Digital Radiography

The use of digital radiography is becoming more common in dentistry. Although there are technical advantages over conventional approaches, the same limitations apply. Overall, digital radiographs are equivalent to conventional radiographs with regard to diagnostic and treatment-related accuracy. These factors and other considerations are discussed further throughout this chapter.

Diagnosis

Diagnostic radiology involves not only identifying the presence and nature of pathosis, but also determining root and pulp anatomy and characterizing and differentiating other normal structures.

Identifying Pathosis

Radiographs must be studied carefully by someone with a working knowledge of the changes that indicate pulpal, periapical, periodontal, or other bony lesions. Many changes are obvious, but some are subtle.

Determining Root and Pulpal Anatomy

Determining the anatomy involves not only identifying and counting the roots and canals, but also identifying unusual tooth anatomy, such as dens invaginatus and a C -shaped configuration, and determining curvatures, canal relationships, and canal location. Identification also includes characterizing the cross-sectional anatomy of individual roots and canals ( Fig. 12.1 ).

A, The facial projection of this premolar gives some limited information about pulp/root morphology. “Fast break” (small arrow) usually indicates canal bifurcation. A double root prominence on the mesial surface (large arrow) indicates two bulges and a concavity; its absence on the distal surface indicates a flat or convex root surface. B, The same premolar from the proximal view. The presence of two definitive canals, each in its own “root bulge,” is confirmed.

Characterizing Normal Structures

Numerous radiolucent and radiopaque structures often lie in close proximity. Frequently, these structures are superimposed over and obscure crowns and roots. These must be distinguished and differentiated from pathosis and from normal dental structures.

Treatment

“Working” radiographs are made while the dental dam is in place, which creates problems in film placement and cone positioning. These radiographs are exposed during the treatment phase and have special applications.

Determining Working Lengths

The distance from a reference point to the radiographic apex is determined precisely. This establishes the distance from the apex at which the canal is to be prepared and obturated.

Moving Superimposed Structures

Radiopaque anatomic structures often overlie and obscure roots and apices. Using special cone angulations, these radiopaque structures can be “moved” to give a clear image of the apex.

Locating Canals

Canal location is obviously essential to success. Standard and special techniques allow the practitioner to determine the position of canals not located during access.

Differentiating Canals and Periodontal Ligament Spaces

Canals end in the chamber and at the apex. A periodontal ligament space ends on a surface and in a furcation (molars) and demonstrates an adjacent lamina dura ( Fig. 12.2 ).

This distal angulation shows the root surface outline (large arrow) and a periodontal ligament space (small arrow) with an adjacent lamina dura. The file is in the mesiobuccal canal (same lingual, opposite buccal [SLOB] rule).

Evaluating Obturation

Postoperative radiographs provide considerable information on canal preparation and obturation. The length from the apex, density, taper, preservation of original canal shape, and general quality of obturation in each canal are determined from these radiographs.

Follow-up Evaluation (Recall)

Ultimate success is verified at specified intervals of months or years after treatment. Because failures often occur without signs or symptoms, radiographs are essential to evaluate periapical status.

Identifying New Pathosis

The presence and nature of lesions that arise after treatment are best detected on radiographs. These lesions may be periapical, periodontal, or nonendodontic. It is important to remember that such lesions frequently present with no overt signs or symptoms and are detectable only on radiographs ( Fig. 12.3 ).

Failed root canal treatment because of missed root or canal. This mesial-angled radiograph clearly shows the untreated lingual root (arrow).

(Courtesy Dr. L. Wilcox.)

Evaluating Healing

Pretreatment lesions should be resolving or should have resolved. In a successful (healed) treatment, restitution of generally normal structures should be evident on recall radiographs ( Fig. 12.4 ).

Same tooth as shown in Fig. 12.3 . Recall radiograph after 9 months shows almost complete regeneration of bone, indicating a healing lesion. A permanent restoration must be placed as soon as possible.

(Courtesy Dr. L. Wilcox.)

Special Applications

Radiographs should be used to their maximum advantage. The following alternative techniques greatly enhance the ability to make an accurate and definitive diagnosis and to control treatment procedures.

Cone-Image Shift

Varying either the vertical or, particularly, the horizontal cone angulation from parallel alters images and enhances interpretation. These shifts reveal the third dimension and superimposed structures. Shifts also permit identification and positioning of objects that lie in the faciolingual plane.

Working Radiographs

Working radiographs are essential aids to treatment and are exposed when necessary but with discretion.

Diagnostic Radiographs

Number

The number of exposures depends on the situation. For diagnosis in most cases, only a single exposure is necessary. A properly positioned film and cone (usually a parallel projection is best) permits visualization at least 3 to 4 mm beyond the apex. The initial diagnostic radiograph is used primarily to detect pathosis and to provide general information on root and pulp anatomy. A bitewing radiograph may aid in determining the extent of caries or in detecting caries or an open margin of a crown. Usually, it is not necessary at this time to make additional films for identification of additional canals except in endodontically treated teeth, in which missed canals can be identified by using a shift.

If other films are at hand, each gives a slightly different view of the same tooth ( Fig. 12.5 ). Examine the tooth on each film in which it appears.

A, Facial projection of incisors suggests a single canal and a single root. B, Distal (canine) projection gives a different perspective. The canals of the lateral and central incisors are seen to bifurcate in the middle third of the root (arrow) and reunite in the apical third.

Angulation

Unquestionably, the most accurate radiographs are made using a paralleling technique. The advantages are (1) less distortion and more clarity and (2) reproducibility of film and cone placement with preliminary and subsequent radiographs. Reproducibility is important when assessing whether changes occurring in the periapex indicate healing or nonhealing. Paralleling devices enhance reproducibility.

There may be special situations in which the paralleling technique is not feasible, such as a low palatal vault, maxillary tori, exceptionally long roots, or an uncooperative or gagging patient; these circumstances may necessitate an alternative technique. A second choice is the modified paralleling technique; the least accurate technique is the bisecting angle.

Working Radiographs

Special situations require special considerations. Although the basic principles of doing everything possible to obtain the best quality radiograph are followed, there are definite limitations in making working films. These require cooperation by the patient if he or she holds the film or sensor in position.

These radiographs are usually neither parallel nor bisecting angle. The technique used is called modified paralleling. Essentially, the film is not parallel to the tooth, but the central beam is oriented at right angles to the film surface. In endodontic working radiographs, a further modification is made by varying the horizontal cone angle. Specific details of film/sensor and cone placement and radiographic interpretation are discussed later in this chapter.

Working Length

In general, establishment of working length should require only a single exposure. If a root contains or may contain two superimposed canals, either a mesial or distal angle projection is absolutely necessary; the straight facial view is not particularly helpful. Additional working length radiographs may be required later for confirmation of working lengths to detect the presence or lengths of newly discovered canals ( Fig. 12.6 ) or for reexposure if an apex has been cut off in the first radiograph.

Identifying and locating a canal. This incisor was rotated, requiring mesially angled working radiographs. A, The file is off-center, as indicated by the mesial root surface (arrows) . Therefore, the file is in the facial canal. B, A search to the lingual locates the lingual canal. There is a common canal apically.

Master Cone

The same principles used with working length films apply. With proper technique, only one radiograph is necessary to evaluate the length of the master gutta-percha cone. The master cones should extend to, or very close to, the corrected working length. (Procedures for the fitting of master cones are discussed in Chapter 18 .)

Other Considerations

Additional working radiographs are often required. For example, they are useful as aids in locating a canal or in determining the occurrence of procedural accidents (perforations, separated instruments, or ledges). Variations in cone positioning and angulation are made as required. When there is doubt about the accuracy of the master cone fit, an intermediary fill radiograph may be exposed after the master cone has been condensed with sealer but before the obturation is finalized. Errors in length or density can be corrected at this stage.

Obturation

The same basic principles used for diagnostic radiographs apply. At least a parallel projection should be made. In teeth with multiple canals, it is desirable to supplement this with an angled projection to visualize separate superimposed canals for separate evaluation of each. However, the radiograph gives only a rough indication of obturation length and quality.

Follow-up Evaluation (Recall)

The same principles used for diagnostic and obturation radiographs (parallel projection and exposure factors) apply to recall radiographs. There is one exception. If treatment is deemed to be questionable or a failure, additional angled radiographs are often required to search for a previously undetected canal or other abnormality.

Principles

Image Shift

Superimposed Structures

The cone-image shift technique separates and identifies the facial and lingual structures. An example is the mesiobuccal root of a maxillary molar that contains two superimposed canals. The cone shift separates and permits visualization of both canals.

Faciolingual Determination

Principles of relative movement of structures and film orientation are applied to the differentiation of object position ( Figs. 12.7 and 12.8 ).

A, The film is positioned parallel to the plane of the arch. The cone has the central ray (arrow) directed toward the film at right angles. This is the basic cone-film relationship used for horizontal or vertical angulations. B, There is a clear outline of the first molar but limited information about superimposed structures (canals that lie in the buccolingual plane). The arrow points to a periodontal ligament space adjacent to a superimposed root bulge, not to a second canal.

(From Walton R: Endodontic radiographic techniques, Dent Radiogr Photogr 46:51, 1973.)

A, The horizontal angulation of the cone is 20-degrees mesial from the parallel, right-angle position (mesial projection). B, The resultant radiograph demonstrates the morphologic features of the root or canal in the third dimension. For example, two canals are now visible in the distal root of the first molar.

(From Walton R: Endodontic radiographic techniques, Dent Radiogr Photogr 46:51, 1973.)

SLOB Rule

When two objects and the film or sensor are in a fixed position buccal and lingual from each other, and the radiation source (cone) is moved in a horizontal or vertical direction, images of the two objects move in the opposite direction ( Fig. 12.9 ). The facial (buccal) object shifts farthest away; the lingual object moves in the direction of the cone movement. The resulting radiograph shows a lingual object that moved relatively in the same direction as the cone and a buccal object that moved in the opposite direction. This principle is the origin of the acronym SLOB (same lingual, opposite buccal).