Dental Radiology for Dental Assisting Exam Study Guide (page 2)

Practice problems for this study guide can be found at:

Dental Radiology for Dental Assisting Exam Practice Problems

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.

Bitewings

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.

Periapicals

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

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.

Paralleling Technique

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

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.

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.

Anode-Tungsten Target

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.

Cathode-Tungsten Filament

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.

Aluminum Filter

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.

Lead Collimator

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.

Contrast

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.

Density

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

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.

Geometric Distortion

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.

Radiation Measurement

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.

Operator Protection

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.

Patient Protection

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 Radiography

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.

Panoramic Radiography

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.

Technique

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.

Cephalometric View

Commonly used by orthodontists to measure arch size changes, this extraoral view is a lateral profile of the entire skull. 

Digital Imaging

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.

Direct Imaging

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.

Indirect 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.

Patient Management

The dental assistant must not only be properly skilled and trained to produce clinically acceptable radiographs, but also be able to communicate with patients. Patients who are informed and educated are more likely to cooperate and comply.

Pediatric Radiographic

Technique Exposing dental radiographs on children can be challenging. Children have a tendency to move and sometimes gag. Some basic tips include:

• Explain in simple terms what you are about to do.
• Show the child the film, the camera, and the film holder, and let him or her touch the film prior to placing it in the child’s mouth.
• Work quickly and communicate the importance of holding perfectly still so that no retakes will be needed.
• Use the smallest film that can record the most accurate image. Smaller films mean less gagging.
• Utilize occlusal films since they induce less gagging and show all teeth in one arch.
• Praise the child for helping you so much and thank him or her for being such a great helper.

Edentulous Radiographic Technique

Patients with no teeth will need to have radiographs taken to determine bone quality, height, and quantity, and to visualize anatomic structures such as the foramen and the maxillary sinuses and pathologic anomalies. Because of the absence of the teeth, proper film placement is determined by the location of anatomical structures on the face.

Techniques used are the bisecting the angle and paralleling. Cotton rolls can also be used to help hold the film in place.

Endodontic Radiographic Technique

The paralleling technique is the best technique available to obtain an accurate anatomical image. The rubber dam clamp and endodontic files may be in the way and not allow for proper film placement. Using a film holder, the patient may be allowed to hold the film using a Snap-A-Ray or hemostat. The bisecting technique is employed so that the film may be placed parallel to the long axis of the tooth. The X-ray beam is directly perpendicular to the imaginary line that bisects the angle formed by the recording plane of the dental X-ray film and the long axis of the tooth. The tooth being treated should be centered on the film, and 5 mm of bone beyond the apex should be present on the film.

Bisecting the Angle Technique

This technique is used when the paralleling technique cannot be used. In other words, if the film holder cannot be employed, as with edentulous patients, small children, and so on, the bisecting angle technique must be used. The film is placed directly against the teeth to be radiographed. The central beam is directed perpendicular to the line that bisects the angle made by the tooth and the film.

Anatomic Variations

Some patients present the operator with a variety of anatomic variations such as palatal tori, mandibular tori, crowded and/or misaligned teeth, narrow palate, and ankyloglossia (tongue tie). Film placement can be altered to accommodate these variations.

Practice problems for this study guide can be found at:

Dental Radiology for Dental Assisting Exam Practice Problems