6: Oral and Maxillofacial Radiology

Oral and Maxillofacial Radiology

Although technology continues to create new diagnostic aids that advance the practice of dental hygiene, oral and maxillofacial radiographs remain an essential tool for comprehensive client care. Oral and maxillofacial radiographs play a key role in the assessment, care planning, and evaluation of oral health and disease. The goal of oral radiography is to obtain the highest quality radiographic image while maintaining the lowest possible radiation exposure risk to the patient.

Knowledge and skill in applying this information are critical for the safe use of ionizing radiation in the oral health care setting. This chapter emphasizes radiation physics, production, protection, and ethics; radiographic imaging techniques and receptor systems; radiographic film processing and quality assurance procedures; and radiographic anatomy and principles of interpretation.

General Considerations

Radiation Physics

Radiation is the emission or movement of energy through space in the form of particles or waves

Types of radiation

1. Particulate radiation

2. Electromagnetic radiation

a. Nonparticulate radiations

b. Energy charges and currents associated with electric and magnetic waves and frequencies

(1) Wavelength—distance from one crest of a wave to the next; the shorter the wavelength, the greater are the energy and penetrating ability of the radiation; the shortest wavelengths are measured in nanometers (nm) (1 × 10−9 meters [m]), and the longer wavelengths are measured in meters

(2) Frequency—number of wavelength crests passing a particular point per unit of time; measured in hertz (Hz); 1 Hz is equal to 1 cycle per second; the frequency of electromagnetic radiation ranges from <3 × 109 to >3 × 1019 Hz

(3) Photon and quantum—terms used to designate a single unit or bundle of energy

(4) Energy—ability to do work; energy of electromagnetic radiation is measured in electron volts (eV); x-ray energy ranges from 100 to 100,000 eV (1 kiloelectron volt [keV] is equal to 1000 eV)

c. Characteristics

d. Examples—radio waves, microwaves, infra-red light, visible light, ultraviolet light, x-rays, gamma rays, and cosmic rays

3. Ionizing radiation

Sources of radiation

1. Naturally occurring (or background) radiation constitutes 50% of the overall exposure to the United States population1

2. Medical applications make up 48% the overall exposure to the United States population1

3. Consumer products and industrial and occupational exposures account for the remaining 2% of total exposure to the United States population

Exposure—defined as the average annual effective dose equivalent of ionizing radiation

Electricity

Definition—flow of electrons through a wire or other electrical conductor

Types

Units of measurement

Power supply to dental x-ray machines is primarily 110 -V, 60-Hz alternating current

X-Ray Machines

Types

X-ray unit components

1. Generator—device that supplies electrical power to the x-ray tube

a. Transformer—device that changes the potential difference of incoming electrical energy to any desired level

b. Types of transformers

c. Rectification

(1) Definition—process of changing an alternating current into a direct current

(2) Rectifier—an electrical device that changes AC to DC

(3) Dental x-ray units are considered to be self-rectified or half-wave rectified

(4) In full-wave rectification, x-rays are generated during both phases of the alternating current cycle

2. X-ray tube (Figure 6-3)

a. Protective housing

b. Cooling system

c. Glass tube

d. Cathode

e. Anode

f. Filtration

g. Collimation

3. Position-indicating device (PID)

4. Control panel

Production of X-Radiation

X-ray machine preparation

1. Initial process for x-radiation production is achieved by the activation of the on–off switch located on the unit control panel; this process completes the filament circuit, and the filament is heated

2. Appropriate mA, kV, and exposure time are set by using the controls located on the unit console; if mA and kV are preset by the manufacturer, the control panel will be labeled with the preset values

a. Milliamperage control, which is connected to the mA-filament circuit (step-down transformer), allows for warming of the cathode filament and determines the number of electrons available for x-ray production; the higher the mA, the hotter the filament becomes, resulting in a greater number of available electrons

b. Kilovoltage control, which is connected to the high-voltage circuit (step-up transformer), establishes the high voltage needed for x-ray production; this control also provides the condition in which the anode is positively charged and the cathode is negatively charged for the attraction and high-speed acceleration of electrons from the cathode to the anode; the higher the kVp setting, the greater is the speed of acceleration of electrons from the cathode to the anode

c. The exposure time establishes the time during which electrons are available for the bombardment of target material

Electronics of x-ray production

1. X-rays are produced by the interactions that occur when high-speed electrons strike a target material

2. The phenomenon of x-ray production occurs only when the exposure switch on the console is pressed, which completes the high-voltage circuit

a. The heated filament provides electrons for x-ray production by thermionic emission

b. Thermionic emission occurs when electrons absorb sufficient thermal energy (from the mA circuit) to allow for the electrons’ short movement away from the filament; commonly referred to as a “boiling off” of electrons; an electron cloud surrounding the filament is formed

c. The closure of the high-voltage circuit creates an electrical potential difference whereby electrons are attracted from the negative cathode to the positive anode

d. The one-directional flow of electrons (from the negative cathode to the positive anode) is influenced by the focusing cup of the cathode; electrons are repelled away from the negatively charged focusing cup because like charges repel; this mechanism controls the size and shape of the electron stream

Electron–target interactions

1. The electron stream is directed at a small portion of the target called the focal spot

2. The actual production of x-radiation occurs by the interaction of accelerating electrons and target atoms

3. Less than 1% of the kinetic energy leaving the cathode is converted into x-ray energy; just over 99% of the kinetic energy leaving the cathode is converted into heat energy

4. Two types of interactions for x-ray production

a. General (braking, or bremsstrahlung) radiation (Figure 6-4)

b. Characteristic radiation (see Figure 6-4)

Characteristics of x-rays

Interactions of X-Rays with Matter

Considerations

1. When x-ray photons interact with matter, they may be either absorbed or scattered

2. Attenuation (removal) of x-ray photons from the beam as they travel through matter (tissue) is determined by the intensity (energy) of the radiation and the density, atomic number, and electrons per gram of the matter

3. Definitions

Types of interactions

1. No interaction

2. Photoelectric effect

3. Compton scatter

4. Coherent scatter

Image formation and differential attenuation

1. Radiographic image formation depends on the differential attenuation of x-ray photons from the primary beam by the client’s tissues

2. If all photons exited the client’s tissues (no absorption), the image receptor would be totally exposed; if all photons were attenuated (absorbed), the image receptor would be unexposed

3. As a result of photoelectric, Compton, and coherent interactions, x-ray photons are removed from the beam

4. Variances in the ability of tissues to absorb x-rays produce radiographic contrast

5. Generally, as density, atomic number, and electrons per gram of tissue increase, the number of absorbed photons increases; metallic dental restorative materials, enamel, dentin, cementum, and bone absorb photons to a great extent and produce radiopaque images; bone marrow spaces, sinuses, pulp chambers, and periodontal ligament spaces do not attenuate photons and produce radiolucent images

Interactions of Ionizing Radiation with Cells, Tissues, and Organs

Definitions

1. Whole-body exposure (total body)—each gram of tissue in the entire body absorbs equal amounts of radiation

2. Specific-area exposure (localized)—each gram of body tissue irradiated in the specific area absorbs equal amounts of radiation (e.g., skin exposure from four bitewing radiographs)

3. Direct effect—transfer of energy by ionization mechanism from an x-ray photon to a biologically critical molecule such as deoxyribonucleic acid (DNA)

4. Indirect effect—transfer of energy by the ionization mechanism from an x-ray photon to a noncritical molecule, which, in turn, delivers the energy to the biologically critical molecule

5. Genetic effect—causes mutations in future generations; results from the exposure of reproductive cells, yielding alterations in genetic coding

6. Somatic effect—injury in the person being irradiated

7. Latent period—time between the exposure and development of the biologic effect

8. Deterministic effect— when the severity of a biological response is dependent on the dose; for example: erythema (redness of the tissue) would be expected to increase in direct proportion to exposure to damaging radiation

9. Stochastic effect—a biologic response that is based on the probability of occurrence rather than its severity; for example: cancer may or may not occur with exposure to damaging radiation

10. Acute effects (short-term or early)—effects that may occur minutes, hours, or weeks after exposure; usually result from high doses of whole-body exposure

11. Chronic effects (long-term or late)—effects observed years after original exposure

Units of radiation measurement—two systems used to define radiation measurement: the metric equivalent system, or Systeme Internationale (SI), adopted in 1985, is preferred;3 an older, traditional system may be found in the research literature published prior to 1985

1. Radiation exposure—measurement of ionization in air produced by x-rays

2. Radiation absorbed dose—amount of radiation absorbed by tissue

3. Dose equivalent—measure of biologic effects produced by different types of radiation

Characteristics of dose–response relationships

Biologic responses to irradiation

1. Considerations

2. Radiation effects on cells

a. Two types of cells in the human body:

b. Nucleus of proliferating somatic and genetic cells—area of the cell most sensitive to ionizing effects

(1) Exposure to the nucleus results in cell inhibition

(2) Most sensitive sites within the cell’s nucleus—DNA and chromosomes

(3) Chromosomal aberrations in somatic cells are observed during the metaphase stage of mitosis (cell division); changes in genetic material can occur during meiosis or reduction division

c. Cells most sensitive to radiation exposure—young, rapidly dividing, nondifferentiated cells such as those of the developing fetus

d. Cell responses to irradiation

3. Radiolysis of water

4. Factors that determine tissue sensitivity

5. Degree of tissue and organ sensitivity4

6. Repair and accumulation of radiation effects

Ethical Considerations Regarding the Use of Ionizing Radiation

Regulatory and recommending agencies (see the unnumbered table “Web Site Information and Resources” at the end of this chapter)

Assessment of need for radiographic procedures

1. A licensed physician or dentist must prescribe radiographic services

2. Dental hygiene assessment for recommendation of need for radiographs

a. Must be based on a review of the client’s health and dental histories; clinical examinations; client signs, symptoms, and complaints

b. Radiographs are recommended only in the presence of reasonable expectation of client benefit2

c. Recommendations based on U.S. Food and Drug Administration (FDA) selection criteria guidelines (Table 6-1)

TABLE 6-1

Guidelines for Prescribing Dental Radiographs

image

image

image

(Data from U.S. Department of Health and Human Services: The selection of patients for dental radiographic examinations, Revised 2004 by the American Dental Association: Council on Dental Benefit Program, Council on Dental Practice, Council on Scientific Affairs.)

d. If no positive findings are noted in the clinical examination or the client’s history, radiography should not be performed

Radiation protection

1. ALARA (“As Low As Reasonably Achievable”) concept—individuals working with radiation should attempt to keep all radiation exposure as low as reasonably achievable

2. Maximum permissible dose (MPD) (Table 6-2)

TABLE 6-2

U.S. Nuclear Regulatory Commission Occupational Dose Limits

Tissue Annual Dose Limit
Whole body 0.05 Sv
Any organ 0.5 Sv
Skin 0.5 Sv
Extremity 0.5 Sv
Lens of eye 0.15 Sv

Sv, Sievert.

(United States Nuclear Regulatory Commission: Standards for protection against radiation, Title 10, Part 20, of the Code of Federal Regulations. December 4, 2007: Available at www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1201.html: Accessed Apr.11, 2010.)

3. Reduction of unnecessary client dose

a. Eliminate unnecessary examinations

b. Eliminate repeat examinations

c. Use proper exposure settings

d. Use safe equipment

e. Use the fastest image receptor system available

f. Position client and image receptor properly

g. Shield specific areas from possible secondary radiation with 0.25-mm–thick lead or lead equivalent apron

4. Reduction of occupational exposure

a. Use protective shielding and apparel

b. Distance from the source of radiation, if structural shielding is not available

c. Never hold the client, the image receptor, or the tube head during exposure

d. Use personnel radiation-monitoring devices

Personnel radiation-monitoring devices4

Documentation of client’s radiographic exposure

Documentation of client’s refusal of radiographic services

Infection Control

See the section on “Prevention of Disease Transmission During Radiographic Procedures” in Chapter 10.

General considerations

Procedure

1. Unit preparation

a. Clean and disinfect the radiographic equipment (e.g., tube head and PID, tube-head support arm, control panel, countertop, treatment chair)

b. Use the barrier technique and disposable plastic wrap to cover items likely to become contaminated (e.g., tube head and PID, tube-head support arm, control panel, countertop, treatment chair)

c. Obtain radiographic supplies (e.g., image receptors, holding devices) before starting the procedure

d. Disposable intraoral film packet barrier wraps may be used to minimize the possibility of disease transmission during the radiographic procedure and processing

c. Digital sensors should be wiped with an Environmental Protection Agency (EPA)–accepted disinfectant before placing the protective sheath barrier and following removal; consult the manufacturers’ recommendations

2. While exposing intraoral radiographs, wear disposable client-care gloves

a. Wipe the film packet with an EPA-accepted disinfectant after removal of the film from the client’s mouth and drop the film into a containment cup6 (Figure 6-7)

b. If using a disposable intraoral film packet barrier wrap, after wiping the packet barrier wrap with an EPA-accepted disinfectant, aseptically tear open the wrap, allowing the protected film to drop aseptically into a containment cup; this same procedure is used for phosphor plates; allow the protected phosphor plates to drop into the light-tight containment box; consult the manufacturer’s recommendations.

3. Process intraoral radiographs under safelight conditions

4. Following radiographic procedures

Image Receptors

Definition

Types

Jan 1, 2015 | Posted by in Dental Hygiene | Comments Off on 6: Oral and Maxillofacial Radiology
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