Oral Radiology - Projection Geometry PDF

CHAPTER 6 Projection Geometry m OUTLINE o...

CHAPTER 6 Projection Geometry m OUTLINE o Image Sharpness and Resolution Image Shape Distortion Object Localization t.c Image Size Distortion Paralleling and Bisecting-Angle Techniques Eggshell Effect A conventional radiograph is made with a stationary x-ray in Chapter 1, the size of the effective focal spot is a function po source and displays a two-dimensional image of a part of of the angle of the target with respect to the long axis of the the body. Such images are often called plain or projection electron beam. A large angle distributes the electron beam over views (in contrast to ultrasound, computed tomography [CT], a larger surface and decreases the heat generated per unit of magnetic resonance imaging, or nuclear medicine). In plain views, target area, thus prolonging tube life; however, this results in gs the entire volume of tissue between the x-ray source and the image a larger effective focal spot and loss of image clarity (Fig. 6-2). receptor (digital sensor or film) is projected onto a two-dimensional A small angle has a greater wearing effect on the target but image. To obtain the maximal value from a radiograph, a clinician results in a smaller effective focal spot and increased image must have a clear understanding of normal anatomy and mentally sharpness. lo reconstruct a three-dimensional image of the anatomic structures 2. Increase the distance between the focal spot and the object by using a of interest from one or more of these two-dimensional views. Using long, open-ended cylinder. Figure 6-3 shows how increasing the high-quality radiographs greatly facilitates this task. The principles focal spot-to-object distance reduces image blurring by reducing y.b of projection geometry describe the effect of focal spot size and the divergence of the x-ray beam. A longer focal spot-to-object relative position of the object and image receptor (digital sensor distance minimizes blurring by using photons whose paths are or film) on image clarity, magnification, and distortion. Clinicians almost parallel. The benefits of using a long focal spot-to-object use these principles to maximize image clarity, minimize distor- distance support the use of long, open-ended cylinders as tion, and localize objects in the image field. aiming devices on dental x-ray machines. r 3. Minimize the distance between the object and the image receptor. Figure IMAGE SHARPNESS AND RESOLUTION 6-4 shows that, as the object-to-image receptor distance is ra reduced, the zone of unsharpness decreases, resulting in Several geometric considerations contribute to image sharpness enhanced image clarity. This is the result of minimizing the and spatial resolution. Sharpness measures how well a boundary divergence of the x-ray photons. llib between two areas of differing radiodensity is revealed. Image spatial resolution measures how well a radiograph is able to reveal small objects that are close together. Although sharpness and reso- IMAGE SIZE DISTORTION lution are two distinct features, they are interdependent, being Image size distortion (magnification) is the increase in size of the influenced by the same geometric variables. For clinical diagnosis, image on the radiograph compared with the actual size of the a it is desirable to optimize conditions that result in images with object. The divergent paths of photons in an x-ray beam cause high sharpness and resolution. enlargement of the image on a radiograph. Image size distortion nt When x rays are produced at the target in an x-ray tube, they results from the relative distances of the focal spot-to-image recep- originate from all points within the area of the focal spot. Because tor and object-to-image receptor (see Figs. 6-3 and 6-4). Increasing these rays originate from different points and travel in straight the focal spot-to-image receptor distance and decreasing the object- lines, their projections of a feature of an object do not occur at to-image receptor distance minimizes image magnification. The de exactly the same location on an image receptor. As a result, the use of a long, open-ended cylinder as an aiming device on an x-ray image of the edge of an object is slightly blurred rather than sharp machine thus reduces the magnification of images on a periapical and distinct. Figure 6-1 shows the path of photons that originate view. As previously mentioned, this technique also improves image at the margins of the focal spot and provide an image of the edges sharpness by increasing the distance between the focal spot and of an object. The resulting blurred zone of unsharpness on an the object. image causes a loss in image sharpness. The larger the focal spot area, the greater the unsharpness. There are three means to maximize image sharpness: IMAGE SHAPE DISTORTION 1. Use as small an effective focal spot as practical. Dental x-ray Image shape distortion is the result of unequal magnification of machines preferably should have a effective focal spot size of different parts of the same object. This situation arises when not 0.4 mm because this greatly adds to image clarity. As described all the parts of an object are at the same focal spot-to-object 84  C H A P T E R 6 Projection Geometry 85 Anode Large focal spot Anode Small focal spot FIGURE 6-1 Photons originating at different places on the focal spot (red) result in a zone of unsharpness on the radiograph. The density of the image changes from a high m Object background value to a low value in the area of an edge of enamel, dentin, or bone. On the left, a large focal spot size results in a wide zone of unsharpness compared with a small o focal spot size on the right, which results in a sharper image (narrow zone of unsharpness). t.c Image receptor Unsharpness Unsharpness po Density Density gs Electron beam Electron beam Anode Anode Actual lo focal spot Actual focal spot y.b FIGURE 6-2 As the angle of the target becomes closer to perpendicular to the long axis of the electron beam (as shown on the right) the actual focal spot becomes smaller, Object which decreases heat dissipation and tube life. The more r perpendicular angle also decreases the effective focal spot ra Effective size, increasing the sharpness of the resulting image. focal spot llib Image receptor Unsharpness a nt distance. The physical shape of the object may often prevent its image receptor. This type of shape distortion is called fore- optimal orientation, resulting in some shape distortion. Such a shortening because it causes the radiographic image to be phenomenon is seen by the differences in appearance of the image shorter than the object. Figure 6-6 shows the situation when de on a radiograph compared with the true shape. To minimize shape the x-ray beam is oriented at right angles to the object but distortion, the practitioner should make an effort to align the tube, not to the image receptor; this results in elongation, with object, and image receptor carefully according to the following the object appearing longer on the image receptor than its guidelines: actual length. 1. Position the image receptor parallel to the long axis of the object. 2. Orient the central ray perpendicular to the object and image receptor. Image shape distortion is minimized when the long axes of Image shape distortion occurs if the object and image receptor the image receptor and tooth are parallel. Figure 6-5 shows are parallel, but the central ray is not directed at right angles to that the central ray of the x-ray beam is perpendicular to the each. This distortion is most evident on maxillary molar views image receptor, but the object is not parallel to the image (Fig. 6-7). If the central ray is oriented with an excessive vertical receptor. The resultant image is distorted because of the angulation, the palatal roots appear disproportionately longer unequal distances of the various parts of the object from the than the buccal roots. 86 PART II Imaging Anode m FIGURE 6-3 Increasing the distance between the focal spot and the object Anode o results in an image with increased sharpness and less magnification of the object as seen on the right. t.c po Image gs receptor lo y.b Anode Anode r ra FIGURE 6-4 Decreasing the distance between the object and the image receptor increases the sharpness and results in less magnification of the object as seen on the left. a llib Image receptor nt 0 5 10 15 20 0 5 10 15 20 25 de The practitioner can prevent shape distortion errors by aligning placed as close to the teeth as possible without deforming it. the object and image receptor parallel with each other and the However, when the image receptor is in this position, it is not central ray perpendicular to both. parallel to the long axes of the teeth. This arrangement inherently causes distortion. Nevertheless, by directing the central ray perpen- PARALLELING AND BISECTING-ANGLE TECHNIQUES dicular to an imaginary plane that bisects the angle between the teeth and the image receptor, the practitioner can make the length From the earliest days of dental radiography, a clinical objective of the tooth’s image on the image receptor correspond to the actual has been to produce accurate images of dental structures that are length of the tooth. This angle between a tooth and the image normally visually obscured. An early method for aligning the x-ray receptor is especially apparent when teeth are radiographed in the beam and image receptor with the teeth and jaws was the bisecting- maxilla or anterior mandible. Although the projected length angle technique (Fig. 6-8). In this method, the image receptor is of a tooth is correct, these images display a distorted image of the  C H A P T E R 6 Projection Geometry 87 Anode o m 10 5 0 t.c Image receptor 0 5 10 FIGURE 6-7 The central ray should be perpendicular to the long axes of both the tooth and the image receptor. If the direction of the x-ray beam is not at right angles to the long axis of the tooth, the appearance of the tooth is distorted, typically by apparent elongation of the po FIGURE 6-5 Foreshortening of a radiographic image results when the central ray is per- length of the palatal roots of upper molars and distortion of the relationship of the height of pendicular to the image receptor but the object is not parallel with the image receptor. the alveolar crest relative to the cementoenamel junction. gs Central axis of tooth Anode lo Imaginary bisector y.b 0 5 10 r 25 ra 15 20 10 llib 0 5 FIGURE 6-6 Elongation of a radiographic image results when the central ray is perpen- FIGURE 6-8 In the bisecting-angle technique, the central ray is directed at a right angle dicular to the object but not to the image receptor. to the imaginary plane that bisects the angle formed by the image receptor and the central axis of the object. This method produces an image that is the same length as the object but results a in some image distortion. nt position of alveolar crest with respect to the cementoenamel junc- loss of sharpness. To overcome these limitations, the paralleling tion of a tooth. In recent years, the bisecting-angle technique has technique also uses a relatively long open-ended aiming cylinder been used less frequently for general periapical radiography as use (“cone”) to increase the focal spot-to-object distance. This “cone” de of the paralleling technique has increased. directs only the most central and parallel rays of the beam to the The paralleling technique is the preferred method for making image receptor and teeth and reduces image magnification, while intraoral radiographs. It derives its name as the result of placing increasing image sharpness. Because it is desirable to position the image receptor parallel to the long axis of the tooth (Fig. 6-9). image receptors near the middle of the oral cavity with the paral- This procedure minimizes image distortion and best incorporates leling technique, image receptor holders should be used to support the imaging principles described in the first three sections of this the image receptor in the patient’s mouth (see Chapter 7). chapter. To achieve this parallel orientation, the practitioner often must position the image receptor toward the middle of the oral cavity, OBJECT LOCALIZATION away from the teeth. Although this allows the teeth and image In clinical practice, the dentist often must derive from a radiograph receptor to be parallel, it results in some image magnification and three-dimensional information concerning patients. For example, 88 PART II Imaging the dentist may wish to use radiographs to determine the location radiograph, the dentist may take a mandibular occlusal view to of a foreign object or an impacted tooth within the jaw. Three identify its mediolateral position. The occlusal film may reveal a methods are frequently used to obtain such three-dimensional calcification in the soft tissues located laterally or medially to the information. The first is to examine two images projected at right body of the mandible. This information is important in determin- angles to each other. The second method is to use the tube-shift ing the treatment required. The right-angle (or cross section) tech- technique employing conventional periapical views. Third, in nique is best for the mandible (see Figs. 22-8, A, 22-15, and 22-23, recent years, the advent of cone-beam imaging has provided a new B). On a maxillary occlusal view, the superimposition of features tool for obtaining three-dimensional information. In this chapter, in the anterior part of the skull frequently obscures the area of we discuss the first two of these methods. These techniques are interest. m valuable because cone-beam CT may not be available or even The second method used to identify the spatial position of an necessary if the dentist already has multiple periapical views of the object is the tube-shift technique. Other names for this procedure region of interest. Cone-beam CT is discussed in Chapters 11-13. are the buccal-object rule and Clark’s rule (Clark described this Figure 6-10 shows the first method, in which two views made method in 1910). The rationale for this procedure derives from the o at right angles to one another localize an object in or about the manner in which the relative positions of radiographic images of maxilla in three dimensions. In clinical practice, the position of two separate objects change when the projection angle at which t.c an object on each radiograph is noted relative to the anatomic the images were made is changed. landmarks; this allows the observer to determine the position of Figure 6-11 shows two radiographs of an object exposed at dif- the object or area of interest. For example, if a radiopacity is ferent angles. Compare the position of the object in question on found near the apex of the mandibular first molar on a periapical each radiograph with the reference structures. If the tube is shifted po and directed at the reference object (e.g., the apex of a tooth) from a more mesial angulation and the object in question also moves mesially with respect to the reference object, the object lies lingual Central axis of tooth to the reference object. gs Alternatively, if the tube is shifted mesially and the object in question appears to move distally, it lies on the buccal aspect of the reference object (Fig. 6-12). These relationships can be easily remembered by the acronym SLOB: same lingual, opposite buccal. lo Thus if the object in question appears to move in the same direc- tion with respect to the reference structures as does the x-ray tube, it is on the lingual aspect of the reference object; if it appears y.b to move in the opposite direction as the x-ray tube, it is on the buccal aspect. If it does not move with respect to the reference object, it lies at the same depth (in the same vertical plane) as the reference object. r ra llib FIGURE 6-9 In the paralleling technique, the central ray is directed at a right angle to the central axes of the object and the image receptor. This technique requires a device to support the film in position. A a nt de B A B FIGURE 6-11 The position of an object may be determined with respect to reference structures with use of the tube shift technique. A, A radiopaque object on the lingual surface FIGURE 6-10 A, Periapical radiograph shows impacted canine lying apical to roots of of the mandible (black dot) may appear apical to the second premolar. B, When another lateral incisor and first premolar. B, Vertex occlusal view shows that the canine lies palatal to radiograph is made of this region angulated from the mesial, the object appears to have moved the roots of the lateral incisor and first premolar. mesially with respect to the second premolar apex (“same lingual” in the acronym SLOB).  C H A P T E R 6 Projection Geometry 89 A o m t.c A B po FIGURE 6-12 The position of an object can be determined with respect to reference structures with use of the tube shift technique. A, An object on the buccal surface of the mandible may appear apical to the second premolar. B, When another radiograph is made of this region angulated from the mesial, the object appears to have moved distally with respect to the second premolar apex (“opposite buccal” in the acronym SLOB). gs lo Examination of a conventional set of full-mouth images with y.b this rule in mind demonstrates that the incisive foramen is located lingual (palatal) to the roots of the central incisors and that the B mental foramen lies buccal to the roots of the premolars. This technique assists in determining the position of impacted teeth, FIGURE 6-13 The position of the maxillary zygomatic process in relation to the roots the presence of foreign objects, and other abnormal conditions. It of the molars can help in identifying the orientation of views. A, The inferior border of r works just as well when the x-ray machine is moved vertically as the zygomatic process lies over the palatal root of the first molar. B, The inferior border horizontally. ra of the zygomatic process lies posterior to the palatal root of the first molar. This difference The dentist may have two radiographs of a region of the in position of the zygomatic process in relation to the palatal root indicates that when the dentition that were made at different angles, but no record image in A was made, the beam was oriented more from the posterior than when the exists of the orientation of the x-ray machine. Comparison of image in B was made. The same conclusion can be reached independently by examining llib the anatomy displayed on the images helps distinguish changes the roots of the first molar. The palatal root lies behind the distobuccal root in the image in horizontal or vertical angulation. The relative positions of in A, but it lies between the two buccal roots in the image in B. osseous landmarks with respect to the teeth help identify changes in horizontal or vertical angulation. Figure 6-13 shows the inferior border of the zygomatic process of the maxilla over a the molars. This structure lies buccal to the teeth and appears to move mesially as the x-ray beam is oriented more from nt the distal. Similarly, as the angulation of the beam is increased than photons traveling at right angles to the surface. Figure 6-14, vertically, the zygomatic process is projected occlusally over B, shows an expansile lesion on the buccal surface of the mandible the teeth. on an occlusal view. The periphery of the expanded cortex is more opaque than the region inside the expanded border. The de EGGSHELL EFFECT cortical bone is not thicker on the cortex than over the rest of the lesion, but rather the x-ray beam is more attenuated in this Plain images—images that project a three-dimensional volume onto region because of the longer path length of photons through the a two-dimensional receptor—may produce an eggshell effect of bony cortex on the periphery. This eggshell effect accounts for corticated structures. Figure 6-14, A, shows a schematic view of why normal structures such as the lamina dura, the border of the an egg being exposed to an x-ray beam. The top photon has a maxillary sinuses and nasal fossa, and abnormal structures, includ- tangential path through the apex of the egg and a much longer ing the corticated walls of cysts and benign tumors, are well path through the shell of the egg than does the lower photon, demonstrated on plain images. Soft tissue masses, such as the which strikes the egg at right angles to the surface and travels nose and tongue, do not show an eggshell effect because they through two thicknesses of the shell. As a result, photons traveling are uniform rather than being composed of a dense layer sur- through the periphery of a curved surface are more attenuated rounding a more lucent interior. 90 PART II Imaging o m A B C t.c FIGURE 6-14 Eggshell effect. A, Radiograph of a hard-boiled egg. Note how the rim of the eggshell is opaque even though it is uniform in thickness. B, Schematic view of the egg being exposed to an x-ray beam. The top photon has a tangential path through the apex of the egg and a longer path through the shell of the egg than the lower photon. As a result, photons traveling through the periphery po of a curved surface are more attenuated than the photons traveling at right angles to the surface. C, An expansile lesion on the buccal surface of the mandible on an occlusal view. The expanded cortex is more opaque than the region inside the border as a result of the eggshell effect. gs BIBLIOGRAPHY lo Ludlow JB: The buccal object rule, Dentomaxillofac Radiol 28:258, 1999. Buccal-Object Rule Richards AG: The buccal object rule, Dent Radiogr Photogr 53:37–56, 1980. 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