Dental Radiology for Dental Assisting Exam Study Guide (page 3)
Practice problems for this study guide can be found at:
The dental assistant is trained to obtain both intraoral (film placed inside the patient’s mouth) and extraoral (film placed outside the patient’s mouth) radiographs. There are two types of intraoral radiographs, which record images of the teeth and the supporting structures. The radiographs show the outline, dimension, and positions of the teeth. The supporting structures viewed are the alveolar bone, the lamina dura, the periodontal ligament, and the membrane space. Radiographs can reveal restorations with amalgam overhangs, restorations that are failing, recurrent decay on a tooth, interproximal caries, calculus levels, crestal bone levels, internal pulp pathology, anatomy and pathology in the root area, and surrounding bony structures and occlusal relationships.
Radiation is used to produce radiographs (X-ray films) and can be biologically damaging. Every exposure has the ability to damage living tissue. Therefore, the operator and patient must be properly protected, and stringent infection control techniques must be followed. The operator must also follow proper exposing and processing techniques for the safety of all.
Concepts and Skills
Radiology is a central tool in dental diagnosis and treatment. This topic is broken down into nine concepts and skills:
- Intraoral Radiographic Technique
- Processing Intraoral Radiographs
- The Generation of X-rays in the X-ray Tube Head
- Characteristics of the Image
- Radiation Biology and Protection
- Radiographic Presentation of Lesions
- Extraoral Radiography
- Digital Imaging
- Patient Management
We will present questions relating to each of these areas of dental radiology. The outline below chronicles current information regarding exposing, processing, mounting, and interpreting both intraoral and extraoral radiographs.
Intraoral Radiographic Technique
There are two intraoral X-ray techniques used in dentistry. The oldest technique is the bisecting angle technique, and the newer is the paralleling technique, which is widely taught in all dental schools.
The film is sealed in a packet to protect it. If this is placed backward in the mouth, the result is an image of low density with a herringbone pattern on the film.
The speed of the film is determined by the size of the silver halide crystals and is classified A through F, F being the fastest.
Types of Intraoral Surveys
The examination of a complete area with radiographs is referred to as a survey. Intraoral surveys could include a full-mouth series of films or a localized area, such as a maxillary cuspid view.
The bitewing radiograph shows both the maxillary and mandibular teeth in occlusion. Bitewings can either be taken horizontally or vertically. The main purpose of a bitewing is to examine the interproximal surfaces, mesial and distal, and the height of the crestal bone level. Other purposes include detection of overhanging restorations, pathology of the pulp, and detection of location of calculus. Bitewings are usually taken once or twice a year depending on the patient’s caries rate and level of oral home care. A bicuspid/premolar bitewing radiograph should be placed to include the distal half of the mandibular cuspid. The standard film size used for bitewings is size 2. Size 0 or 1 can be used for children with primary teeth. Size 3 is specially made for an extra long bitewing; however, it is seldom used.
The periapical radiograph shows the most accurate image of crowns, roots, and supporting structures of a particular area of the oral cavity. Supporting tissues examined in a periapical radiograph include the alveolar bone, lamina dura, periodontal ligament, periodontal membrane space, and 2–3 mm of supporting tissue beyond the apex of the tooth. Periapicals are used to examine the anatomy and pathology of a particular area and generally use size 1 or 2 film. Size 1 film would most likely be used to radiograph the incisors and cuspids of adult patients.
Occlusal films examine the complete arch of teeth, maxillary or mandibular, all in one view. The occlusal film, a size 4 film, is much larger than a bitewing or periapical film. This film is used to locate objects present in the oral cavity, along with locating supernumerary teeth (extra teeth), impacted teeth, root tips of extracted teeth that were left behind, tumors, and cysts. Other uses of the occlusal film include the examination of the maxillary sinuses, large sections of the jaw, and to determine the presence of any jaw fractures or pathologies such as cysts and malignancies. The two main techniques for exposing an occlusal film are topographical and cross-sectional.
Basic Principles of Intraoral Survey
The intraoral survey, full mouth X-ray (FMX) series, consists of 18 to 20 individual films showing the entire oral cavity. The full mouth series consists of bitewings and periapicals. Areas of the oral cavity are grouped together and films are taken of each quadrant, usually only every three to five years. Occlusal films are another intraoral film; however, they are not included in a full-mouth series.
The paralleling technique is widely used in dental radiology because it produces a quality, anatomically correct image so that no retakes are needed. Having no retakes or minimal retakes reduces the patient’s exposure to radiation. The paralleling technique is an exposure technique in which the film is placed parallel to the long axis of the tooth and the central beam is directed at a right angle to both the tooth and the film.
Infection control must be followed in exposing radiographs. PPE—gloves, mask, lab coat, and eyewear—is always worn by the operator. Protective barriers are placed on the exposure button, the X-ray unit buttons, and on the X-ray tube head and position indicating device (PID).
Disposable film holders are used along with reusable film holders that are sterilized. The treatment room is disinfected, but no antiseptic agents are used. Infection control measures must be followed while processing the films. However, immersing a contaminated exposed film packet in a disinfecting solution will destroy the image.
Processing Intraoral Radiographs
Conventional intraoral radiographs require a process that brings out the latent image, making it visible.
A latent image is an unseen image that is on the film from the exposure time to the time that the image appears on the film. This invisible image is made when the X-radiation in the X-ray tube head strikes the silver halide crystals on the film.
Dental X-ray film is covered with an emulsion on both sides of the film. The emulsion consists of a mixture of silver halide crystals in a gelatin base. These halide crystals change when they are exposed to radiation. When the film is processed, the developer and fixer chemicals react with these exposed and unexposed silver halide crystals to produce an image.
Chemicals Involved in Processing
The three forms of processing chemicals are ready-to-use, concentrate, and powder. The concentrate form is most commonly used in dental offices.
The developer reacts with the exposed silver halide crystals and is responsible for creating the black or dark tones on the image. The reducing agents in the developer are metol and hydroquinone.
The fixer reacts with unexposed silver halide crystals and removes them from the film, which causes a white or clear area. The fixing agent is ammonium thiosulfate.
Water is used in the processing of dental X-rays to rinse and remove any developer or fixer chemicals on the film.
Automatic Film Processing
Automatic processing systems can develop intraoral films in four to five minutes or less. There is much less room for operator error as the temperature and developing and fixing time are controlled by the processor and not the operator. Some automatic processors have a daylight loader attached to them so that a darkroom is not necessary. The solutions in the automatic processors are much warmer than in manual processing. The solutions are approximately 85–105° F (30–40° C). The films move through the processor on rollers. Maintenance of the automatic processor is very important. Solutions must be replenished daily and changed every two to six weeks, depending on the rate of use. The processor rollers must be cleaned each time the solutions are changed. Chemicals must be disposed of according to local regulation, and documentation should be filed. There is less chance of error using automatic processors.
Manual Film Processing
Films may be processed manually in a darkroom with an X-ray utility “red” safety light over the traditional tank. Intraoral films are clipped to a film rack and processed on the rack. Accurate temperature of the developer and fixer solutions and timing is mandatory to obtain a diagnostic image. Optimum developing time and temperature for manual processing is four-and-a-half to five minutes at 68° F (20° C). Increasing the time that the films are in the developer and increasing the temperature of the developer cause a denser, darker image. Fixing time is usually twice the developing time. The films are then washed for at least 20 minutes in a fresh running water tank, then hung to dry. The process includes developing, rinsing, fixing, washing, and drying.
There are 18–20 individual films mounted into a full mouth survey mount. A dot (pimple, not a dimple) is utilized to determine the front side of the film. This is known as labial mounting. Anatomical landmarks are used to aid in mounting. Note that on an edentulous patient, the maxillary tuberosity and the outer corner of the eye would serve as the landmarks of the maxillary molar radiograph.
Helpful hints include:
- Maxillary molars have three roots.
- Mandibular molars have two roots.
- Maxillary central incisor films show the median palatine suture, a radiolucent line between the maxillary central incisors.
- Anterior films are orientated vertically.
- Posterior films are orientated horizontally.
- Maxillary films have large radiolucent areas: nasal fossa and maxillary sinuses.
- Maxillary molar films show the maxillary tuberosity.
- Mandibular molar films show the retromolar pad and the external oblique ridge.
- The overall appearance of the full mouth survey is in an upward curve (smile).
- Radiolucent structures, such as dental pulp, permit the passage of X-rays and thus appear dark on the film. On the other hand, radiopaque materials, such as gold and amalgam, block the penetration of X-rays and therefore appear lighter on the film.
- If the operator uses too much vertical angulation, the result is an X-ray image that is shorter than the actual tooth; this is known as foreshortening.
- The correct vertical angulation in a bitewing radiograph is +10 degrees.
Intraoral film is available in double-pack films, which creates two originals simultaneously. However, duplicating is completed using a duplicating film. Duplication is completed in a darkroom using a duplicator that shines a bright ultraviolet light onto the films that then produces an image onto the duplicating film.
The Generation of X-rays in the X-ray Tube Head
X-rays are produced in the tube head and are the result of high-speed electrons stopping or slowing. The electron kinetic energy is changed into electromagnetic energy by Bremsstrahlung radiation. The X-ray photons produced in the tube head have many different wavelengths. Photons striking a living organism break molecules into smaller pieces, disrupt molecular bonds, and form new ones within molecules and between new molecules.
Properties of X-rays
X-rays are energy that travels in a wave-like motion. They penetrate matter, produce fluorescence in some materials, cause ionization of matter, and produce a latent image on the film. Ionization is the loss of electrons from a substance.
Parts of the Dental X-ray Tube
There are five main parts in a dental X-ray tube: anode-tungsten target, cathode-tungsten filament, aluminum filter, lead collimator, and position indicating device (PID). Each is discussed in turn. The X-ray tube creates the X-ray production conditions of: a source of electrons, high voltage for electron speed, and a target that can stop the electrons.
This is the positively charged end of the X-ray tube head. The kilovoltage (kV) setting controls the current in the anode or the quality.
This is the negatively charged side of the X-ray tube head where the electrons are boiled off of the tungsten filament. The milliampere (mA) setting controls the number of electrons or quantity.
This filter is located in the position indicating device (PID) between the PID and the X-ray tube head. This filter removes the long wave—low energy wavelengths that are not needed to produce X-rays.
Also referred to as the lead disc, the lead collimator restricts the spread of the X-ray beam to no more than 2.75 inches (70 mm) at the patient’s face. Lead is used because it is resistant to the penetration of ionizing radiation.
Position Indicating Device (PID)
The position indicating device (PID) is also referred to as the cone. The PID is used for aiming the central beam toward the patient’s face and anatomical landmarks.
Production of X-rays
Thermionic emission occurs at the tungsten filament in the cathode. A cloud of electrons is boiled off the filament. The negatively charged electrons are attracted to the tungsten target in the anode, the positive side of the tube. When the electrons collide with the target, energy in the form of X-rays and heat is produced. Ninety-nine percent of the energy is heat and only 1% of the energy produced is X-rays. The X-rays then escape the tube head through the aluminum filter and collimator and travel down the PID to strike the matter and the film.
Characteristics of the Image
There are four characteristics of an X-ray image. These are: contrast, density, detail, and geometric distortion. Each is discussed in turn. The dentist requires properly processed radiographs with minimal distortion to be diagnostically acceptable. For example, a film that has been exposed to radiation twice, known as double exposure, is of no use.
This refers to the varying shades of gray present in the image. Contrast is dependent upon density and can be influenced by processing. Contrast is difference in densities. It is controlled by the voltage (kV) setting.
Radiographic density is the degree of darkness in the image. Density depends on the total amount of radiation that the film receives, the thickness of the bone, the developing/processing conditions, and the distance between the X-ray tube head and the patient. Density is controlled by the amperage (mA) setting.
Detail is the sharpness and clarity of the image. Detail is affected by patient movement or X-ray tube head movement. Any movement during the exposure of an X-ray will cause the image to appear blurry and out of focus.
By increasing the object-to-film distance, a penumbra will be present. A penumbra is the lack of sharpness that surrounds the shadow. This results in an inaccurate duplication of the tooth since it is geometrically distorted. Also, if the patient makes any slight movement, the result will be a blurry image.
Radiation Biology and Protection
Radiation is extremely dangerous. So, it is essential that dental assistants have a complete knowledge and understanding of how it works, and the proper protection from harmful rays for both the patient and dental staff. Five topics covered are: cellular and molecular changes, radiation measurement, operator protection, patient protection, and the benefits and risks of radiographs. There are several factors that determine radiation injury: total dose, dose rate, area exposed, age, sensitivity of the individual, and cell and tissue sensitivity.
Cellular and Molecular Changes
The energy of the X-ray beam is transferred to the matter that it passes through. This is called absorption. The molecules are affected by the X-rays. A molecule can break into smaller molecules, form new bonds with other molecules, form new bonds within itself, or be disrupted. Reproductive cells, bone marrow, small lymphocytes, and internal mucosa are the cells most sensitive to radiation. Muscle and nerve cells are the least sensitive. The harmful effects of radiation are cumulative and sometimes do not show up immediately. The latent period is the period of time from when the tissue is exposed to radiation to the first signs of biologic damage.
Rad is the term that is used to describe the absorbed radiation dose. The International System of Units (SI) uses the unit gray (Gy). One gray is equal to 100 rad (1 rad is equal to 0.01 gray). Rem is the equivalent or effective radiation dose. The SI unit is sievert (Sv). One sievert is equal to 100 Rem (1 Rem is equal to 0.01 sievert). Roentgen (R) is the term that describes radiation exposure. Generally, exposure of 1 R will result in an absorbed dose of 1 rad, or 0.01 Gy.
The exposure MPD limit for an occupational worker is the maximum permissible dose, or MPD. For an occupational worker, MPD is 5.0 Rem per year.
The operator must be behind a barrier while exposing films. If no barrier is available, the operator should be at least six feet away from the patient and in an area that is between 90º and 135º to the primary beam of X-ray. Radiation measuring devises such as film badges, film rings, and dosimeters should be worn by the operator to measure radiation exposure. Film holders should be utilized, and an operator should never hold a film for the patient.
When it comes to exposure to radiation, the ALARA concept should be followed. ALARA stands for As Low As Reasonably Achievable. X-rays should be prescribed by the doctor and not taken as a matter of routine. X-ray machines rated at over 70 kilovoltage peak (kVp) must use a one inch (2.5 cm) aluminum filter. The collimator is the lead disc that restricts the size of the X-ray beam to 2.75 inches (70 mm) at the patient’s face. Long cone PIDs (12–16 inches long) should be utilized as they cause less spread of the X-ray beam. Patients should wear a thyroid collar and lead apron impregnated with 0.25 mm of lead to shield out radiation.
Benefits and Risks of Radiographs
When addressing the patient’s concerns regarding dental X-rays, it is important to explain the benefits versus the risks. Dental radiographs taken using the prescribed precautions pose little risk to the patient. Radiographs should only be taken when the benefits far outweigh the risks.
Radiographic Presentation of Lesions
Doctors use radiographs to locate and view the extent of many lesions. Lesions are growths that are not coordinated with a body system, and therefore serve no useful purpose. Not all lesions are malignant. Some lesions that can be seen in a dental X-ray include periapical infection, caries or decay, trauma, periodontal disease, oral lesions, and dental abnormalities. This allows for the early detection of precancerous or cancerous lesions. Without the benefit of dental radiographs, the dentist does not have the tools necessary to detect growths that cannot be seen with a clinical examination. Early detection can save lives.
Extraoral films are placed outside of the patient’s mouth. They are much larger than intraoral films and are utilized to view large areas of the skull and jaw. Extraoral radiography includes panoramic radiography, as well as views of the temporomandibular joint (TMJ), maxillary sinuses, and a cephalometric view.
A panoramic film is placed into a cassette that contains an intensifying screen on either side of the film. The intensifying screen is made up of phosphor, which emits blue or green light onto the film when X-rays are exposed to it. The panoramic film has less detail than an intraoral periapical or bitewing.
Note that a patient placed in a panoramic unit with chin too low will result in X-ray images with an exaggerated curve of Spee. Prior to placing an implant, the panoramic technique is preferred when exposing intraoral films.
The patient must remove all appliances in her or his mouth and any facial jewelry, large necklaces, earrings, hearing aids, and glasses as they will interfere with the radiograph and show a ghost image. The patient is draped with a lead apron that covers both front and back of the body to the waist. No thyroid collar is utilized as it would block the X-rays. The patient is instructed to bite down on a bite tab and the operator then exposes the film. The film in the cassette and the panoramic tube head rotate around the patient to obtain the radiograph. The patient is instructed to place his or her tongue on the roof of the mouth and hold very still. The operator must align the patient’s Frankfort plane parallel to the floor and the patient’s midsagittal plane perpendicular to the floor.
Temporomandibular Joint (TMJ) Views
There are many extraoral radiographs available that view the temporomandibular joint (TMJ) at different views. Submentovertex projection views the joint from under the patient’s chin looking up at the TMJ. The panoramic projection can view the TMJ; however, sometimes the glenoid fossa is not in clear view. The anthrogram views the soft tissues of the TMJ. Magnetic resonance imaging (MRI) can also be used to view soft tissue. Other TMJ views include the transcranial, transpharyngeal, tomographic, and transorbital.
Maxillary Sinuses Views
The maxillary sinuses can be viewed in the water’s view extraoral radiograph.
Commonly used by orthodontists to measure arch size changes, this extraoral view is a lateral profile of the entire skull.
More and more dental practices are moving away from traditional film radiography toward digital imaging. Both hardware and software for digital imaging are widely available. There are many advantages to digital radiography. There is less radiation and no chemicals are needed. Over the long term, digital radiographs cost less, and there is no processor maintenance. Images can be manipulated, which allows for better patient understanding when these tools are used while explaining interpretation. Easy duplication allows radiographs to be sent electronically to referred doctors and when fi ling insurance claims.
Charged couple devices (CCDs) are used in direct imaging. They are made of a silicon chip embedded with an electronic circuit. CCDs come in area array detectors and linear array detectors. The area array detectors are used with intraoral digital X-rays and video cameras. They are the size of an intraoral dental film size—0, 1, and 2. The electronic signals that are received by the CCDs are displayed on a computer screen. They are exposed to X-rays. Linear array detectors are used with digital panoramic imaging.
Scanners are used to obtain an image or digitize an existing radiograph. Phosphor Storage Plates Phosphor storage plates are made in the same size as number 0, 1, and 2 size intraoral films. They are placed in plastic barriers, then put in the patient’s mouth and exposed to X-rays. The plate is then placed into a device that reads the light signal stored on the plate, and the image is displayed on a computer monitor. This uses the same technique as intraoral X-rays, but with less radiation and with digital processing.