Dental radiology textbook pdf

Show less. About the Authors. Tim Reynolds has worked for many years as a radiographer in the NHS. Outside of clinical work Tim has worked in radiographic education for more than 20 years. Tim was a nationally elected member of the College of Radiographers Council. During his time on Council he was a member of the team that liaised with the Dental Nurse Standards Advisory Board to jointly develop the syllabus of study which provided the model for dental nurse radiography courses.

Formerly Principal Tutor for the Diagnostic Radiography undergraduate course at Keele University, Tim now runs his own educational consultancy specialising in radiography training for dental nurses. Download Links. Describe how kilovoltage, milliamperage, exposure time, and source-to-receptor distance in uence the intensity of the x-ray beam. Calculate an example of radiation intensity using the inverse square law.

Explain how the half-value layer determines the penetrating quality of the x-ray beam. Radiation characteristics include x-ray beam quality, quantity, and intensity. Variations in the character of the x-ray beam in uence the quality of the resulting images. The dental radiographer must have a working knowledge of radiation characteristics and exposure factors. The purpose of this chapter is to 1 detail the concepts of x-ray beam quality and quantity, 2 de ne the concept of beam intensity, and 3 discuss how exposure factors in uence these characteristics.

Current dental x-ray units have control panels with preset, predetermined exposure factors kV, mA, time for the various anatomic areas of the maxilla and mandible, so no manual operator adjustment choices are needed Figure On older x-ray units, the exposure factors could be manually adjusted by the operator.

Whereas all current dental x-ray units allow for the adjustment of time, the ability to adjust kilovoltage and milliamperage varies from model to model. On many current dental x-ray units, manual adjustments of kilovoltage and milliamperage are not an option. X-rays with shorter wavelengths have more penetrating power, whereas those with longer wavelengths are less penetrating and more likely to be absorbed by matter.

In dental imaging, the term quality is used to describe the mean energy or penetrating ability of the x-ray beam. The quality, or wavelength and energy of the x-ray beam, is controlled by kilovoltage. Voltage is a measurement of force that refers to the potential difference between two electrical charges.

Inside the dental x-ray tubehead, voltage is the measurement of electrical force that causes electrons to move from the negative cathode to the positive anode. Voltage determines the speed with which they move. When voltage is increased, the speed of the electrons is increased. The electrons strike the target with greater force and energy, resulting in a penetrating x-ray beam with a short wavelength.

Voltage is measured in volts or kilovolts. The volt V is the unit of measurement used to describe the potential that drives an electrical current through a circuit. Dental x-ray equipment requires the use of high voltages. Most radiographic units operate using kilovolts; 1 kilovolt kV is equal to volts.

The term kilovoltage kV is the maximum voltage, or peak voltage of an alternating current AC Figure In older units, the kilovoltage uctuated depending on the voltage waveform applied to the tube. In current dental x-ray units, this uctuation is so very small that the kilovoltage can be considered as a xed value during exposure. In the past, dental x-ray units were available with adjustable settings ranging from 65 to kV. With these units, the kilovoltage could be adjusted according to the individual diagnostic needs of patients.

For example, a higher kilovoltage setting was used when the area to be examined was dense or thick. The use of higher kV produces more penetrating dental x-rays with greater energy, whereas the use of lower kV produces less penetrating dental x-rays with less energy. Current intraoral x-ray units include adjustable settings that range from 60 to 70 kV, or. FIG Kilovoltage kV control and m illiam pe rage m A of the unit is locate d on the de ntal x-ray m achine.

Courte s y Ins trum e ntarium De ntal, Inc. FIG A, Diagnos tic radiograph. B, Incre as e in kilovoltage re s ults in an im age that e xhibits incre as e d de ns ity; the im age appe ars darke r. FIG Kilovoltage kV controls the quality of the x-ray be am and m e as ure s the pe ak voltage of the curre nt. If the unit has a xed kV, that kV number is found imprinted on the face of the control panel.

On units that allow for the adjustment of kilovoltage, a kV button is found on the control panel. The quality, or wavelength and energy, of the x-ray beam is controlled by the kilovoltage.

The kilovoltage regulates the speed and energy of the electrons and determines the penetrating ability of the x-ray beam. Increasing the kilovoltage results in a higher energy x-ray beam with increased penetrating ability.

Density and Kilovoltage Density is the overall darkness or blackness of an image. An adjustment in kilovoltage results in a change in the density of a dental image. If the kilovoltage is increased while other exposure factors milliamperage, exposure time remain constant, the resultant image exhibits an increased density and appears darker Figure If the kilovoltage is decreased, the resultant image exhibits a decreased density and appears lighter Figure Table summarizes the effect of kilovoltage on density.

With digital imaging, special image enhancement software can be used to change the density by adjusting the brightness. For example, if a digital image is too dark or too light, the brightness can be adjusted so that the image is readable. In comparison, if lm is used and the density is nondiagnostic, the image must be retaken with adjusted exposure factors. Contrast and Kilovoltage Contrast refers to how sharply dark and light areas are differentiated or separated on an image.

An adjustment in kilovoltage results in a change in the contrast of a dental image. When lower kilovoltage settings are used, a high-contrast image will result. An image with high contrast is useful for detecting and determining the progression of dental caries.

With higher kilovoltage settings, low contrast results. An image with low contrast is useful for the detection of periodontal or periapical disease. In dental imaging, a compromise between high and low contrast is desirable.

See Table for a summary of the effect of kilovoltage on contrast also see Chapter 8. With digital imaging, special image enhancement software can be used to change the contrast by altering the distribution of the gray levels seen in the image. A digital image can be adjusted so that the contrast is higher, which is desirable in. FIG Im age produce d w ith low e r kilovoltage e xhibits high contras t; m any light and dark are as are s e e n, as de m ons trate d by the us e of the s te pw e dge.

FIG Im age produce d w ith highe r kilovoltage e xhibits low contras t; m any s hade s of gray are s e e n ins te ad of black and w hite. B, De cre as e in kilovoltage re s ults in an im age that e xhibits de cre as e d de ns ity; the im age appe ars lighte r. In comparison, if lm is used and the contrast is incorrect, the image must be retaken with adjusted exposure factors.

Exposure Time and Kilovoltage Exposure time refers to the interval of time during which x-rays are produced. The timer controls the length of exposure time and determines how long the x-rays will be emitted from the machine. The longer the exposure time, the more x-rays are delivered, and a darker image results.

Every x-ray machine has a timer. The timer is the exposure factor that is recommended to adjust in order to lighten or darken an image. For example, to get the same end result, a larger patient may require more x-ray exposure time, whereas a smaller patient may require less x-ray exposure time. The timer may be calibrated in either seconds or impulses, depending on when the unit was manufactured.

One impulse. Kilovoltage and exposure time are inversely related. On older x-ray units, if the kilovoltage was changed, the exposure time needed to be adjusted in order to maintain the diagnostic density of an image.

When kilovoltage was increased, the exposure time was decreased in order to compensate for the penetrating power of the x-ray beam. When kilovoltage was decreased, the exposure time was increased.

Amperage and Milliamperage Amperage determines the amount of electrons passing through the cathode lament. An increase in the number of electrons available to travel from the cathode to the anode results in production of an increased number of x-rays. The quantity of the x-rays produced is controlled by milliamperage. The ampere A is the unit of measure used to describe the number of electrons, or current owing through the cathode lament.

The number of amperes needed to operate a dental x-ray unit is small; therefore, amperage is measured in milliamperes. It is common to abbreviate milliamperage as mA, the same as its unit, the milliampere. Some dental x-ray units have a xed milliamperage setting, whereas others have a milliamperage adjustment on the control panel see Figure In the past, dental x-ray units were available with adjustable setting choices of 7 mA or 15 mA.

With these units, the mA setting could be chosen according to the individual diagnostic needs of patients. For example, the higher mA setting was used when the area to be examined was dense or thick.

The use of 15 mA produced more dental x-rays, whereas the use of 7 mA produced less dental x-rays. Current intraoral x-ray units may include adjustable settings that range from 6 to 8 mA, or else a xed setting. If the unit has a xed mA, that mA number is found imprinted on the face of the control panel. Milliamperage regulates the temperature of the cathode lament. A higher milliampere setting increases the temperature of the cathode lament and consequently increases the number of electrons produced.

An increase in the number of electrons that strike the anode increases the number of x-rays emitted from the tube. The quantity, or number of x-rays emitted from the tubehead, is controlled by milliamperage. Milliamperage controls the amperage of the lament current and the amount of electrons that pass through the lament.

As the milliamperage is increased, more electrons pass through the lament, and more x-rays are produced. For example, if the milliamperage is increased from 7 to 15 mA, approximately twice as many electrons travel from the cathode to the anode, and approximately twice as many x-rays are produced.

Density and Milliamperage Milliamperage, as with kilovoltage, has an effect on the density of a dental image. An increase in milliamperage increases the overall density and results in a darker image. Conversely, a decrease in milliamperage decreases the overall density and results in a lighter image. Table summarizes the effect of milliamperage on density. As previously described, with digital imaging, special image enhancement software can be used to change the density by adjusting the brightness.

In comparison, if lm is used and the density is nondiagnostic, the image will need to be retaken with adjusted exposure factors. As described in this chapter, changing any one of these three exposure factors changes the appearance of the resultant image.

On dental x-ray units, only the exposure time setting is always adjustable. Most manufacturers recommend that once an x-ray unit has been installed, calibrated, and inspected, it is best not to adjust the kV and mA. Instead, only adjust the exposure time to make any needed changes.

There is less potential for confusion, errors, and retakes when only the exposure time is adjusted. The exposure time adjustment is based on patient size; it should be decreased with small children and increased with adults with large jaws. Quality and quantity are described together in a concept known as intensity. Exposure Time and Milliamperage Milliamperage and exposure time are inversely related. On older units, if the milliamperage was changed, the exposure time needed to be adjusted in order to maintain the diagnostic density of an image.

When milliamperage was increased, the exposure time was decreased. When milliamperage was decreased, the exposure time was increased. Table lists guidelines for adjusting kilovoltage, milliamperage, and exposure time.

Intensity of the x-ray beam is affected by a number of factors, including kilovoltage, milliamperage, exposure time, and distance. Kilovoltage Kilovoltage regulates the penetrating power of the x-ray beam by controlling the speed of the electrons traveling between the cathode and the anode.

Higher kilovoltage settings produce an x-ray beam with more energy and shorter wavelengths; higher kilovoltage levels increase the intensity of the x-ray beam. Milliamperage Milliamperage controls the penetrating power of the x-ray beam by controlling the number of electrons produced in the x-ray tube and the number of x-rays produced.

Higher milliampere settings produce a beam with more energy, increasing the intensity of the x-ray beam. Exposure time, as with milliamperage, affects the number of x-rays produced. A longer exposure time produces more x-rays. FIG Dis tance s to cons ide r w he n e xpos ing de ntal radiographs : targe t-s urface , targe t-obje ct, and targe t-re ce ptor dis tance.

Distance The distance traveled by the x-ray beam affects the intensity of the beam. As x-rays travel from their point of origin or away from the target anode, they diverge like waves of light and spread out to cover a larger surface area.

FIG The inve rs e s quare law s tate s that the inte ns ity of radiation is inve rs e ly proportional to the s quare of the dis tance from the s ource. Note that as the s ource -to-re ce ptor dis tance is double d, the radiation is one fourth as inte ns e. The inverse square law is used to explain how distance affects the intensity of the x-ray beam.

Inverse Square Law The inverse square law is stated as follows: The intensity of radiation is inversely proportional to the square of the distance from the source of radiation.

As x-rays travel away from their source of origin, the intensity of the beam lessens. Unless a corresponding change is made in one of the other exposure factors kilovoltage , the intensity. When the source-to-receptor distance is increased, the intensity of the beam is decreased. According to the inverse square law, when the target-receptor distance is doubled, the resultant beam is one fourth as intense Figure When the target-receptor distance is reduced by half, the resultant beam is four times as intense.

The following mathematical formula is used to calculate the inverse square law. This mathematical formula reveals that the beam will be one fourth as intense if the target-receptor distance is changed from 8 to 16 inches assuming that kilovoltage and milliamperage remain constant.

In this example, the inverse square law reveals that doubling the distance from the source of radiation to the receptor results in a beam that is one fourth as intense. This is als o true for lighting us e d in profe s s ional photography. Half-Value Layer To reduce the intensity of the x-ray beam, aluminum lters are placed in the path of the beam inside the dental x-ray tubehead.

Aluminum lters are used to remove the low-energy, less penetrating, longer-wavelength x-rays. Aluminum lters increase the mean penetrating capability of the x-ray beam while reducing the intensity. When placed in the path of the x-ray beam, the thickness of a speci ed material e. For example, if an x-ray beam has an HVL of 4 mm, a thickness of 4 mm of aluminum would be necessary to decrease its intensity by half.

Measuring the HVL determines the penetrating quality of the beam. The higher the half-value layer, the more penetrating the beam. Filtration of the x-ray beam is discussed further in Chapter 5. Instead, adjust only the exposure time to make any needed changes. Johnson ON: Producing quality radiographs. In dental imaging, the quality of the x-ray beam is controlled by: a.

Identify the kilovoltage range for current dental x-ray machines: a. A higher kilovoltage produces x-rays with: a. Identify the unit of measurement used to describe the amount of electric current owing through the x-ray tube: a. Radiation produced with high kilovoltage results in: a. In dental imaging, the quantity of radiation produced is controlled by: a. Increasing milliamperage results in an increase in: a.

Identify the milliamperage range used for current dental x-ray machines: a. The overall blackness or darkness of an image is termed: a. If kilovoltage is decreased with no other variations in exposure factors, the resultant image will: a. Identify the term that describes how dark and light areas are differentiated on an image:.

De ne the terms associated with radiation injury. Describe the mechanisms and theories of radiation injury. De ne and discuss the dose-response curve and radiation injury. Describe the sequence of radiation injury and list the determining factors for radiation injury. Discuss the short-term and long-term effects as well as the somatic and genetic effects of radiation exposure. Describe the effects of radiation exposure on cells, tissues, and organs and identify the relative sensitivity of a given tissue to x-radiation.

De ne the units of measurement used in radiation exposure. List common sources of radiation exposure. Discuss risk and risk estimates for radiation exposure. Discuss dental radiation and exposure risks.

Discuss the risk versus bene t of dental images. All ionizing radiations are harmful and produce biologic changes in living tissues. The damaging biologic effects of x-radiation were rst documented shortly after the discovery of x-rays.

Since that time, information about the harmful effects of high-level exposure to x-radiation has increased based on studies of atomic bomb survivors, workers exposed to radioactive materials, and patients undergoing radiation therapy. Although the amount of x-radiation used in dental imaging is small, biologic damage does occur. The dental radiographer must have a working knowledge of radiation biology, the study of the effects of ionizing radiation on living tissue, to understand the harmful effects of x-radiation.

The purpose of this chapter is to describe the mechanisms and theories of radiation injury, to de ne the basic concepts and effects of radiation exposure, to detail radiation measurements, and to discuss the risks of radiation exposure. The kinetic energy of such electrons results in further ionization, excitation, or breaking of molecular bonds, all of which cause chemical changes within the cell that result in biologic damage Figure Ionization may have little effect on cells if the chemical changes do not alter sensitive molecules, or such changes may have a profound effect on structures of great importance to cell function e.

Absorption, as de ned in Chapter 2, refers to the total transfer of energy from the x-ray photon to patient tissues. What happens when x-ray energy is absorbed by patient tissues?

Chemical changes occur that result in biologic damage. Two speci c mechanisms of radiation injury are possible: 1 ionization and 2 free radical formation. Ionization X-rays are a form of ionizing radiation; when x-rays strike patient tissues, ionization results. As described in Chapter 2, ionization is produced through the photoelectric effect or Compton scatter and results in the formation of a positive atom and a dislodged negative electron. The ejected high-speed electron is set into motion and interacts with other atoms within.

Free Radical Formation X-radiation causes cell damage primarily through the formation of free radicals. Ionization of water results in the production of hydrogen and hydroxyl free radicals Figure A free radical is an uncharged neutral atom or molecule that exists with a single, unpaired electron in its outermost shell.

It is highly reactive and unstable; the lifetime of a free radical is approximately seconds. To achieve stability, free radicals may 1 recombine without causing changes in the molecule, 2 combine with other free radicals and cause changes, or 3 combine with ordinary molecules to form a toxin e. Theories of Radiation Injury Damage to living tissues caused by exposure to ionizing radiation may result from a direct hit and absorption of an x-ray photon within a cell or from the absorption of an x-ray photon by the water within a cell accompanied by free radical formation.

Two theories are used to describe how radiation damages biologic tissues: 1 the direct theory and 2 the indirect theory. A free radical with a charge is an ion. FIG The x-ray photon inte racts w ith tis s ue s and re s ults in ionization, e xcitation, or bre aking of m ole cular bonds , all of w hich caus e che m ical change s that re s ult in biologic dam age.

FIG Fre e radicals can com bine w ith e ach othe r to form toxins s uch as hydroge n pe roxide. Direct Theory The direct theory of radiation injury suggests that cell damage results when ionizing radiation directly hits critical areas, or targets, within the cell.

For example, if x-ray photons directly strike the DNA of a cell, critical damage occurs, causing injury to the irradiated organism. Direct injuries from exposure to ionizing radiation occur infrequently; most x-ray photons pass through the cell and cause little or no damage. Indirect Theory The indirect theory of radiation injury suggests that x-ray photons are absorbed within the cell and cause the formation of toxins, which in turn damage the cell.

For example, when x-ray photons are absorbed by the water within a cell, free radicals are formed. The free radicals combine to form toxins e. An indirect injury results because the free radicals combine and form toxins, not because of a direct hit by x-ray photons.

Indirect injuries from exposure to ionizing radiation occur. If all ionizing radiations are harmful and produce biologic damage, what level of exposure is considered acceptable? To establish acceptable levels of radiation exposure, it is useful to plot the dose administered and the damage produced.

When dose and damage are plotted on a graph, a linear, nonthreshold relationship is seen. A linear relationship indicates that the response of the tissues is directly proportional to the dose. A nonthreshold relationship indicates that a threshold dose level for damage does not exist.

A nonthreshold doseresponse curve suggests that no matter how small the amount of radiation received, some biologic damage does occur Figure Consequently, there is no safe amount of radiation exposure.

In dental imaging, as mentioned earlier, although the doses received by patients are low, damage does occur. Most of the information used to produce dose-response curves for radiation exposure comes from studying the effects of large doses of radiation on populations, for example, atomic bomb survivors.

In the low-dose range, however, minimal information has been documented; instead, the curve has been extrapolated from animal and cellular experiments. Stochastic and Nonstochastic Radiation Effects The deleterious effects of ionizing radiation on human tissue can be divided into two types: stochastic and nonstochastic.

Stochastic effects occur as a direct function of dose. The probability of occurrence increases with increasing absorbed dose; however, the severity of effects does not depend on the magnitude of the absorbed dose.

As in the case of nonthreshold radiation effects, stochastic effects do not have a dose threshold. Stochastic effects occur due to the effect of ionizing radiation on chromosomes that result in genetic mutations.

Examples of stochastic effects include induction of leukemia and other cancers i. Nonstochastic effects deterministic effects have a threshold and increase in severity with increased absorbed dose. Nonstochastic effects only occur after a threshold of exposure has been exceeded. The severity of deterministic effects increases as the dose of exposure increases. Because of an identi able threshold level, appropriate radiation protection mechanisms and occupational exposure dose limits can be put in place to reduce the likelihood of these effects occurring.

FIG A, Thre s hold curve : This curve indicate s that be low a ce rtain le ve l thre s hold , no re s pons e is s e e n. Line ar curve : This curve indicate s that re s pons e is proportional to dos e. B, Line ar nonthre s hold curve : This dos e -re s pons e curve indicate s that a re s pons e is s e e n at any dos e. The physical effects occur when the cell death burden is large enough to cause obvious functional impairment of a tissue or organ.

Examples of nonstochastic effects include skin erythema, loss of hair, cataract formation, decreased fertility, radiation sickness, teratogenesis, and fetal death. Compared with stochastic effects, nonstochastic effects require larger radiation doses to cause serious impairment of health. Sequence of Radiation Injury Chemical reactions e.

However, varying amounts of time are required for these changes to alter cells and cellular functions. As a result, the observable effects of radiation are not visible immediately after exposure.

Instead, following exposure, a latent period. A latent period can be de ned as the time that elapses between exposure to ionizing radiation and the appearance of observable clinical signs. The latent period may be short or long, depending on the total dose of radiation received and the amount of time, or rate, it took to receive the dose. The more radiation received and the faster the dose rate, the shorter the latent period. After the latent period, a period of injury occurs.

A variety of cellular injuries may result, including cell death, changes in cell function, breaking or clumping of chromosomes, formation of giant cells, cessation of mitotic activity, and abnormal mitotic activity. The last event in the sequence of radiation injury is the recovery period.

Not all cellular radiation injuries are permanent. With each radiation exposure, cellular damage is followed by repair. Depending on a number of factors, cells can repair the damage caused by radiation. Most of the damage caused by low-level radiation is repaired within the cells of the body.

The effects of radiation exposure are additive, and unrepaired damage accumulates in the tissues. The cumulative effects of repeated radiation exposure can lead to health problems e. Table lists disorders that may result from the cumulative effects of repeated radiation exposure on tissues and organs. Determining Factors for Radiation Injury In addition to understanding the mechanisms, theories, and sequence of radiation injury, it is important to recognize the factors that in uence radiation injury.

More damage occurs when tissues absorb large quantities of radiation. More radiation damage takes place with high dose rates because a rapid delivery of radiation does not allow time for the cellular damage to be repaired. Total-body irradiation produces more adverse systemic effects than if small, localized areas of the body are exposed.

An example of total-body irradiation is the exposure of a person to a nuclear energy disaster. Extensive radiation injury occurs when large areas of the body are exposed because of the damage to the blood-forming tissues. Somatic and Genetic Effects All the cells in the body can be classi ed as either somatic or genetic. Somatic cells are all the cells in the body except the reproductive cells.

The reproductive cells e. Depending on the type of cell injured by radiation, the biologic effects of radiation can be classi ed as somatic or genetic. Somatic effects are seen in a person who has been irradiated. Radiation injuries that produce changes in somatic cells produce poor health in the irradiated individual.

Major somatic effects of radiation exposure include the induction of cataracts and cancer, including leukemia. These changes, however, are not transmitted to future generations Figure Genetic effects are not seen in the irradiated person but are passed on to future generations.

Radiation injuries that produce changes in genetic cells do not affect the health of the exposed individual. Instead, the radiation-induced mutations affect the health of the offspring see Figure Genetic damage cannot be repaired. Radiation effects can be classi ed as either short-term or longterm effects. Following the latent period, effects that are seen within minutes, days, or weeks are termed short-term effects. Short-term effects are associated with large amounts of radiation absorbed in a short time e.

Acute radiation syndrome ARS is a short-term effect and includes nausea, vomiting, diarrhea, hair loss, and hemorrhage. Short-term effects are not applicable to dentistry. Effects that appear after years, decades, or generations are termed long-term effects. Long-term effects are associated with small amounts of radiation absorbed repeatedly over a long period.

Repeated low levels of radiation exposure are linked to the induction of cancer, birth abnormalities, and genetic defects. FIG A s om atic m utation produce s poor he alth in the e xpos e d anim al but doe s not produce m utations in s ubs e que nt ge ne rations.

In contras t, a ge ne tic m utation doe s not affe ct the e xpos e d anim al but produce s m utations in future ge ne rations. Radiation Effects on Cells The cell, or basic structural unit of all living organisms, is composed of a central nucleus and surrounding cytoplasm.

Ionizing radiation may affect the nucleus, the cytoplasm, or the entire cell. The cell nucleus is more sensitive to radiation than is the cytoplasm.

Damage to the nucleus affects the chromosomes containing DNA and results in disruption of cell division, which, in turn, may lead to disruption of cell function or cell death. Not all cells respond to radiation in the same manner. A cell that is sensitive to radiation is termed radiosensitive; one that is resistant is termed radioresistant. Cells that are radiosensitive include blood cells, immature reproductive cells, and young bone cells.

The cell that is most sensitive to radiation is the small lymphocyte. Radioresistant cells include cells of bone, muscle, and nerve Table Cells are organized into the larger functioning units of tissues and organs. As with cells, tissues and organs vary in their sensitivity to radiation. Radiosensitive organs are composed of radiosensitive cells and include the lymphoid tissues, bone marrow, testes, and intestines.

Examples of radioresistant tissues include the salivary glands, kidney, and liver. The term exposure refers to the measurement of ionization in air produced by x-rays. The traditional unit of exposure for x-rays is the roentgen R.

The roentgen is a way of measuring radiation exposure by determining the amount of ionization that occurs in air. A de nition follows:.

Radiation can be measured in the same manner as other physical concepts such as time, distance, and weight. Just as the unit of measurement for time is minutes, for distance miles or kilometers, and for weight pounds or kilograms, the International Commission on Radiation Units and Measurements ICRU has established special units for the measurement of radiation.

Such units are used to de ne three quantities of radiation: 1 exposure, 2 dose, and 3 dose equivalent. The dental radiographer must know radiation measurements to discuss exposure and dose concepts with the dental patient. In addition, the dental radiographer must be familiar with a number of physics terms used in the de nitions of both traditional and SI units of radiation measurement Table Unit of e le ctrical charge ; the quantity of e le ctrical charge trans fe rre d by 1 am pe re in 1 s e cond.

Am pe re A Unit of e le ctrical curre nt s tre ngth; curre nt yie lde d by 1 volt agains t 1 ohm of re s is tance. Erg e rg Unit of e ne rgy e quivale nt to 1. J oule J SI unit of e ne rgy e quivale nt to the w ork done by the force of 1 ne w ton acting ove r the dis tance of 1 m e te r. Kilogram kg Unit of m as s e quivale nt to gram s or 2. Roentgen: The quantity of x-radiation or gamma radiation that produces an electrical charge of 2. In measuring the roentgen, a known volume of air is irradiated.

The interaction of x-ray photons with air molecules results in ionization, or the formation of ions. The ions electrical charges that are produced are collected and measured. One roentgen is equal to the amount of radiation that produces approximately 2 billion, or 2.

The roentgen has limitations as a unit of measure. It measures the amount of energy that reaches the surface of an organism but does not describe the amount of radiation absorbed. The roentgen is essentially limited to measurements in air. By de nition, it is used only for x-rays and gamma rays and does not include other types of radiation. No SI unit for exposure that is equivalent to the roentgen exists.

The coulomb C is a unit of electrical charge. The radiation absorbed dose, or rad, is the traditional unit of dose. Unlike the roentgen, the rad is not restricted to air and can be applied to all forms of radiation.

Using SI units, 1 rad is equivalent to 0. Dose Equivalent Measurement Different types of radiation have different effects on tissues. The dose equivalent measurement is used to compare the biologic effects of different types of radiation. The traditional unit of the dose equivalent is the roentgen equivalent in man, or rem. A de nition follows: Rem: The product of absorbed dose rad and a quality factor speci c for the type of radiation. To place the exposure effects of different types of radiation on a common scale, a quality factor QF , or dimensionless multiplier, is used.

Each type of radiation has a speci c QF based on different types of radiation producing different types of biologic damage. For example, the QF for x-rays is equal to 1.

The SI unit equivalent of the rem is the sievert Sv. Measurements Used in Dental Imaging In dental imaging, the gray and sievert are equal, and the roentgen, rad, and rem are considered approximately equal.

Smaller multiples of these radiation units are typically used in dentistry because of the small quantities of radiation used during imaging procedures. This knowledge can then be used to better understand the radiation risks associated with dentistry. Humans are exposed daily to radiation from both natural and synthetic sources.

Natural, or background, radiation sources include radon in the air; uranium, radium, and thorium in the earth; cosmic rays from outer space and the sun; radioactive potassium in food and water; and radioactive material found within the human body.

Radon gas arising from the soil is the single greatest source of exposure to background radiation in the United States. Exposure to background radiation varies depending on where a person lives. The cosmic exposure depends on the elevation above sea level; the higher the altitude, the more exposure to cosmic rays.

Terrestrial exposure comes from the ground; an example includes naturally occurring uranium-enriched soil. Type of home construction also effects exposure; a brick home has a higher natural radiation level than a home made of wood. Internal radiation exposure depends on the food and water that a person ingests. Foods such as bananas and Brazil nuts naturally contain higher levels of radiation than other foods, and most water supplies naturally contain radon.

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