PRESCRIBING DENTAL RADIOGRAPHS
An advisory panel sponsored by the Public Health Service of the U.S. Dept. of Health and Human Services has developed guidelines for the prescription of dental radiographs. Using these guidelines the dentist can decide when, what type, and how many radiographs should be taken. Dental radiographs should not be taken as a matter of routine, but should always be undertaken after evaluating the overall health needs of the patient after a clinical examination and history. Guidelines are shown on the next page.
GUIDELINES FOR THE USE OF DENTAL RADIOGRAPHS
Radiographs are frequently needed during oral examinations when historical and clinical findings are inconclusive. Previously, professional judgment provided the sole guide for determining which radiographs were needed and when they should be obtained. However, the introduction of FDA guidelines has provided clinicians with well-defined and fact-supported recommendations.
New Patients When evaluating new patients, the type of baseline radiographic study required depends on the dental age and status of the patient. In children with complete primary dentition (including teeth whose proximal surfaces cannot be evaluated clinically) and in whom no evidence of disease is found, bitewing examinations are recommended to assess caries activity. Additional periapical studies should be done only if manifest disease is noted. In children with transitional dentition, all teeth and tooth-bearing areas can be assessed with periapical or occlusal films or a panoramic radiograph, together with posterior bitewings. Further studies may be needed in the presence of positive findings.
Adolescent and adult patients can be evaluated similarly, using posterior bitewing and selected periapical films as needed. If generalized dental disease or a history of extensive dental treatment is noted, a full-mouth periapical survey may be appropriate. In edentulous patients, full-mouth surveys are required, using periapical or occlusal films or a panoramic radiograph.
Recall Patients Bitewing or selected periapical radiographs are recommended for assessment of caries activity or the status of alveolar bone in recall patients. Growth and development also can be assessed in children with transitional dentition and in adolescents with developing third molars using a panoramic radiograph. Appropriate radiographic studies and their frequency are determined based on the age of the patient, positive clinical findings, and risk categories to which the patient has been assigned.
Special Circumstances Because patients who have received radiation therapy for malignant disease may be anxious about further exposure, the need for radiographs should be explained carefully. Patients should be informed that the amount of radiation necessary to produce diagnostically sound radiographs is insignificant compared with that received during radiation therapy. In patients who have been treated for malignancies of the head and neck, thorough clinical and radiographic monitoring is essential because radiation related xerostomia may increase caries susceptibility. Pregnant patients also may be concerned about fetal radiation exposure. Although studies have shown that such exposure can lead to birth defects and increase risk for childhood malignancy, the amount of radiation needed to produce these effects far exceeds that produced by oral radiographs.
Radiographs are inappropriate in the absence of specific clinical indications, except in patients with dental caries, which may not be detected clinically, and in edentulous patients, who generally have a higher occurrence of disease without clinical signs or symptoms. Although the FDA guidelines are not meant to replace professional judgment, they can facilitate decisions concerning the choice and timing of radiographic studies in specific patients.
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Source: Frederiksen NL: Guidelines for prescribing dental radiographs. Texas Dent J 112 (2): 63-67, 1995.
A recent study evaluated the accuracy of caries detection with the use of a limited set of radiographs, per the FDA guidelines vs. a complete set of periapical radiographs. A total of 490 patients, from the age of 33 to 88 years, participated in the study. The patients did not have a recent full mouth radiograph. Three general dentists examined the patients using an explorer, mouth mirror, and periodontal probe. The clinical findings were entered into a database program to identify the periapical radiographs that the clinicians should request, per the FDA guidelines.
Of the 490 patients, 436 (89%) had at least one carious tooth revealed radiographically; most of the lesions (81%) appeared in posterior teeth, primarily in proximal surfaces. Most anterior lesions were detected on periapical radiographs. The patients had a total of 13,159 teeth; 2,213 lesions were found radiographically in 1,949 teeth, compared with 1,564 carious teeth noted on clinical examination. The average number of periapical radiographs ordered based on the FDA guidelines was 9.7, a 43% reduction from the 17 generally ordered at the clinic where the study was conducted. Full-mouth radiography revealed 93 lesions that were not evident on the limited set of radiographs; most were in anterior teeth and restricted to enamel.
Clinical and radiographic examinations must be used in conjunction with each other, since neither method reveals all lesions. The practical importance of the missed lesions is slight because of the stage of caries and the wide variation in dentists' diagnostic and treatment decisions. More research is needed to determine the FDA guidelines' efficacy in the diagnoses of periodontal and periapical disease.
Source: White SC, Atchison KA, Hewlett AR, Flack VF: Efficacy of FDA guidelines for ordering radiographs for detection of caries. Oral Surg Oral Med Oral Pathol 77; 531-540, 1994.
RADIOGRAPHY FOR CHILDREN
Children have the same basic needs for dental treatment as do adults, and, therefore, radiography is just as important for children as it is for their parents. Dental radiography would help detect dental infections, dental caries and disfigurations produced by premature loss or prolonged retention of the teeth. Radiography also plays an important role in the area of prevention. Infected teeth can seriously affect a child's health, and it is important to detect the source of hidden infections to prevent further damage to the child's dental health in later years. Deciduous teeth that are lost too early or retained too long may cause severe damage to the occlusion and other conditions that are difficult to correct later. Hidden lesions can only be detected by frequent periodic examinations including radiographs.
Radiography should be employed for both prevention and treatment during the formative years which are between birth and 6 years of age. A radiograph would detect dental caries and also show the roots of the deciduous teeth as well as the developing permanent teeth within the alveolar bone.
Moreover, disturbances in normal development, such as amelogenesis imperfecta (failure of the enamel to develop fully), anodontia (absence of teeth), the presence of dentigerous cysts (sacs containing fluid or producing teeth) supernumerary (extra) teeth, and a host of other conditions can be discovered only through radiographs.
In the absence of an accident or other unusual circumstances, the schedule of radiographic survey in children should take the following course:
| First survey at the age of 3 soon after all the deciduous teeth have erupted |
| Second survey when the child is about 6 years old, when the first deciduous teeth are shed and the first of the permanent teeth erupt |
| Third survey at the age of 9 when the child has a combination of deciduous and permanent teeth |
| Fourth survey when the child is between the age of 12 and 14, after the deciduous teeth are lost |
MOUNTING THE RADIOGRAPHS
The mounting of dental radiographs is an extremely important function of the dental auxiliary. Radiographs mounted in their proper anatomical relationship facilitate viewing and diagnosis by the clinician. Attaching the radiographs to the film mounts is called mounting.
The task of mounting the radiographs is not difficult once the novice acquires a thorough understanding of radiographic anatomy and masters a few simple procedures. First, the auxiliary must be able to distinguish which films are of the patient's right and which are of the left side. Every radiograph has an embossed or raised dot to help indicate the film orientation: the dot will appear convex or concave depending on the side from which the film is viewed. If the radiograph is mounted so that the raised portion of the dot (convex) is toward you, you are looking at the film as if you are facing the patient; the patient's left side is on your right. This is called labial mounting, the method recommended by the American Dental Association.
Mounting procedure
The following procedure is recommended for beginners. As they become more skillful, many auxiliaries develop simpler and faster techniques.
1. First, wash hands to prevent smudging the films and lay a clean towel over the work bench in front of an illuminator.
2. Auxiliaries must always work on a light colored table top so they can see the radiographs easily when they are laid out.
3. So as not to mix up the films, you should always work with only one patient's films.
4. Arrange the films so that all the identification dots face in the same direction with the convex side toward the viewer. Always handle films by their edges to avoid smudging them.
5. The films are divided into three groups: the anterior periapical, posterior periapical and bitewings. The bitewing films can be identified easily because the crowns of both the upper and lower teeth can be seen. The anterior and posterior periapical films are differentiated by the vertical orientation on the film for anterior teeth and the horizontal orientation for posterior teeth.
6. The radiographs must be arranged in the same fashion as the teeth in the mouth are; the anterior teeth must be mounted in the middle frames and the posterior teeth in the frames on either side of the mouth. The maxillary teeth must be positioned so that the incisal edges or occlusal surfaces point downward and the roots upward. The mandibular teeth are mounted the opposite way, with the incisal or occlusal surfaces pointing up and the roots pointing to the bottom of the mouth.
Following generalizations about crown and root shapes make the mounting of radiographs easier:
1. The roots and crowns of upper anterior central and lateral incisors are larger than those of lower central lateral incisors.
2. Maxillary premolars usually have two roots; mandibular premolars have one root.
3. Maxillary molars generally have three roots and the mandibular molars have two.
4. Most roots curve distally.
5. The large radioluscent areas denoting the nasal fossa or the maxillary sinus indicate that the radiograph is of a maxillary area.
6. The radioluscent mental foramen indicates that the film belongs in the mandibular premolar (bicuspid) area.
7. The body of the mandible has a distinct upward curve toward the ramus in the molar area.
PANORAMIC RADIOGRAPHY
Panoramic radiography refers to a technique, developed and refined within the last 3 decades, which produces a radiograph that shows the patient's mandible and maxilla on one film. It is also known as pantomography (making graphic recordings of contours on radiographic film), tomography (making body sections on radiographic film), and laminography (from the word lamina meaning layer as this is a layered radiographic technique.) The term panoramic radiography is most commonly used because the resulting radiograph shows a panoramic view of a large area of the face and large portion of the head.
Rotational panoramic radiographic technique allows radiographing in one plane of an object while blurring or eliminating images from structures in other planes. It accomplishes this by moving the tube and film in opposite directions simultaneously while the specific tissue layer remains in a fixed relationship to the tube. Thus, when the tissues of the right side are in the path of the x-ray beam, the tissues of the left side are out of focus and do not superimpose on those of the right side and vice versa. This blurring of the other layer is necessary to prevent interference from the structures of the other layers that were not selected for viewing.
The plane of the object that is not blurred on the radiograph is called the plane of acceptable detail or focal trough. It is also called the image layer.
Tomographic Technique
Panoramic x-ray machines operate with the patient positioned between the tube head and the cassette that holds the film. The exposure is a continuous one, made as the tube head and cassette rotate slowly above the patient's head during the operational cycle (usually about 15 - 20 seconds).
By placing an elongated screened film, varying in width from 5-6 inch wide and 12 inches long, in a rigid or flexible cassette that is positioned extraorally, the operator is able to produce an image of the entire dentition, the surrounding alveolar bone, the sinuses, and the temporomandibular joints on a single film. Instead of exposing 16 or more intraoral radiographs, the entire panoramic exposure can be made in less than 3 minutes instead of the 15 minutes usually required for the full-mouth intraoral series. The points of rotation around which the tube head travel can be either inside or outside the focal trough. The width or thickness of the focal trough is governed by many factors, including the angle of movement of the x-ray beam, the width of the x-ray beam, and the size of the focal spot. Any object that lies in the focal plane is shown clearly and objects above and below it appear blurred.
Diagram showing the relationship of the moving x-ray beam as it passes through the center of rotation in a horizontal plane toward the path of the moving x-ray film. As the beam scans the object (usually the dental arches), a continuous image is registered on the moving film.
Source: From a syllabus prepared for a symposium on panoramic radiography presented at Anaheim, California, October 3, 1983, by the American Dental Association in cooperation with the University of Texas Health Science Center at San Antonio Dental School. (de Lyre WR, Johnson ON: Essentials of Dental Radiography. Norwalk, CT: Appleton & Lange, 1995.)
There are many domestic and imported machines available on the market for panoramic radiography. They differ primarily in the number and locations of the centers of rotation, the choice of a fixed or adjustable focal trough, and the type and shape of the film transport mechanism. All units use intensifying screens, with a film size of either 5 x 12 or 6 x 12 inches. Design differences include head positioning devices, bite blocks, kVp and mA range, standing or sitting patient position, and wall-mounted or freestanding units.
Operational Procedure
The following are general rules of technique for preparing and positioning the patient for panoramic radiography.
1. Explain the procedure to the patient, pointing out the importance of not moving during the procedure.
2. Have the patient remove a jacket or any other bulky piece of clothing that might interfere with the movement of the cassette holder.
3. Ask the patient to remove glasses, earrings, or other appliances that might become superimposed on the image. This includes necklaces, napkin chains, or any other metal objects on the back of the neck.
4. Seat or stand the patient in the most erect position possible so that the spinal column is straight.
5. Swing the head position or assembly into place. Align the patient's head so that the midsagittal plane is perpendicular to the floor.
6. Place the patient's chin on the chin rest (if present) so that the ala-tragus line is tilted slightly down, about 5 degrees, from a parallel line to the floor.
7. Alert the patient to the fact that certain parts of the machine will revolve around the head and that the film cassette may touch the shoulder or ear gently during the exposure rotation.
8. Drape the patient's abdomen with the lead apron. Do not use a thyroid collar or a bib chain,because it would be superimposed on the image.
9. Place a cotton roll or a bite stick, if the unit has one, between the patient's upper and lower incisors.
10. Have the patient close the lips and place the tongue against the roof of the mouth. This prevents the formation of an airspace that is represented as a radiolucent area above the apices of the upper teeth.
11. Take any readings or measurements called for and set exposure factors.
12. Stand behind protective covering or an adequate distance away while making the exposure. The exposure can be as long as 22 seconds as the tube and film cassette travel around the patient. Watch the patient during this time to make sure that there has been no undesired movement.
13. Swing the head positioning assembly out of the way. Remove the protective apron or collar, and release the patient. Return glasses, ear rings, or appliances.
14. Deactivate the x-ray machine, and swing the head positioner back into place. Remove the cassette from the cassette holder or drum.
15. Unload the cassette under proper safelight conditions, and process the film according to the manufacturer's instructions.
Advantages and Disadvantages
Like any technique, panoramic tomography has its advantages and disadvantages compared with conventional full-mouth radiography, and these are summarized below:
Advantages
1. Field size is one of the major advantages of the pantomogram. The full- mouth series is not composed of radiographs of the entire mouth but only of the teeth, alveolar ridges, and part of the supporting bone. The pantomogram covers an area that includes all of the mandible from condyle to condyle and the maxillary regions extending superiorly to the middle third of the orbits.
2. The procedure for panoramic radiography is relatively simple to perform and requires considerably less time. With a minimum amount of training and strict attention to detail, the dentist or dental auxiliary can become very proficient in taking these films.
3. Since panoramic radiography is an extraoral procedure, it requires a minimum amount of patient cooperation when compared with intraoral techniques, especially when gagging problems exist. No film packet is placed in the patient's mouth. The patient is asked to bite on a rod and is only required to sit or stand still for the 12 - 22 seconds of exposure. In panoramic radiography, the radiation dose to the patient is no more than in intraoral radiography.
4. Less time is required to do pantomographic examination than an intraoral
survey, as only one film instead of a series must be handled. Similarly,
mounting time is eliminated. Panoramic film is easier to file and store and
there is no danger of losing or damaging a film of the series.
5. A panoramic radiograph is less confusing to the patient than a series of small separate intraoral radiographs, making it easier for the dentist to explain the diagnosis and the proposed treatment plan in a manner that is clear and understandable to the patient.
Disadvantages
1. Compared to intraoral radiographs, panoramic radiographs show magnifica- tion, geometric distortion, and poor definition, which is a result of factors such as external placement of the film with resulting increased object-film distance, the use of intensifying screens, and faster film with large grain size.
2. Areas of diagnostic interest out of the focal trough may be visualized poorly or not at all. The focal trough or plane of acceptable detail is not as wide as either the mandible or maxilla, and only structures or changes that lie within the trough are visualized clearly.
3. Pantomographic units have a tendency to produce overlapping images, particularly in the premolar area.
4. Frequently superimposition of the spinal column shows up on the anterior position of the pantomogram which may be due to the improper positioning of the patient.
5. The amount of vertical and horizontal distortion varies from one part of the film to another, resulting in an uneven magnification of the image; struc- tures, spaces, and distances may appear larger than they actually are.
6. Incipient caries are difficult to detect and are frequently missed. Supplementary films are required for this purpose.
DIRECT DIGITAL RADIOGRAPHY
Direct digital radiographic systems are being used with greater frequency in dental practice over the past 10 years, especially in Europe. These systems, however, are not as popular in America as in Europe, primarily due to their high initial cost (more than $20,000). Direct digital systems are bound to become more popular as costs fall, and benefits over the conventional film-based systems become apparent.
Two types of filmless systems are available. One type uses a charge-coupled device (CCD) to sense the x-ray image and display a radiograph within seconds. The second system, Soredex Digora, uses memory phosphor screens in minicassettes. Both types have the advantages of fast display (less than 5 seconds for CCD and about 30 seconds for the Digora system) and 4 to 10 times less radiation exposure than with D-speed films. Filmless radiographs also eliminate the cost and handling problems of film developing. Computers ensure that copies are inexpensive and quick to make. Copies can be stored on tape or hard disk, sent to other locations inexpensively and quickly, or manipulated during case presentation.
Direct digital radiographic systems offer several benefits over current film-based systems. No darkroom or processing equipment is needed. Infection control procedures and radiation exposure are also reduced. In addition, duplicates are instantly made with absolutely no loss in image quality. Patient acceptance and approval of these systems has also proven to be excellent. Technology exists to send images via phone transmissions to other dentists or insurance companies for consultations or instant approval.
Source: Vandre RH, Webber FL: Future trends in dental radiology. Oral Surg Oral Med Oral pathol Oral Radiol Endod 80:471-478, 1995
Source: Khademi JA: Direct digital radiography: Preliminary review of three FDA-approved systems. J Calif Dent Assoc 22(11): 48-56, 1994.
RADIATION PROTECTION
At present the maximum permissible dose (MPD) of whole-body radiation for persons occupationally concerned with ionizing radiation, such as dental auxiliaries and dentists, is 5000 mrem (50 mSv) per year, or 100 mrem per week. This is in contrast to the recommended MPD of 500 mrem (5 mSv) for the general public. In addition, the operator should not receive more than 3000 mrem (30 mSv) in any 13-week period. Dental personnel also should not exceed an accumulated lifetime dose of (N-18) x 5000 mrem. In this formula, N is the operator's age.
In recent years, patients and health care personnel have become increasingly aware about the long-term effects of unnecessary and excessive radiation. The issue of radiation protection should be addressed from the perspective of the patient and the dental auxiliary or the dentist. The objective in any dental office is to use the least amount of radiation to achieve the maximum diagnostic results. Dentists have the obligation to protect the patient and his staff from the hazards of radiation. Toward this objective the following measures for radiation protection should be taken:
Equipment The use of outdated and malfunctioning equipment is often the cause of unnecessary radiation; however, all dental x-ray machines manufactured after 1974 must meet federal diagnostic equipment performance standards, and this has resulted in superior safety record. In an x-ray machine, radiation emanates from the focal spot on the tube of the x-ray unit. Useful radiation is the primary beam that emerges from the housing of the tube and is limited by the aperture lead diaphragm (collimator), or other collimating device such as a lead-lined position indicating device (PID). The size of the useful beam is restricted by the size of the opening in the collimator; this opening should not be any larger than necessary to produce a beam big enough to expose all parts of the film.
In a defective equipment, radiation may escape through the protective shielding of the x-ray tube head which is referred as head leakage. The tube head of the dental x-ray machine should not move or drift in any direction after positioning for an exposure. The patient or the dental auxiliary should never hold the tube head in place during an exposure. Holding the tube head subjects the operator of the machine to possible exposure from leakage radiation, and holding the PID places the hand right in the path of the primary beam.
Film The use of high-speed film means shorter exposure times which, in turn, results in less radiation to the patient and operator. Film slower than group D should never be used. Group E film, when compared with group D film, reduces the radiation exposure by 50%. Fast film has been the single most important factor in reducing unnecessary radiation.
Another radiation safeguard is the use of the largest film that can be placed comfortably in the patient's mouth, so that fewer films and exposures are needed. This may or may not be possible in every situation.
Lead aprons and collars Several states have laws requiring the use of a lead apron for all exposures and a thyroid collar for intraoral exposures. The use of lead aprons and thyroid collars can reduce radiation to the thyroid and gonads up to 94%.
Exposure technique The dental auxiliary or dentist should never be in the path
of the primary beam. The operator should not hold the drifting tube head nor
should
the film packets be held in the patient's mouth. The use of film holders affords
the patient additional protection by reducing the number of retakes and avoids
having the patient hold the film with their fingers. One of the major sources of
unnecessary exposure to radiation in the dental office is retaking films which is the
result of carelessness or inexperience on the part of the operator.
Distance is an important factor for operator protection. The intensity of radiation diminishes the farther the x-rays travel. The operator should stand as far away as practical from the source of the radiation unless protected by shielding. At six feet and behind the barrier, the occupational exposure to radiation is 0.
Scatter radiation The radiation that is deflected from its path by impact upon the tissues upon the patient's head is scattered in all directions. This is known as scatter radiation. The technician should be taught the areas of minimum and maximum scatter. The areas at right angles to the x-ray beam and toward the back of the patient are minimum scatter, while the areas of highest scatter are in back of the tube head and behind the patient. This is why the operator should stand at least 6 feet from the head of the patient when exposing x-ray film and why the patient should be draped with a lead apron.