binding energy of tungsten electron shells ranges from approximately 69 keV in the K-shell, to 12 keV in the L-shell, 3 keV in the M-shell, and 1 keV or less in the remaining shells.17

Characteristic radiation is produced as an effect of projectile electron interaction with inner-shell atomic electrons. The incoming projectile electron has suf-ficient kinetic energy to eject an inner-shell electron. Following the normal laws of physics, the void left by the ejected electron must be filled by an outer-shell electron. Recall that atoms must have full K-shells before they fill an L-shell, and full L-shells before they fill an M-shell, and so on. Outer-shell electrons migrate to inner shells to fill the void. When the electrons transition from an outer shell to an inner shell, they emit energy in the form an x-ray photon. The actual energy of the photon might be calculated by establishing the difference in binding energies between the shells. For example, an L-shell electron moving to fill a K-shell void would release a photon with 57 keV of energy. The photon energy is determined by subtracting the L-shell binding energy (12 keV) from the K-shell binding energy (69 keV). The process is the same regardless of which orbital shell electron is removed, and which migrates; however, only K-shell electrons produce photons of sufficient energy to be of diagnostic imaging value.17,19,22

To summarize useful characteristic radiation, a high-velocity projectile electron collides with a K-shell electron, which is ejected from the tungsten atom. An outer-shell electron moves to fill the void, and in doing so releases an x-ray photon at specific energy.19 However, the peak energy that characteristic radia-tion can produce is 57 kVp. Many imaging procedures require sufficiently higher energy photons to be diag-nostic; therefore, the majority of the useful photons in


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characteristic x-rays produced from the photoelectric effect are low-energy photons that contribute nothing to the imaging process and generally do not make it to the receptor.24 However, because the newly created pho-tons are not exiting the body, they increase the risk of biologic damage.23

Pair production and photodisintegration do not occur in the diagnostic energy range. Pair production occurs at energies over 1.02 MeV, an energy range never seen in imaging. However, pair production is unique in that the energy of the incident x-ray is suf-ficiently strong to cause the photon to completely disappear and to cause the creation of 2 electrons in its place. Photodisintegration occurs at energies at or above 10 MeV. When energies exceed 10 MeV, the incident photon can interact directly with the nucleus of an atom. The nucleus becomes unstable when it absorbs that much energy, and emits some nuclear fragment in an attempt to stabilize. Typically, the fragment is a neutron; however, protons or alpha particles also can be emitted during this type of interaction.23,24

RadiobiologyThe known effects of radiation include surface

burns, cataracts, and cancer. Each of the known effects of radiation involve cellular damage in the tissue with which it interacts. To fully appreciate how that damage occurs, a review of basic human biology is important before advancing to the more in-depth examination of radiation biology and the effects of photon interactions in human tissue.

General Human BiologyA cell is the basic structural and functional unit of

all biological life forms. The human body contains tril-lions of individual cells working together to form tissue, which builds organs that constitute body systems. Cells cause muscles to contract, conduct nerve impulses, transport nutrients and waste in the body, and provide support to the organs of which they are composed. Cells are capable of movement, growth and repair, reacting to stimuli, protection, and reproduction. All human cells are self-contained in a cellular membrane that encloses the cytosol and organelles.25

Coherent scattering occurs when x-rays below 10 keV are produced. First described by JJ Thompson, this type of scattering occurs when the incident x-ray interacts with an atom and induces excitation in the atom. The excitation is released in the form of energy, which is equal to that of the incoming photon. Coherent radia-tion always ejects itself from the secondary atom in a different direction than the incident photon. However, little effect is elicited in the diagnostic range of ener-gies.17,23 Effectively, “the process of coherent scattering is of no importance in any energy range…[because] no ionization of the biological atom occurs.”23

Compton scattering reduces the energy of the x-ray and ionizes the secondary atom. In fact, Compton scat-ter is responsible for the majority of scatter produced during diagnostic imaging procedures.23 Compton scatter originates with an incident photon interact-ing with the orbital electrons of a target atom. The incident photon has sufficient energy to remove an outer-shell electron from the atom. When the electron is dislodged from the atom, the energy of the incident photon is reduced, and the atom is ionized. The energy of the Compton radiation that is produced is equal to the difference in energy of the incident photon and the ejected electron.17 The Compton electron then is free to interact with other electrons, causing secondary ioniza-tion, until it depletes enough energy to combine with an atom that needs an outer-shell electron to stabilize.23

Photoelectric absorption, sometimes called pho-toelectric effect, is the process of incident photons interacting with inner-shell electrons. Photoelectric absorption, as the name implies, does not result in scat-ter; rather, the incident photon is absorbed completely by the atom when it transfers its energy to an inner-shell electron, causing it to be ejected from the atom. The exiting photoelectron possesses kinetic energy equal to the difference between the incoming photon and its binding energy.23,24 When the inner-shell electron has been vacated, it fills from the outer-shell electrons. This process creates additional characteristic x-ray photons, just as in the x-ray tube. The characteristic of each new photon (energy) directly is correlated to the atom from which it is created. In general, as each electron gives up its place on an outer shell and moves inward, addi-tional characteristic photons are emitted. However, the


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Mitochondria (the plural form of mitochondrion) are membranous, bean-shaped structures that func-tion in the transformation of energy in the cell. Inside a mitochondrion, proteins, enzymes, and other molecules provide the basis for cellular respiration. They accom-plish this by converting stored energy into adenosine triphosphate. Adenosine triphosphate is the source of usable cellular energy. The more a cell is metabolically active, the more mitochondria it has. For example, bone cells have relatively few mitochondria, but muscle cells are full of them.25

Microtubules function as pathways that chromo-somes and other cellular components use to facilitate communication in the cell and among cells. One pair of microtubules, the centrioles, lie near the nucleus and assist in cellular reproduction and division. Other smaller organelles in the cytoplasm include the per-oxisomes, which are responsible for the production and storage of enzymes that are useful in cellular defense (particularly against poisons such as alcohol). Technologists should know peroxisomes also are responsible for neutralizing hydrogen peroxide and free radicals, both of which are known cellular responses to radiation.25

Basic Principles of RadiobiologyThe branch of biology that is focused on the effects

of ionizing radiation is radiation biology. Understanding that the human body is composed of cells that, when combined, constitute a living system is crucial to rec-ognizing the foundational principles of radiobiology. Within a few years of Roentgen’s discovery, the possibil-ity of radiation-induced damage to human tissue was known. The radiosensitivity of living cells has been well known, described, and discussed since at least 1906.

In 1906 Bergonie and Tribondeau described the con-cept that any immature cell, which was undifferentiated and actively dividing, was much more radiosensitive than a standard cell. By linking the maturity of a cell to its radiosensitivity these French scientists laid the foundation for radiobiology.26 Although their influence on diagnostic imaging is not as important as it is to radiation oncology, they remind us that a fetus is more radiosensitive than a child, who is more radiosensitive than an adult.27

Cytosol acts as the medium in which cellular reactions can take place. The organelles are membrane-enclosed bodies that perform a unique function. Together, cytosol and organelles comprise the cellular cytoplasm. The nucleus is a well-known organelle and contains the cellular DNA.25

Three of the main organelles in the human cell—the Golgi apparatus, vesicles, and endo-plasmic reticulum—work in conjunction with 1 another to form parts of the endomembrane system (see Figure 6). The endoplasmic reticulum is either rough or smooth, and functions as a transport system in the cell, providing passageways through the major-ity of the cell. Rough endoplasmic reticulum is lined with ribosomes that give it the rough appearance, and these ribosomes are the site of protein synthesis, a primary export product for the cell. Smooth endoplas-mic reticulum does not have the ribosomes, but does engage in the synthesis of lipids. In addition, smooth endoplasmic reticulum aids in the synthesis of phos-pholipids, which are crucial in the production of steroid hormones and acts as a regulator for cellular calcium, which aids in neurotransmission. A secondary function of smooth endoplasmic reticulum is the detoxification of the cell. The Golgi apparatus sorts, modifies, and ships the products that the endoplasmic reticulum pro-duces. The Golgi apparatus does this by repackaging the cellular export products into vesicles that are either retained in the cell or migrate to the surface of the cell for excretion.25

Some digestive enzymes packaged by the Golgi apparatus remain inside the cell for use in its own function. The enzymes are packaged and stored in vesicles that might alter their composition and become lysosomes. Alternatively, they might fuse with exist-ing lysosomes; regardless, a lysosome is an organelle that detoxifies the cell by breaking down and digesting unneeded components. The body uses lysosomes as a defense mechanism, as in the case of phagocytosis, when they are responsible for digesting invading bacte-ria. Lysosomes also are responsible for autolysis—the process of self-destructing in the case of damaged or unhealthy cells. Autolysis is accomplished by the lyso-somes opening up and releasing their digestive enzymes directly into the cytoplasm.25


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as halogenated pyrimidines and vitamin K, though they are more resistant to radiation in the presence of chemi-cal agents that contain a sulfhydryl group (such as cysteine). Radioprotective agents that have been found to be effective also are lethal to humans at the effective dose, so application of these agents has been limited.27

The physical factors that affect radiosensitivity include linear energy transfer (LET), relative biologic effectiveness, and protraction and fractionation. LET hinges on the fact that ionizing radiation traveling through matter gradually loses energy as it interacts with the atoms along its course of travel. Effectively, the higher the LET, the greater its ability to produce biologic damage. This is because the higher the LET, the greater the rate of ionizations.8 Relative biologic effectiveness is a measure of the ability of radiation to produce damage. Relative biologic effectiveness is linked with LET; specifically, as the LET increases, the ability to produce damage increases. That ability to produce damage is quantified by the relative bio-logic effectiveness.27 Protraction and fractionation are

RadiosensitivityWell-known biologic adverse effects are elicited after

ionizing radiation exposure. This is a direct result of the physical interaction of the photon and biologic tissue, which loses energy as it interacts with tissue and trans-fers that energy to the tissue as damage. There are many factors that affect the sensitivity of tissue, including bio-logic and physical concerns.

Biologic factors that affect radiosensitivity include oxygenation, age, recovery, and the presence of radio-sensitizers or radioprotectors. Tissue is more sensitive to radiation when it is oxygenated. Sensitivity to radiation also is dependent on age. Humans are most sensitive to irradiation in utero, with decreasing sensi-tivity through maturity and increasing sensitivity from maturity to old age; however, age in this context is a continuum, not clearly defined by relative age ranges. Cells can recover from the effects of irradiation if they are not killed in interphase death. In addition, cells are sensitive in the presence of some chemical agents, such

Nucleolus

Mitochondria

Lysosome

Smooth endoplasmic

reticulum

Rough endoplasmic reticulum

Intermediate �lament

Secretory vesicle

Ribosomes

Vacuole

Micro�lament

Cytoplasm

Nucleus

Chromatin

Golgi aparatus

Golgi vesicle

Peroxisome

Centrosome

Microtubule

Figure 6. Prototypical human cell containing the primary organelles and internal struc-tures. © 2019 ASRT.


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Linear quadratic dose-response models demonstrate that responses at low levels of exposure are linear, but that the higher the dose, the line becomes quadratic. There is no threshold in a linear quadratic model, and it models the multiple target/single hit theory of cellular reaction to ionizing radiation exposure.26 The multiple target/single hit theory of cellular reaction is used for complex biologic entities such as human beings.28 The response that is being modeled in humans is stochastic or nonstochastic (deterministic).

Deterministic EffectsDeterministic effects are responses to ionizing

radiation exposure that require a threshold to occur. The effects increase as dose increases, once the thresh-old is surpassed. Deterministic effects often are the most egregious responses to radiation exposure and typically are elicited after a high dose of radiation has been received. Possible deterministic effects include erythema, epilation, cataracts, and sterility. The most commonly observed effect of radiation exposure is ery-thema. Erythema occurs after doses of 3 Gyt. The dose required to cause erythema in 50% of the exposed pop-ulation is 5 Gyt, which also is the dose at which sterility becomes a concern. Although sterility requires a dose of 5 Gyt, suppressed fertility is relatively common with doses as low as 100 mGyt. Death by radiation exposure is possible and can be caused by acute radiation syn-dromes such as hematologic, gastrointestinal, or central nervous system.29,30

Acute radiation syndromes are dependent on the dose received and occur after exposures of at least 2 Gyt. At doses of 2 Gyt to 10 Gyt, the patient experi-ences nausea, vomiting, diarrhea, anemia, leukopenia, hemorrhage, fever, and infection. These are symptoms of hematologic syndrome, and likely lead to hemato-logic death. The mean survival time for patients with exposure in this range is 10 to 60 days. With aggressive clinical support, humans have survived; the maximum reported dose was 8.5 Gyt.29 Patients who receive a dose of 10 Gyt to 50 Gyt likely experience gastroin-testinal death. They suffer all the same symptoms as patients with hematologic syndrome, with the addition of electrolyte imbalance, lethargy, fatigue, and shock. Typically, gastrointestinal death occurs as a result of

simply measures of how the radiation is being deliv-ered. Delivering the same total dose at a lower rate protracts the exposure. Delivering the same total dose in increments fractionates the exposure. Each of these approaches, protraction and fractionation, allow suffi-cient time for cellular recovery to take place prior to the completion of exposure, thus lowering the probability of long-term damage.

Dose-Response RelationshipsScientists continually try to establish dose-response

relationships. Dose-response models are used to explain the effect of a given radiation exposure. Dose-response models are salient in planning radiation therapy treat-ments and in providing information on low-dose exposure to determine probable effects of diagnostic imaging studies. There are 3 primary dose-response models used in radiobiology that pertain to diagnostic imaging: linear nonthreshold, linear threshold, and lin-ear quadratic. That is not to say that nonlinear models are nonexistent; rather, nonlinear responses mostly are near-term injuries, such as skin burns, and the focus generally is more on late-term effects. Late-term effects follow linear response models.27

Linear response models might be used to extrapo-late dose effects at any range, given information on any other range, of exposures. Linear nonthreshold models ref lect the idea that any exposure, regardless of amount, might cause detrimental effects, and those effects increase in direct relation and proportion to the exposure Because of this concept, linear nonthresh-old models are used for regulatory purposes, which often is codified by asking technologists to keep their exposure as low as reasonably achievable (ALARA). However, linear threshold models often intuitively make the most sense to technologists because, for example, some might think that adverse effects from a single-projection chest radiograph will not be notice-able. Linear threshold models demonstrate an increase in response directly proportional to the exposure, but require a minimum amount of radiation to elicit the initial response. Regardless of the regulatory or intuitive allure of linear nonthreshold models, linear quadratic models are best suited to human response modeling.


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protection is the universal term used to acknowledge the concepts, requirements, technologies, and pro-cesses used to protect ourselves, patients, and lay people from the harmful effects of ionizing radiation.12 Radiographers should keep radiation exposure and the consequences of that exposure as low as reasonably achievable. The ALARA concept is a seminal practice in radiography. This is particularly salient because there are no known safe dose levels for radiation-induced malignancy.32

Radiographers use 3 basic principles to enact ALARA. Time, distance, and shielding are used to minimize patient exposure to radiation. First, limit-ing or minimizing the exposure time and exposures reduces the dose the patient receives from the source. Second, increasing the distance from the source reduces the exposure exponentially. Finally, using sufficient barriers and protective devices to preclude nonessential areas from being exposed greatly reduces or eliminates unnecessary exposure.33

Technologists use the same principles to limit their own exposure. Reducing the time spent in the path of ionizing radiation reduces the exposure, particularly in f luoroscopy. The inverse square law shows that the fur-ther away from the source of radiation one can get the better. Shielding is the best way to provide occupational protection from radiation. Gloves, aprons, eyeglasses, thyroid shields, lead, and concrete generally are avail-able to provide technologists protection from ionizing radiation.

Dose limitsThe National Council on Radiation Protection and

Measurements publishes the standards that outline occupational dose limits for radiation workers, includ-ing radiologic technologists. They note that the annual occupational effective dose limit for technologists should not exceed 50 mSv of whole-body exposure. The council added a recommendation that the lifetime dose must not exceed the technologist’s age in years multiplied by 10 mSv. Pregnant technologists should not receive a monthly dose that exceeds 0.5 mSv to the fetus, nor should they receive more than 5 mSv over the course of the entire pregnancy, as measured after their declaration of pregnancy.34

damage to the interstitial linings in the intestines with-in about 4 to 10 days of exposure.4

Central nervous system death is inevitable in indi-viduals who receive a dose of 50 Gyt or more. Central nervous system syndrome elicits all of the effects of gas-trointestinal syndrome, but with the addition of ataxia, edema, systemic vasculitis, and meningitis. The typical cause of death in central nervous system syndrome is increased intracranial pressure resulting from excess f luid in the brain. Central nervous system death might occur from a few hours after exposure up to about 3 days.29

Chromosomal aberrations also are a deterministic effect. High doses of radiation cause these effects; however, it is highly probable that low doses also cause aberrations, but they are not as observable. It can be difficult to unequivocally link the aberrations to the radiation exposure, given that they can be caused by so many other factors.29

Stochastic EffectsUnlike deterministic effects, which are com-

pletely attributable to a radiation exposure, stochastic effects are random effects that might or might not be attributable to radiation.26 The effects elicited in the diagnostic imaging range of exposures are stochastic. This is because people might experience an effect, or they might not; there is no predicting what can occur. Cancer and hereditary concerns are stochastic effects because there is no discrete way to determine whether they will occur.26,30

The primary stochastic effect observed in humans is cancer. However, DNA mutagenesis in somatic cells has been observed.31 Radiation protection guidelines are based on stochastic effects, using the premise that any radiation might cause harm. However, it should be noted that diagnostic imaging is relatively safe for the patient, and that radiologic technology is a safe profes-sion for the radiographer.31

Radiation ProtectionBecause background radiation surrounds us at all

times, a medical imaging professional’s role is to limit a patient’s exposure to man-made radiation, particularly radiation that is generated in the profession. Radiation


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6. Bushong SC. Electromagnetic energy. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:44-59.

7. Sherer MAS, Visconti P, Ritenour ER, Haynes KW. Radiation: types, sources, and doses received. In: Radiation Protection in Medical Radiography. 7th ed. St Louis, MO: Elsevier; 2014:15-38.

8. Wondergem J. Radiation Biology: A Handbook for Teachers and Students. Vienna, Austria: International Atomic Energy Agency; 2010.

9. Radiation sources and doses. Environmental Protection Agency website. https://www.epa.gov/radiation/radiation -sources-and-doses. Updated October 18, 2018. Accessed November 11, 2018.

10. Radiation basics. Environmental Protection Agency website. https://www.epa.gov/radiation/radiation-basics. Updated January 30, 2018. Accessed November 11, 2018.

11. Basics R. United States Nuclear Regulatory Commission website. https://www.nrc.gov/about-nrc/radiation/health -effects/radiation-basics.html. Updated October 2, 2017. Accessed November 24, 2018.

12. Radiation protection overview: international aspects and perspective. Nuclear Energy Agency website. http://oecd -nea.org/brief/brief-10. Published December, 1994. Updated 2008. Accessed November 28, 2018.

13. Radiation protection. United States Nuclear Regulatory Commission website. https://www.nrc.gov/about-nrc /radiation.html. Updated September 25, 2017. Accessed November 24, 2018.

14. Bushong SC. The structure of matter. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:26-43.

15. Whaites E, Cawson R. Essentials of Dental Radiography and Radiology. St Louis, MO: Elsevier; 2002.

16. Novikov I. Production of x-rays. https://www.wku.edu/ehs /radiation/module-9_production_of_x-rays.pdf. Published 2015. Accessed November 12, 2018.

17. Bushong SC. X-radiation. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:83-160.

18. Bushong SC. The x-ray imaging system. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:88.

19. Garz R. The x-ray machine. http://www.austincc.edu /rudygarz/xRayMachine/xRayMachine.pdf. Accessed November 30, 2018.

20. Bushong SC. The x-ray tube. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:106.

ConclusionThe process of producing photons from electrons

in the x-ray tube for use in diagnostic imaging proce-dures is something every newly certified radiographer can explain. However, these concepts tend to fade into the recesses of the practicing technologist’s memory. A periodic review of these concepts is paramount to continued professional development and maintaining a professional identity. Although the overviews provided in this article are cursory, they should refresh the hours devoted to learning the concepts during initial training.

Eric P Matthews, PhD, R.T.(R)(CV)(MR), ENT, completed his doctorate at Southern Illinois University with an emphasis in adult, vocational, and technical education (workforce education and development). He also holds graduate degrees in education (administration and supervision) and museum studies. He serves as associate professor for the College of Graduate Health Studies at A.T. Still University in Mesa, Arizona.

Reprint requests may be mailed to the American Society of Radiologic Technologists, Publications Department, 15000 Central Ave SE, Albuquerque, NM 87123-3909, or emailed to [email protected].

© 2019 American Society of Radiologic Technologists.

References1. American Registry of Radiologic Technologists. Content

Specifications: Radiography Examination. St Paul, MN: American Registry of Radiologic Technologists; 2017.

2. Sherer MAS, Visconti P, Ritenour ER, Haynes KW. Radiation Protection in Medical Radiography. 7th ed. St Louis, MO: Elsevier; 2014.

3. Matter. Merriam-Webster Dictionary website. https://www .merriam-webster.com/dictionary/matter. Accessed March 12, 2019.

4. Bushong SC. Essential concepts of radiologic science. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:2-25.

5. The Nuclear Regulatory Commission’s science 101: what are the different types of radiation? United States Nuclear Regulatory Commission website. https://www.nrc.gov /reading-rm/basic-ref/students/science101.html. Updated August 10, 2017. Accessed November 11, 2018.


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21. Behling R. Modern Diagnostic X-Ray Sources, Technology, Manufacturing, Reliability. Boca Raton, FL: Taylor and Francis; 2015.

22. Carlton R, Adler A. Principles of Radiographic Imaging: An Art and a Science. 5th ed. Clifton Park, NY: Delmar-Cengage; 2011.

23. Sherer MAS, Visconti P, Ritenour ER, Haynes KW. Interaction of x-radiation with matter. In: Radiation Protection in Medical Radiography. 7th ed. St Louis, MO: Elsevier; 2014:39-60.

24. Bushong SC. X-ray interaction with matter. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:149-150.

25. OpenStax. Human Anatomy and Physiology, p3.2. https://opentextbc.ca/anatomyandphysiology/chapter/3-2-the -cytoplasm-and-cellular-organelles/. Rice University: OpenTextBC; 2013. Accessed December 2, 2018.

26. Bolus NE. Basic review of radiation biology and terminol-ogy. J Nucl Med Technol. 2017;45(4):259-264. doi:10.2967 /jnmt.117.195230.

27. Bushong SC. Fundamental principles of radiobiology. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:484-488.

28. Bushong SC. Cellular radiobiology. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:503.

29. Bushong SC. Deterministic effects of radiation. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:507-521.

30. Sherer MAS, Visconti P, Ritenour ER, Haynes KW. Dose limits for exposure to ionizing radiation. In: Radiation Protection in Medical Radiography. 7th ed. St Louis, MO: Elsevier; 2014:215-216.

31. Bushong SC. Stochastic effects of radiation. In: Radiologic Science for Technologists: Physics, Biology, and Protection. 11th ed. St Louis, MO: Elsevier; 2017:522-540.

32. Sherer MAS, Visconti P, Ritenour ER, Haynes KW. Introduction to radiation protection. In: Radiation Protection in Medical Radiography. 7th ed. St Louis, MO: Elsevier; 2014:5.

33. Protecting YFR. Environmental Protection Agency website. https://www.epa.gov/radiation/protecting-yourself-radia tion. Published February 15, 2008. Accessed November 11, 2018.

34. National Council on Radiation Protection and Measurements. NCRP Report no. 116: Limitation of Exposure to Ionizing Radiation. Bethesda, MD: National Council on Radiation Protection and Measurements Publications; 1993.

https://doi.org/10.2967/jnmt.117.195230

https://doi.org/10.2967/jnmt.117.195230


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a. 1 and 2b. 1 and 3c. 2 and 3d. 1, 2, and 3

5. A(n) ______ can travel a great distance and typically requires a hydrogen shield to contain it.a. protonb. alpha particlec. neutrond. beta particle

6. Radiation quantity is governed by the voltage used and regulates the x-ray penetrability of the beam.a. trueb. false

1. The substance of which a physical object is composed is known as:a. mass.b. matter.c. weight.d. energy.

2. ______ energy describes the ability of an object to do work simply by virtue of its position.a. Potentialb. Chemicalc. Electricald. Thermal

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12. Radiographers use which of the following basic principles to enact ALARA?

1. time2. distance3. shielding

a. 1 and 2b. 1 and 3c. 2 and 3d. 1 ,2, and 3

7. ______ is responsible for the majority of scatter produced during diagnostic imaging.a. Photoelectric absorptionb. Pair productionc. Compton scatterd. Photodisintegration

8. The most commonly observed effect of radiation exposure is:a. erythema.b. epilation.c. cataracts.d. sterility.

9. Symptoms of hematologic syndrome include all of the following except:a. nausea.b. constipation.c. leukopenia.d. fever.

10. Symptoms of central nervous system syndrome include:

1. ataxia.2. meningitis.3. systemic vasculitis.

a. 1 and 2b. 1 and 3c. 2 and 3d. 1, 2, and 3

11. The primary stochastic effect observed in humans is:a. sterility.b. cataracts.c. cancer.d. erythema.

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