Intraoral radiographic (imaging) examinations are the backbone of imaging for the general dentist. Intraoral images can be divided into three categories: (1) periapical projections, (2) bitewing projections, and (3) occlusal projections. Periapical radiographs should show all of a tooth, including the surrounding bone. Bitewing images show only the crowns of teeth and the adjacent alveolar crests. Occlusal images show an area of teeth and bone larger than periapical images.
A full-mouth set of radiographs consists of periapical and bitewing projections (Fig. 7-1). These projections, when well exposed and properly processed, can provide considerable diagnostic information to complement the clinical examination. As with any clinical procedure, the operator must clearly understand the goals of dental radiography and the criteria for evaluating the quality of performance.
Images should be made only when a clear diagnostic need exists for the information the radiograph may provide. The frequency of such examinations varies with the individual circumstances of each patient (see Chapter 16).
Criteria of Quality
• Radiographs should record the complete areas of interest on the image. In the case of intraoral periapical images, the full length of the roots and at least 2 mm of periapical bone must be visible. If evidence of a pathologic condition is present, the area of the entire lesion plus some surrounding normal bone should show on one radiograph. If this is not possible to achieve on a periapical radiograph, an occlusal projection may be required as well as an extraoral projection. Bitewing examinations should demonstrate each posterior proximal surface at least once.
• Radiographs should have the least possible amount of distortion. Most distortion is caused by improper angulation of the x-ray beam rather than by the curvature of the structures being examined or inappropriate positioning of the receptor. Close attention to proper positioning of the receptor and x-ray tube results in diagnostically useful images.
• Images should have optimal density and contrast to facilitate interpretation. Although milliamperage (mA), peak kilovoltage (kVp), and exposure time are crucial parameters influencing density and contrast, faulty processing can adversely affect the quality of a properly exposed radiograph.
When evaluating images and considering whether to retake a view, the practitioner should consider the initial reason for making the image. When a full-mouth set is indicated, it is not necessary to retake a view that fails to open a contact or show a periapical region if the missing information is available on another view. If a single view or only a few views are needed, they should be repeated only if they fail to reveal the desired information.
Two intraoral projection techniques are commonly used for periapical imaging: (1) the paralleling technique and (2) the bisecting-angle technique. Most clinicians prefer the paralleling technique because it provides a less distorted view of the dentition. The paralleling technique is the most appropriate technique for digital imaging. The following discussion describes the principles and uses of the paralleling technique to obtain a full-mouth set of radiographs. When anatomic configuration (e.g., palate and floor of the mouth) precludes strict adherence to the paralleling concept, slight modifications may have to be made. If the anatomic constraints are extreme, some of the principles of the bisecting-angle technique may be used to accomplish the required receptor placement and determine the vertical angulation of the tube. The bisecting-angle technique is described later.
The term image receptor refers to any medium that can capture an image, including film, charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) sensors, or storage phosphor plates. The principles for making images are the same for each of these receptor types; thus this chapter uses the general term receptor to refer any of the image receptors.
General Steps for Making an Exposure
• Prepare unit for exposure. Place barriers for universal infection control (see Chapter 15) and have receptors and receptor-holding instruments ready at chairside.
• Greet and seat the patient. Position the patient upright in the chair with the back and head well supported, and briefly describe the procedures that are to be performed. Position the dental chair low for maxillary projections and elevated for mandibular projections. Ask the patient to remove eyeglasses and all removable appliances. Drape the patient with a lead apron regardless of whether a single image or a full series is to be made. Do not comment on any discomfort the patient may feel during the procedure. If it seems necessary to apologize for any discomfort, do it after the examination.
• Wash hands thoroughly. Wash your hands with soap and water, preferably in front of the patient or at least in an area where the patient can observe or be aware of the washing. Put on disposable gloves.
• Examine the oral cavity. Before placing any receptors in the mouth, examine the teeth to estimate their axial inclination, which influences the placement of the receptor. Also note tori or other obstructions that modify receptor placement.
• Position the receptor. Insert the receptor into the holding device, and position the receptor and holding device in the region of the patient’s mouth to be examined. Angle the receptor holder such that the film or sensor is parallel to the occlusal plane. Leading with the apical end of the receptor, rotate it into the oral cavity. This technique avoids asking the patient to open wide. For maxillary views, place the receptor in the mouth as far from the teeth as possible, near the midline of the palate. This places the receptor in the midline where there is the maximal space available. The added space allows the receptor to be oriented parallel to the long axis of the teeth. With the receptor now in the mouth, place it gently on the palate or floor of the mouth. For all bitewing and mandibular periapical images, it is useful to shift the mandible to the side being imaged; this provides more patient comfort because there is now more room for the sensor on the lingual side of the mandible. After the sensor is positioned, rotate the receptor-holding instrument until the bite-block rests on the teeth to be radiographed, and place a cotton roll between the bite-block and the opposing teeth. The cotton roll helps stabilize the receptor-holding instrument and in many cases contributes to the patient’s comfort. Ask the patient to close the mouth gently, holding the instrument and receptor in place. When placing a CCD or CMOS sensor, remember to account for the 2- to 4-mm nonimaging area found at the end of the sensor where the cord attaches.
• Position the x-ray tube. Adjust the vertical and horizontal angulation of the tube head to correspond to the receptor-holding instrument. The end of the aiming cylinder of the x-ray machine must be flush or parallel to the guide ring instrument. Alignment is satisfactory when the aiming cylinder covers the port and is within the limits of the face shield. Caution the patient not to move.
• Make the exposure. Make the exposure with the preset exposure time. If the receptor is a film or storage phosphor plate, remove the receptor from the patient’s mouth after exposure, dry it with a paper towel, and place it in an appropriate receptacle outside the exposure area. If the receptor is a CCD or CMOS sensor, you may be able to keep it in the patient’s mouth and reposition it for the next view. Encourage the patient along the way.
A typical full-mouth set of radiographs consists of 21 images (Box 7-1; see also Fig. 7-1). Establish a regular sequence when making exposures to avoid overlooking individual projections. Make the anterior views before the posterior views because the former views cause less discomfort for the patient. The following description of procedures pertains to the paralleling technique.
The central concept of the paralleling technique (also called the right-angle technique or long-cone technique) is that the x-ray receptor is supported parallel to the long axis of the teeth, and the central ray of the x-ray beam is directed at right angles to the teeth and receptor (Fig. 7-2). This orientation of the receptor, teeth, and central ray minimizes geometric distortion and presents the teeth and supporting bone in their true anatomic relationships. To reduce geometric distortion, the x-ray source should be located relatively distant from the teeth. The use of a long source-to-object distance reduces the apparent size of the focal spot, thus increasing image sharpness, and provides images with minimal magnification. The paralleling method works equally well for film, CCD or CMOS sensors, or storage phosphor plates.
Use instruments to allow precise positions of the receptor in the patient’s mouth. Many of these receptor holders are specific for various brands of digital sensors, storage phosphor plates, or film. It is also important to use a receptor-holding instrument that has an external guiding ring. This guiding ring is used to align the x-ray aiming cylinder and ensures that the receptor is centered in the beam behind the teeth of interest and that the receptor and teeth are perpendicular to the x-ray beam (Fig. 7-3). These should be used with rectangular collimators to reduce patient exposure (see Chapter 3).
For the best images, the receptor should be positioned parallel to the teeth and deep in the patient’s mouth; this is particularly important when rigid sensors are used because they may be larger than film. For maxillary projections, the superior border of the receptor generally rests at the height of the palatal vault in the midline. Similarly, for mandibular projections, the receptor should be used to displace the tongue posteriorly or toward the midline to allow the inferior border of the receptor to rest on the floor of the mouth away from the mucosa on the lingual surface of the mandible. Especially for digital sensors, patient acceptance and comfort are best when the receptor is placed in the center of the mouth.
Angulation of the Tube Head
Orient the aiming cylinder of the x-ray machine in the vertical and horizontal planes to align with the aiming ring. The horizontal direction of the beam primarily influences the degree of overlapping of the images of the crowns at the interproximal spaces (Fig. 7-4).
The bisecting-angle technique was used often in the first half of the twentieth century but has been largely replaced by the paralleling technique. This method may be useful when the operator is unable to apply the paralleling technique because of large rigid sensors or the anatomy of the patient. The bisecting-angle technique is based on a simple geometric theorem, Cieszynski’s rule of isometry, which states that two triangles are equal when they share one complete side and have two equal angles. Dental radiography applies the theorem as follows. The receptor is positioned as close as possible to the lingual surface of the teeth, resting in the palate or in the floor of the mouth (Fig. 7-5). The plane of the receptor and the long axis of the teeth form an angle with its apex at the point where the receptor is in contact with the teeth along an imaginary line that bisects this angle and directs the central ray of the beam at right angles to this bisector. This forms two triangles with two equal angles and a common side (the imaginary bisector). Consequently, when these conditions are satisfied, the images cast on the receptor theoretically are the same length as the projected object. To reproduce the length of each root of a multirooted tooth accurately, the central beam must be angled differently for each root. Another limitation of this technique is that the alveolar ridge often projects more coronally than its true position, thus distorting the apparent height of the alveolar bone around the teeth.
Several methods can be used to support receptors intraorally for bisecting-angle projections. The preferred method is to use a receptor-holding, bisecting-angle instrument that provides an external device for localizing the x-ray beam. The bisecting-angle instrument uses a fixed average bisecting angle. It is undesirable to have the patient support the receptor from the lingual surface with his or her forefinger. Patients often use excessive force and bend the receptor, causing distortion of the image. Also, the receptor might slip without the operator’s expertise, resulting in an improper image field. Finally, without an external guide to the position of the receptor, the x-ray beam may miss part of the receptor, resulting in a partial image (cone cut).
Positioning of the Patient
For images of the maxillary arch, the patient’s head should be positioned upright with the sagittal plane vertical and the occlusal plane horizontal. When the mandibular teeth are to be radiographed, the head is tilted back slightly to compensate for the changed occlusal plane when the mouth is opened.
The projections described for the paralleling technique may also be used for the bisecting-angle technique. The receptor is positioned behind the area of interest, with the apical end against the mucosa on the lingual or palatal surface. The occlusal or incisal edge is oriented against the teeth with an edge of the receptor extending just beyond the teeth. If necessary for the patient’s comfort, the anterior corner of a film can be softened by bending it before it is placed against the mucosa. Care must be taken not to bend the film excessively because this may result in considerable image distortion and pressure defects in the emulsion that are apparent on the processed film. Such bending is impossible with CCD or CMOS sensors or storage phosphor plates.
Angulation of the Tube Head
When a receptor-holding device with a beam-localizing ring is used, the instrument is positioned horizontally so that when the tube is aligned with the ring, the central ray is directed through the contacts in the region being examined. If the receptor-holding device does not have a beam-localizing feature, the tube is pointed so as to direct the central ray through the contacts. In this situation, the radiation beam is also centered on the receptor. This angulation usually is at right angles (in the horizontal projection) to the buccal or facial surfaces of the teeth in each region.
In practice, the clinician’s goal is to aim the central ray of the x-ray beam at right angles to a plane bisecting the angle between the receptor and the long axis of the tooth. This principle works well with flat, two-dimensional structures, but teeth that have depth or are multirooted show evidence of distortion. Excessive vertical angulation results in foreshortening of the image, whereas insufficient vertical angulation results in image elongation. The angle that directs the central ray perpendicular to the bisecting plane varies with the individual’s anatomy. Several measurements can be used as a general guide when the occlusal plane is oriented parallel with the floor (Table 7-1).