5: Radiology

CHAPTER 5 Radiology

FUNDAMENTAL RADIATION PHYSICS

Understanding radiation physics requires a basic knowledge of atomic structure, radiation types, properties of radiation.

image • See CD-ROM for Chapter Terms and WebLinks.
A. Atomic structure: nucleus and electrons in orbiting shells of an atom function in state of equilibrium until ionization occurs (Figure 5-1):

1. Nucleus:

a. Contains protons, with positive charge.
b. Contains neutrons, with NO electrical charge.

2. Orbiting shells:

a. Contain electrons, with negative charge, and circle the nucleus in shells.
b. Have binding energy, which keeps electrons in their shells as a result of centripetal force and attraction of opposites.

3. Ionization:

a. Occurs when an atom loses an electron from one of its shells and becomes part of ion pair (not unlike the celebrity world with its changing pairs); this process elicits chemical changes in matter.
b. Ion pair is created when an x-ray photon ejects a negative electron from its shell and neutral atom becomes positive.
c. Ionizing radiation is any radiation that is capable of producing ions.

B. Types of radiation: particulate and electromagnetic radiation are types of ionizing radiation:

1. Particulate (corpuscular) radiation:

a. Involves mass and particles that travel in straight lines at high speeds.
b. Four types:

(1) Electrons: originate from radioactive atoms; called beta particles.
(2) Protons: heavily charged particles (namely, hydrogen nuclei).
(3) Neutrons: have mass but NO electrical charge.
(4) Alpha particles: large and emitted from nucleus of heavy metals; usually unable to penetrate tissue.

2. Electromagnetic radiation:

a. Comprises electric and magnetic fields of energy that move through space in wavelike motion.
b. Some forms are ionizing and others are not, depending on their energy (wavelengths) (e.g., x-rays have short wavelengths and are ionizing).
c. Arranged in a spectrum according to its energies, which is demonstrated by its wavelengths and frequencies; wave concept suggests that electromagnetic radiation is much like waves or ripples of water and involves wave frequency and length.

(1) Wavelength:

(a) Distance between the peaks or crests of a wave.
(b) Determines energy of radiation; longer the distance, LESS energetic the radiation, LESS ability it has to penetrate objects.
(c) Measured according to distance between peaks; longer wavelengths are measured in meters; shorter wavelengths are measured in nanometers (0.01 to 0.05 nm).

(2) Frequency:

(a) Number of peaks or crests that occur in a given amount of time.
(b) Frequency of the peaks determines energy of the wavelength; MORE frequent the peaks, shorter and MORE energetic the radiation.

C. Properties of x-rays:

1. X-rays are ionizing forms of radiation that have unique properties.
2. Bundles of pure energy (short wavelengths) that have NO electrical charge.
3. Travel at speed of light (3 × 108 m/sec) (The Flash!).
4. Invisible, weightless, undetectable by any of the senses.
5. Absorbed by matter according to atomic structure of material that is exposed.
6. Cause ionization, which produces biological change in interaction with matter.

image

Figure 5-1 Atomic structure.

X-Ray Machine and Components

X-ray machine has basic operable components and internal components that are important for safe operation and functioning.

A. Basic operable components: include control panel, tubehead, position-indicating device (PID):

1. Control panel:

a. Operated by visible on-off switches, exposure button, indicator light.
b. Controls milliamperage, time, kilovoltage settings by dials on panel.

2. Tubehead (Figure 5-2):

a. Heavy metal housing for transformers and x-ray tube that serves as attachment for PID, filter, collimator.
b. Contains insulating oil, which surrounds x-ray tube and transformers to prevent overheating.
3. Position-indicating device (PID):

a. Attaches to the tubehead.
b. Lead-lined, open-ended cylinder or rectangle that directs x-ray beam to object and film.

B. Internal components: include x-ray tube, power supply, circuit, transformer:

1. X-ray tube:

a. Lead glass vacuum tube that is 1 inch by several inches long.
b. Contains two types of electrodes:

(1) Cathode (negative) electrode:

(a) Has tungsten filament, which is a wire that is source of electrons when heated.
(b) Has molybdenum cup, semicircular in shape, to focus direction of electrons toward anode.
(c) Contains milliamperage (mA) control.

(2) Anode (positive) electrode:

(a) Has tungsten plate (or target) that electrons hit (when they leave the cathode) to produce x-rays and heat.
(b) Has copper stem with an anode embedded in it to dissipate excessive amount of heat produced.
(c) Has focal spot, hit by the electrons leaving cathode.
(d) Contains kVp control

2. Power supply:

a. Electricity that provides energy to unit to produce x-rays and is described by its current, amperage, voltage.
b. Electric current (or flow) of electrons through a wire in a given point of time:

(1) When a direct current (DC), electrons flow in one direction.
(2) When an alternating current (AC), electrons flow in one direction and then change to flow in another direction.

c. Usually measured in amperes; however, in dentistry, milliampere (image of ampere) is used.
d. Uses voltage as the unit of force between two points; measured in kilovolts (1000 volts).

3. Circuit:

a. Route the electrical current takes.
b. Filament and high-voltage are two types of circuits:

(1) Filament circuit:

(a) Adjusts flow through a low-voltage circuit.
(b) Controls the heating of a filament and quantity of available electrons.
(c) Controlled by milliamperage (mA) setting.

(2) High-voltage circuit, uses 65,000 to 100,000 volts to provide high voltage necessary to propel electrons and produce x-rays.

4. Transformer:

a. Mechanism used in electrical circuit to increase or decrease the voltage.
b. May be a step-up, step-down, or autotransformer type:

(1) Step-down transformer:

(a) Needed to produce electrons by heating the tungsten filament in a cathode.
(b) Decreases incoming line voltage (110 to 220) to required 3 to 5 volts.
(c) Has MORE wires in primary coil (input coil) than in secondary coil (output coil).

(2) Step-up transformer:

(a) Provides energy needed to propel electrons from the negative pole to positive pole to produce x-rays.
(b) Increases incoming line voltage (110 to 220) to required 65,000 to 100,000 volts.
(c) Has MORE wires in secondary coil than in primary coil.

c. Autotransformer: changes voltage input into primary coil of step-up transformer.

image

Figure 5-2 Components of x-ray machine.

Generation of X-Rays

X-rays are produced in x-ray tube through complex process that involves negative and positive poles and differing types of x-rays produced.

A. X-ray process: involves use of both cathode and anode:

1. Cathode: negative pole in x-ray tube:

a. Contains tungsten filament that is heated by filament circuit and generates electrons:

(1) Heating temperature is controlled by milliamperage.
(2) Available electrons are “boiled off” from filament by process called thermionic emission.

b. Includes focusing cup, made of molybdenum; cup surrounds filament and directs electrons to focal spot on anode.

2. Anode: positive pole in x-ray tube:

a. Has copper stem that serves as thermal conductor and dissipates heat away from the anode.
b. Includes target made of tungsten, embedded in the copper stem:

(1) Target’s purpose is to convert the electrons from cathode into x-rays.
(2) Process results in generation of x-rays (1%) and heat (99%).

B. X-ray production:

1. Produced in an x-ray tube when kinetic energy from electrons hits the target in the anode.
2. Types include Bremsstrahlung and characteristic radiation:

a. Bremsstrahlung radiation:

(1) Means “braking action” in German; MAJOR source of x-rays produced in dentistry.
(2) Results when high-energy electrons come close to the nuclei of tungsten atoms but are slowed down by the positive pull of the nuclei.
(3) X-ray production results from the slowing down of the electrons, which releases energy.

b. Characteristic radiation:

(1) NOT generated frequently; involves electrons dislodging electrons from the K/L shell of tungsten atom.
(2) Requires filling of a vacancy by lower energy shell electron, resulting in an x-ray that comprises the binding energy between these two different shell electrons.

Interaction of X-Rays with Matter

An X-ray weakens whenever reacts with matter. When x-rays pass through atoms of tissue but do NOT react with the tissue, NO interaction occurs.

A. Photoelectric effect:

1. Ionization that occurs when x-ray photon interacts with inner shell electron.
2. When photons are absorbed and electrons are ejected, effect ceases to exist (NO penetrating power).
3. Responsible for ~30% of all interactions in dental x-rays.

B. Compton scatter (effect):

1. Ionization that occurs when x-ray photon interacts with an outer shell electron.
2. Occurs when electron is ejected and x-ray photon scatters in a different direction.
3. Responsible for ~62% of all scatter in dental x-rays.

C. Thompson (coherent) scatter:

1. Results without the occurrence of ionization.
2. Occurs when low-energy photon reacts with an outer shell electron and NO loss of energy occurs.
3. X-ray photon scatters in a different direction.

Units of Radiation Measurement

Two units of radiation were established to measure exposure, dose, and dose equivalent by the International Commission on Radiation Units and Measurements. Two systems are used for each unit: Standard and Système International (SI).

A. Exposure:

1. Radiation quantity that measures ionization in air; described in roentgens or SI units.

a. Intensity measured by the roentgen (R).
b. In other SI units, equivalent to coulombs per kilogram (C/kg).

B. Dose:

1. Amount of radiation energy absorbed by tissue:

a. Rate is amount of radiation absorbed per unit of time.
b. Often identified with radiation absorbed dose (RAD):

(1) Traditional unit of dose.
(2) The SI equivalent is the gray (Gy) or J/kg.
(3) Conversion:

(a) 1 Gy = 100 rad
(b) 1 rad = 0.01 Gy
(c) 1 J/kg = 100 rad

C. Dose equivalent:

1. Has differing biological effects when compared with different types of radiation.
2. Quality factor is a specific number, according to exposure effect, that is given to each type of radiation.
3. Roentgen-equivalent-man (REM):

a. Traditional unit of absorbed dose.
b. The SI equivalent is the sievert (Sv).
c. Conversion:

(1) 1 Sv = 100 rem
(2) 1 rem = 0.01 Sv

Mechanisms of Radiation Injury

Two theories attempt to explain biological effects of radiation that result in injury to living tissues, and dose-response curve is a graphic means of showing those effects.

A. Direct theory:

1. Radiation damage to tissues is caused by a direct hit on the DNA molecule of a cell, which causes cell death.
2. Seldom causes radiation injury.

B. Indirect theory:

1. Radiation damage to tissues involves the formation of free radicals.
2. Applies when x-ray photon is absorbed in a cell, which creates toxins (free radicals) and damages the cell.
3. Indirect injuries are common because of the reaction with water (70% to 80%) in cells (i.e., formation of hydrogen peroxide).

C. Dose-response curve:

1. Graphic means of plotting biological response (damage) to radiation exposure (dose) to determine acceptable levels of radiation exposure.
2. For low doses of radiation, very little information is available about acceptable levels.
3. Graphically, linear nonthreshold curve occurs, which indicates that response is seen at any dose.
4. Results indicate that there is NO true safe level of radiation exposure.

Factors That Determine Radiation Injury

Several factors determine amount of injury that occurs from radiation: total dose, amount of area exposed, types of cells exposed, cell sensitivity.

A. Total dose: amount of radiation absorbed in the tissue that determines effect.

1. Acute exposure: ionizing radiation given in shorter amount of time; results in MORE dramatic biological effects.
2. Chronic exposure: small amounts of ionizing radiation given over longer period; results in less effect on tissue.
3. Latent period: time (hours, days, or months) between radiation exposure and observed clinical effect.

B. Exposure area: amount of the body exposed to radiation, also determines biological effect of radiation on the tissues.

1. Whole-body exposure: exposure of entire body, results in MORE severe biological effects; can be acute or chronic.

a. Acute: occurs in nuclear warfare and with occupational hazards; can result in such symptoms as nausea, vomiting, bleeding, diarrhea.
b. Chronic: exposure to natural background radiation; largest single cause is cosmic rays.

2. Limited-area exposure: irradiation to specific body area; can be acute or chronic.

a. Acute: to higher doses directed at specific area, such as occurs during radiation therapy (see effects on cells and organs listed below).
b. Chronic: to lower doses, such as occurs during dental radiography.

C. Cell types: effects of radiation are determined by cell maturity and cell type:

1. Law of Bergonié and Tribondeau states that the greatest radiation effects occur in cells that are immature, are NOT highly specialized, and frequently divide.

a. Cell maturity:

(1) Immature cells are nonspecialized; experience rapid cell division.
(2) Mature cells are specialized in function; divide at slower rate or do NOT divide at all.

b. Cell types:

(1) Somatic tissue cells are NOT inherited and NOT involved in reproduction (e.g., skin, kidney cells).
(2) Genetic tissue cells are reproductive tissue cells (e.g., ovaries in woman and testes in man).

D. Radiosensitivity: human tissue cells have different sensitivities to radiation:

1. Radiosensitive tissues and cells are highly sensitive to radiation (e.g., bone marrow, reproductive, small lymphocyte).
2. Radioresistant tissues and cells are MORE resistant to radiation (e.g., salivary glands, lungs, kidneys, muscle).
3. In order of MOST sensitive to least sensitive: blood-forming and reproductive cells, gastrointestinal tract, immature bone, epithelial (glandular) and muscle tissues, adult bone and nerve tissue; blood type does NOT affect sensitivity.

E. Critical organs and radiation:

1. When sensitive organs and tissues are exposed to radiation during dental irradiation, significant decrease in quality of person’s life may occur.
2. Include skin, eye lens, thyroid, bone marrow (hematopoietic):

a. Skin irradiation may lead to erythema: FIRST clinical sign of overexposure (250 rad in 14-day period).
b. Eye irradiation can lead to cataract formation if exposure to eyes is 200,000 mrad.
c. Thyroid irradiation can result in increased cancer risk, particularly when radiation exposure of 6000 mrad or more occurs.
d. Bone marrow irradiation poses increased cancer risk (particularly leukemia) at exposures of 5000 mrad or more of radiation.
e. Organs MOST affected by continued low doses of radiation in order from MOST sensitive to least sensitive are thyroid, skin, brain.

Radiation Exposure and Risks

Exposure and risks associated with radiation include those from environment (background) and those associated with direct irradiation of organs. (See the WebLink image on the CD-ROM for the ADA and FDA Guide to Patient Selection for Dental Radiographs.)

A. Background radiation: GREATEST contribution to radiation is from naturally occurring sources in environment:

1. External radiation: from cosmic and terrestrial sources; comprises ~15% of the radiation exposure to the population.
2. Internal radiation: from exposure to radionuclides through inhalation and ingestion; comprises ~67% of radiation exposure; radon is the LARGEST single contributor to natural radiation.

B. Artificial radiation: includes such sources as medical and dental radiation:

1. Comprises ~17% of radiation exposure.
2. Exposure is equivalent to few days’ worth of background radiation or environmental radiation exposure, or similar to dose received during cross-country airplane flight.

Radiation Protection

Proper use of equipment, patient and clinician positioning, and technique can provide acceptable limits to incidental radiation exposure. Advances in dental radiograph technology have reduced scatter radiation, the reason for protective boxes; lead-lined radiograph storage boxes are thus unnecessary and should be discarded, since they contaminated the film. There is controversy over need for lead (or lead equivalent) aprons for patient protection. Note that term “clinician” is used for operator.

A. Patient: patient exposure to radiation MUST BE reduced by using filtration, collimation, positioning, shielding:

1. Filtration involves filtering-out of nonproductive x-rays; inherent filtration plus added filtration equals total filtration for the machine:

a. Inherent filtration is inside tubehead (i.e., oil, glass window, tubehead seal).
b. Added filtration includes aluminum disc (disk) placed around seal of machine:

(1) Used to filter out longer, nonproductive wavelengths, resulting in primary beam with MORE energy.
(2) Federal and state laws dictate thickness of total filtration according to kilovolt peak (kVp) setting of the machine (≤70 kVp = 1.5-mm thickness of aluminum; ≥70 kVp = 2.5-mm thickness).

2. Collimation restricts size and shape of x-ray beam (reducing film fog) and therefore restricts patient exposure (scatter exposure):

a. Collimator: lead diaphragm placed over opening of tubehead.
b. Exposure reduction is possible by limiting size of x-ray beam at end of PID to no more than 2.75 inches in diameter, using rectangular PID, and placing tubehead 1½ to 2 inches from patient.

3. The PID (beam-indicating device [BID]): aiming device that is used to direct the x-ray beam:

a. Cone-shaped PID: closed, pointed, plastic device that increases scatter radiation and should NOT be used.
b. Round PID: open ended, circular, and lead lined; standard lengths are 8 inches and 16 inches; 16 inches is MAINLY used because provides MORE parallel rays.
c. Rectangular lead-lined PID: highly recommended because offers MOST reduction in radiation exposure to the patient; standard lengths are 8 inches and 16 inches.

4. Lead aprons or lead equivalent (contains 0.25 to 0.3 mm of lead thickness) shields; aprons worn by patients to protect tissues from scatter radiation; mandated by law in many states; should be used in conjunction with thyroid collar for intraoral films (questionable use now).
5. Fast film: one of the MOST effective means of reducing patient exposure; F-speed film is fastest and is MORE than twice as fast as D-speed film; F-speed film needs 60% LESS exposure time and E-speed film needs 50% LESS exposure time than D-speed film; thus E- and F-speed films recommended.
6. Film-holding devices: method necessary to stabilize film and ultimately reduce additional exposure to patient.

B. Clinician: measures MUST be taken to protect clinician from unnecessary radiation; include proper positioning, use of monitors, attention to proper methods of taking radiographs:

1. Positioning:

a. Involves awareness of distance; remaining at least 6 feet from the source of radiation and in a safe quadrant (at right angles to the primary beam) is one of the MOST effective ways to eliminate unnecessary clinician exposure.
b. Clinician SHOULD never hold the tubehead or film in patient’s mouth during exposure.
c. Clinician should stand behind lead shield or barrier whenever possible (cinderblock or 2½ inches of drywall provides adequate protection).

2. Monitoring:

a. Involves using personnel monitoring devices and following national guidelines when measuring occupational radiation exposure; film badges that measure exposure to low doses of radiation should be worn at waist level; two types of monitoring available.

(1) Metal filters: located inside badge, measure amount of radiation but can be worn for ONLY 1 month; hard radiation shows faint shadow; soft radiation casts pronounced shadow.
(2) Thermoluminescent dosimeter (TLD): device that contains lithium fluoride crystals that absorb x-ray energy; MORE sensitive and can be worn for 3 months but is MORE expensive.

b. Maximum permissible dose (MPD): developed by National Council on Radiation Protection (NCRP):

(1) Relates to amount of radiation exposure that is permissible through occupational exposure.
(2) Considered to be maximum dose that does NOT produce significant injury in a lifetime.
(3) Formula represents whole-body dose equivalent of ionizing and electromagnetic radiations and is expressed in sieverts.
(4) The MPD for occupationally exposed persons:

(a) Per year = 50 mSv (5 rem, 5000 mrem).
(b) Per month = 4 mSv (400 mrem).
(c) Per week = 1 mSv (100 mrem).

(5) The MPD for nonoccupational (whole-body) exposure: 10% of that for worker, or 5 mSv (500 mrem) per year.
(6) Maximum accumulated dose = 5 rem × N −18 (N = age).

c. The ALARA concept: As Low As Reasonably Achievable.

(1) Philosophy of radiation protection that is currently practiced by all radiation workers.
(2) Implies that every effort will be made to keep radiation exposure, whether occupational or nonoccupational, as LOW as possible (see later discussion of guidelines for patient radiographic examination).

Digital Imaging

With digital imaging, nonfilm sensor is placed inside the mouth; electronically attached to computer, immediately producing image on monitor.

A. Principles:

1. Requires conventional equipment to generate x-rays; see earlier discussion.
2. Image captured is SIMILAR to conventional film.
3. When sensor is exposed to radiation, electronic chart is produced on its surface.
4. Direct or indirect (scanned):

a. Direct: sensor in rigid case, either charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS), is used; immediate image and immediate reuse.
b. Indirect: photostimulable phosphor (PSP) plate is used to store image, which is then scanned, read, and erased; conventional films can also be scanned and converted to digital.

B. Advantages:

1. Images can be stored, retained as a hard copy, or sent to a different site.
2. Images can be displayed immediately, which is useful for intraoperative procedures such as endodontic treatment; eliminates darkroom procedures.
3. Provides less radiation exposure to the patient but poorer resolution, costs MORE, large sensors, manipulated films, MORE exposures because of small sensor size.
4. Associated software using a database can analyze changes in radiographic density to identify demineralized areas and determine probability that caries is present.

C. Disadvantages: initial cost, uncomfortable sensors, infection control of sensors by plastic sleeves and NOT heat sterilization.

CLINICAL STUDY

Age 42 YRS SCENARIO
Sex image Male imageFemale The new patient’s intraoral examination reveals several suspect areas and fractured restorations on the maxillary arch. It has been at least 6 or 7 years since patient has had radiographs taken. The dental hygienist recommends a full-mouth series (FMX). However, the patient is reluctant because of what she has heard about the dangers and misuse of radiation.
Height 5′4″
Weight 170 LBS
BP 118/76
Chief Complaint “Boy, the fillings in my upper back teeth are so sensitive! They feel like they are moving when I bite down.”
Medical History
Has had a bad cold for 2 weeks with postnasal drip
Mother died of breast cancer
Has mammograms every 6 months because of high risk

Current Medications None Social History
Bus driver
Single mom with two children
Likes bowling

1. What is the most probable diagnosis of the patient’s condition? Identify all possible causes for her symptoms. What other information should be gathered to determine the cause of the sensitivity?
2. What issue must be addressed to provide this patient with the best treatment?
3. What procedures are used to protect the patient from unnecessary radiation?

1. Sinus infection and carious lesions are present. Sensitivity of maxillary teeth could be related to the closeness of the roots of maxillary teeth to enlarged, inflamed sinuses; such sensitivity should dissipate as sinusitis clears up. Movement of restorations is more serious in nature and is indicative of fractured restorations. Ill-fitting restorations often have marginal leakage and/or recurrent caries. Other information can be gained by asking: “What are the teeth sensitive to (e.g., temperature, pressure, sweets)?”, “How long have they been sensitive?”, and “Does anything seem to exacerbate or alleviate the discomfort?”
2. Benefit versus risk of having radiographs taken should be discussed with patient. Clinical examinations warrant FMX radiographs to confirm exact diagnosis and direction of proper treatment. Although minimal amount of radiation exposure occurs, benefit of diagnosis far outweighs risk of allowing carious lesions to continue without diagnosis and then treatment.
3. Devices on x-ray machine such as filter, collimator, lead-lined PID reduce radiation exposure to patient. Using lead apron or lead equivalent with thyroid collar and film-holding devices with fast-speed film (F-speed film is fastest) keeps patient exposure to minimum while providing excellent-quality diagnostic radiographs.

CHARACTERISTICS OF RADIATION IMAGES

Radiation images are characterized by their quantity, quality, intensity, clarity.

A. Quantity: number of x-rays that are produced; measured in amperes, milliampere-seconds, density of the radiographs.

1. Ampere: number of electrons that flow through a filament:

a. Milliampere (mA) is equal to 1–1000 of an ampere.
b. Milliamperage controls temperature of the cathode filament.

2. Milliampere-seconds (mA-s): combination of milliamperes and seconds:

a. Milliamperes have a direct effect on the number of electrons produced.
b. Seconds have the same effect on the quantity of electrons produced.
c. To maintain similar density:

(1) Increased seconds, milliamperage must be decreased.
(2) Decreased seconds, milliamperage must be increased.

3. Density: overall darkness of radiograph:

a. Affected by milliamperage and by quantity of electrons produced.

(1) Increased mA, film will be darker.
(2) Decreased mA, film will be lighter.

B. Quality: energy and penetrating power of the beam; measured in kilovolts; affects image contrasts:

1. Kilovolt (kV): equal to 1000 volts and is measurement used to determine energy in a tubehead:

a. Kilovoltage peak (kVp) represents peak or maximum voltage; determines speed and ultimately energy of electrons.
b. With increased kVp, primary beam will have MORE energy and density will increase.

2. Contrast: difference between lighter and darker shades (grays) on radiograph:

a. Increased kVp (≥90) results in many shades of gray on the film (long-scale contrast); BEST for detecting bone abnormalities.
b. Decreased kVp (65 to 70) results in MORE black and white areas on a radiograph (short-scale contrast); BEST for detecting caries.

C. Beam intensity: affected by mA, kVp, exposure time, distance:

1. The mA affects intensity by controlling the number of electrons produced; higher the mA, MORE intense the beam.
2. The kVp controls energy of electrons traveling from the cathode to the anode; higher the kVp, MORE intense the beam.
3. Exposure time affects the intensity in the same manner as mA; increase in time = MORE intense beam.
4. Distance also affects beam intensity; as beam travels longer distance to film or tooth, becomes LESS intense:

a. Inverse square law states that “intensity of radiation is inversely proportional to square of the distance from the source of radiation”; explains how the beam loses its intensity as it travels farther from the source.
b. Half-value layer describes the reduction in beam intensity by one half with the use of aluminum filters; filters remove LESS penetrating, longer wavelengths in the beam.

D. Accurate image (geometric) formation: BEST produced by controlling image sharpness, magnification, distortion:

1. Sharpness (detail, definition): umbra is increased clarity or distinctness of outlines of object; penumbra is fuzziness or LACK of sharpness in image.

a. Influenced by focal spot size; tungsten target of anode should be small to increase sharpness.
b. Associated with film composition; size of crystals on film determines sharpness; larger the crystals, LESS sharp the image.
c. MAINLY influenced by patient and tubehead movement; either will reduce image sharpness.

2. Magnification: enlargement of actual image:

a. Influenced by target-film distance; increased distance from target in the x-ray tube to film allows MORE parallel rays to hit film, LESS magnification.
b. Affected by object-film distance; closer the tooth to film, LESS the magnification.

3. Distortion: disfigurement of the shape and size of an image:

a. Affected by the object-film alignment; film and long axis of the tooth MUST be parallel to each other (so that rays will hit at a right angle) to decrease distortion.
b. Influenced by beam direction; beam MUST be directed perpendicular to both the film and the tooth to record accurate, distortion-free image.

4. Pneumatization: with free airspace in a structure or tissue, one area is MORE radiolucent than another area (frequently seen when maxillary sinus has dropped).

RADIOGRAPHIC EXAMINATION

Radiographic examination involves intraoral, extraoral, and special imaging techniques to produce high-quality radiographs for use in examination, interpretation, diagnosis of dental conditions and diseases. (See the CD-ROM image for WebLink to the ADA/FDA Guide to Patient Selection for Dental Radiographs, which includes the chart Guidelines for Prescribing Dental Radiographs.) These guidelines will allow for ALARA concept, although each case is subject to clinical judgment.

Intraoral Radiographic Views

Views include periapical (PA), bitewing (BW), occlusal, full-mouth series (FMX).

A. The PA view: captures from cementoenamel junction (CEJ) to root on the film; included in FMX; uses size 1 to 2 films:

1. Used for diagnosis of a pathological changes in a specific tooth by its root and surrounding bone.
2. Especially if there is a need for endodontic therapy (late finding).

B. The BW view: visualizes crowns, contacts, height of alveolar bone in relation to CEJ; included in FMX; uses size 1 to 2 films:

1. Used for diagnosis of a series of teeth as they contact on the occlusal.
2. Used for diagnosis of dental caries (ONLY inter-proximal early, with occlusal and root late) and periodontal disease (ONLY bone loss on interproximal alveolar crest is shown, NOT facial or lingual plates).
3. Can be either horizontal or vertical in placement; vertical used to evaluate alveolar bone loss.

C. Occlusal view: reveals bone surrounding tooth, floor of the mouth, or presence of sialolith (stone) in parotid (Stensen’s) duct; NOT included in FMX; uses size 4 films.
D. FMX: complete set of intraoral radiographs: number depends on size of films:

1. Use size 1 for anteriors and size 2 for others: 20 films, 4 BWs and 16 PAs.
2. Use only size 2: 20 films, 4 BWs and 14 PAs.

Intraoral Techniques

Intraoral radiography includes bisecting, paralleling, and occlusal techniques.

A. Bisecting (angle) technique (Figure 5-3):

1. Based on rule of isometry: two triangles are equal if they have equal angles and share common side.
2. Requires bisection of the angle, forms two equal triangles; angle formed by plane of film and plane of long axis of tooth.
3. If technique is strictly followed, directs primary beam 90° to bisected line, resulting in accurate image.
4. MORE distortion than paralleling technique because of position of the palate.
B. Paralleling technique:

1. Based on geometric figures of parallelism.
2. Requires that film be placed parallel to long axis of the tooth.
3. Requires film holder for proper film placement.
4. Directs primary beam perpendicular to film and long axis of the tooth.

C. Occlusal technique:

1. For anterior topographical mandibular occlusal:

a. Head tilted backward, using negative 55° vertical angle.
b. Central ray directed through point of the chin.

2. For anterior topographical maxillary occlusal:

a. Sagittal plane perpendicular to floor; occlusal plane parallel to floor.
b. Positive vertical angle of 65°; central ray is directed just superior to tip of nose.

3. For cross-sectional maxillary occlusal:

a. Sagittal plane perpendicular to floor; occlusal plane parallel to floor.
b. Central ray directed to film at 90°, with PID centered between the eyebrows.

4. For cross-sectional mandibular occlusal:

a. Head tilted backward until ala-tragus line is almost perpendicular to floor.
b. A 90° vertical angle, central ray directed 3 cm inferior to the chin.

D. Buccal object (SLOB: Same Lingual, Opposite Buccal) rule: shows whether artifact or object is lingual or buccal; in second radiograph, if central ray is moved and object moves in same direction, then lingual, if it moves in opposite direction, buccal; used mainly in endodontics, since on PAs, roots are often superimposed on one another and require separation for proper identification so as to visualize lengths and anatomy.
image

Figure 5-3 Bisecting (angle) technique.

Intraoral Technique Errors

Proper recognition of errors can help clinician correct a technique problem before it is repeated, possibly subjecting patient to unnecessary radiation and producing nondiagnostic radiographs.

A. Overlap (Figure 5-4):

1. Adjacent tooth structures superimposed on each other.
2. Corrected by redirecting primary beam through contacts (change in horizontal angulation).
B. Foreshortening (Figure />

Related posts:

  1. 10: Medical and Dental Emergencies
  2. 7: Nutrition
  3. 8: Microbiology and Immunology
  4. 13: Periodontology
  5. 14: Pain Management
  6. 6: General and Oral Pathology

Stay updated, free dental videos. Join our Telegram channel
Tags:
Jan 1, 2015 | Posted by in Dental Hygiene | Comments Off on 5: Radiology

VIDEdental - Online dental courses
Get VIDEdental app for watching clinical videos