It is a technique that produces an image of the teeth and jaws on a single film, but it also depicts a somewhat bewildering array of hard and soft tissues in the head and neck. Some of these structures are also seen on intraoral radiographs but appear different on panoramic films, whereas other landmarks are found on panoramics but not intraoral films. Because of the unique physical nature of panoramic image formation, some structures are projected in areas of the film where they don’t exist.
Although dentists may concentrate only on the teeth and their supporting tissues when examining panoramic radiographs, they should also be able to identify anatomic landmarks and other images that appear on the film. This may increase the chance of detecting abnormalities in other parts of the head and neck and minimize the possibility of mistaking a normal structure for disease. In addition, knowledge of the structures that appear on the panoramic radiograph, and how they appear, will enable dentists to understand the indications and contraindications for this radiographic technique.
To recognize the anatomic features that are demonstrated on panoramic radiographs, it is helpful to understand how the image is created. This article will present a brief description of the generation and characteristics of the panoramic image, followed by a discussion of the efficacy of panoramic radiography for a variety of tasks. Illustrations of anatomic landmarks and other structures will be provided along with some examples of abnormalities in that are visualized well on panoramic radiographs.
How is the image made?
There are many panoramic units on the market today. They differ in some features, but they all operate on the same basic principles:
1) The X-ray source rotates behind the patient’s head, emitting radiation that is limited to a narrow vertical beam by a lead collimator at the front of the tube head.
2) The film cassette holder simultaneously passes in front of the patient’s head; the film moves in the contrary direction to the X-ray beam behind a lead shield that allows exposure of only a small part of the film through a narrow slit.
3) The rate of film motion is correlated to the rate of X-ray beam motion as it sweeps through the patient’s tissues, equalizing the vertical and horizontal magnification of certain structures in the image and thereby minimizing distortion.
The point around which the X-ray source and cassette holder rotate is called the center of rotation. Most modern machines employ a system that creates a continuously moving rotation center in a sliding path that changes as the X-ray source and cassette holder rotate.
The farther away from the curved plane points in the patient’s tissue are, the more blurred and distorted they appear on the radiograph. Although a certain amount of blurring and distortion is acceptable (and in fact is unavoidable in all radiographic images), some structures are so far removed from the plane that they are blurred beyond recognition. Therefore, there is a limited thickness of tissue on either side of the central curved plane that is imaged with sufficient sharpness and dimensional accuracy that it can be recognized and can be examined for the presence of disease. This thickness is called the image layer or focal trough (Figure 3). Ideally the patient is positioned in the panoramic unit so that the teeth and jaws are located in the image layer.
The actual location and contour of the image layer is determined by the design of the panoramic machine. The speed of film movement past the slit in the cassette holder’s lead shield affects the position of the image layer. The distance between the center of rotation and the central curved plane, and the width of the X-ray beam affects image layer thickness. By varying these parameters, manufacturers of panoramic units can create image layers of various thickness, contour, and position. There are differences between the machines currently on the market, but in general the image layer is widest in the posterior regions (premolar, molar, and TMJ areas) and narrowest in the anterior parts of the jaws. Some panoramic units permit the operator to modify the position and thickness of the image layer to accommodate patients of different sizes.
Objects in or near the image layer, i.e., between the rotation center and the film, appear on the radiograph with varying degrees of sharpness and distortion but in the proper location; these are the “true” images of the objects. However, some structures also can be projected onto the film during the stage in the rotation of the apparatus when they are between the center of rotation and the X-ray source. In this location the structures are far from the image layer, so their images are quite blurred. Because of their greater distance from the film, they are projected to a more superior level on the film than their actual location, the result of the negative vertical angulation of the X-ray beam. Most interestingly, the images appear on the opposite side of the film from their true location because the film is on the contralateral side when the X-rays pass through the objects. These excessively blurred, distorted images of things that don’t exist in those locations are called ghost images—images without substance. Objects that are located laterally in the patient’s maxillofacial anatomy, such as the mandibular rami, earrings, and lead markers in the head positioners of some panoramic units, or centrally, such as the cervical vertebrae and the hyoid bone, can produce ghost images. These structures normally also appear as “true” images. Some structures are scanned twice by the rotating X-ray beam, but they are between the center of rotation and the film in both projections. As a result, they cast images on both sides of the film, but the images are not excessively blurred or distorted as ghost images are. These are referred to as double images, and are made by structures including the body of the hyoid bone, the hard palate, and the epiglottis. In some patients the cervical vertebrae can appear as double images at either edge of the film.
Characteristics of the Panoramic Image
Panoramic radiographs are subject to the same geometric characteristics that affect intraoral images:
Blurring — Even when patients are positioned correctly with teeth in the image layer, panoramic images are generally not as sharp as intraoral radiographs. Panoramic radiography is a technique of controlled blurring: objects distant from the image layer are so blurred that they do not have recognizable form, but even in the image layer structures are somewhat unsharp. This is partly because only points in the thin central plane of the image layer are projected sharply. Points at any distance from the plane become blurred, even in the image layer. Teeth and the alveolar processes have finite thickness and are, therefore, subject to these problems.
Blurring is also due to the use of intensifying screens that are positioned on each side of the film in the cassette. Intensifying screens absorb the X-rays that have passed through the patient and convert them into large numbers of visible light photons that expose the film and make the image. Intensifying screens are beneficial because they permit the use of small amounts of X-rays, reducing patient radiation dose. However, light diverges as it is emitted from the screens, therefore structures represented by small points of X-ray photons appear as lines (i.e., blurs) on the film.
Distortion — At all points except for the curved plane in the middle of the image layer, the vertical and horizontal magnifications are unequal. As a result, panoramic images are distorted. Objects between the image layer and the film (facial to the image layer) will have smaller horizontal than vertical magnification because the X-ray beam passes through them with greater speed than the movement of the film. If the patient’s anterior teeth are positioned too far forward, they will appear blurred and narrow. Conversely, structures between the image layer and the center of rotation (lingual to the image layer) are scanned more slowly than the motion of the film and therefore are stretched out more in the horizontal dimension. This explains why positioning of the patient’s teeth too far away from the film produces an image in which the teeth are blurred and wide. Because the image layer thickness is generally smaller in the anterior aspect of the jaws than in the posterior regions, errors in patient positioning are more likely to move anterior teeth out of the image layer. This accounts for the fact that anterior teeth are often blurrier and more distorted than premolars or molars. Tilting the chin up too high displaces the apical ends of the maxillary anterior teeth behind the image layer, giving them a wide, unsharp appearance. If the chin is tilted down, the roots of the lower anterior teeth are moved behind the layer, with the same results.
Magnification — Image magnification is related to the distance between the X-ray source and the central plane of the image layer. In the horizontal dimension, the effective source of the X-ray beam is the center of rotation, which slides continuously throughout the projection in most panoramic units. Therefore, horizontal magnification varies from point to point on the image and for this reason horizontal measurements on panoramic radiographs are not accurate. In the vertical dimension, the source of X-rays is not the center of rotation but the tubehead itself. Since the distance between the tubehead and central plane is large and fairly constant compared to the distance between the rotation center and central plane, vertical magnification is smaller and less variable than horizontal magnification. However, objects that are oblique (with both vertical and horizontal components) can appear slightly curved because of the significant change in magnification of the horizontal component. Since most anatomic structures are somewhat oblique, vertical measurements are not truly dependable.
Overlapping Teeth — Proximal surfaces of teeth are depicted with minimal overlapping only when the horizontal angulation of the central ray is directed through the contact points. On panoramic radiographs the horizontal alignment of the beam to the alveolar processes varies considerably. In most panoramic units horizontal direction is most favorable in the anterior teeth; there is usually little overlapping of the incisors. The discrepancy between ideal and actual horizontal angulation is usually greatest in the premolar regions, explaining the frequent proximal overlapping of these teeth on panoramics.
Advantages and Limitations in Diagnosis
On the basis of knowledge of how the image is formed on panoramic radiographs, the strengths and drawbacks of this technique can be assessed in relation to various diagnostic tasks.Advantages of panoramic radiographs:
1) Large areas of the jaws that can’t be seen with intraoral radiographs can be visualized with panoramics. This makes these radiographs valuable for detecting
Some anatomic structures can look disturbing on panoramic images. The prominent radiolucency at the apices of the mandibular incisors (above) is simply the mental fossa (3).
The image on the right demonstrates the dorsal surface of the tongue (1), separated from the hard and soft palates by the palatoglossal air space (2). This space is eliminated by having the patient place the tongue against the palate during exposure.
Panoramic radiography is a very useful technique that can be an essential component of dental practice. Knowledge of the concept of the image layer and the manner in which the image is formed helps dentists understand the indications and contraindications of panoramic radiographs. Anatomic landmarks, including those that produce ghost and double images, are presented, along with some examples of abnormalities that can be seen on panoramic images.