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Dental Radiology for Dental Assisting Exam Study Guide (page 3)

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Updated on Jun 23, 2011

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.

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