Radiology Lecture 7.docx

Lecture 7 Chapter 25 – Panoramic Imaging It is often difficult, if not impossible, to obtain adequate diagnostic information from a series of intraoral images alone. Impacted 3rd molars, jaw fractures, and large lesions in the posterior mandible cannot be adequately examined on intraoral project...

Lecture 7 Chapter 25 – Panoramic Imaging It is often difficult, if not impossible, to obtain adequate diagnostic information from a series of intraoral images alone. Impacted 3rd molars, jaw fractures, and large lesions in the posterior mandible cannot be adequately examined on intraoral projections; in such cases, the panoramic image is preferred. Panoramic imaging is an extraoral technique that is used to examine the maxilla and the mandible on a single projection Use/Purpose To evaluate the dentition and supporting structures To evaluate impacted teeth To evaluate eruption patterns, growth, and development To detect diseases, lesions, and conditions of the jaws To examine the extent of large lesions To evaluate trauma Fundamentals The images on a panoramic projection are not as defined or sharp as the images produced with intraoral projections. Consequently, a panoramic image should not be used to diagnose caries, periodontal disease, or periapical lesions. In panoramic imaging, the receptor and the x-ray tubehead move around the patient. The x-ray tube rotates around the patient's head in one direction, while the receptor rotates in the opposite direction The movement of the receptor and the tubehead produces an image through the process known as tomography. Tomo – refers to section; tomography is an imaging technique that allows the imaging of one layer, or section, of the body while blurring the images of structures in other planes. In panoramic imaging, this image conforms to the shape of the dental arches. Rotation Center The pivotal point, or axis, around which the receptor and the x-ray tubehead rotate is termed the rotation center. This rotational change allows the image layer to conform to the elliptical shape of the average dental arches. The moving x-ray source and receptor generate a zone known as the focal trough. Focal Trough In panoramic imaging, the focal trough is a theoretical concept used to determine where the dental arches must be positioned to obtain the sharpest image. The focal trough (also known as the image layer) can be defined as a three-dimensional curved zone in which structures are clearly demonstrated on a panoramic image. The size and shape of the focal trough vary, depending on the manufacturer of the panoramic x-ray unit. The closer the rotation center is to teeth, the narrower the focal trough. In most panoramic x- ray machines, the focal trough is narrow in the anterior region and wide in the posterior region, conforming to the average patient size. Panoramic X-ray Units A variety of panoramic x-ray units are available on the market today. Panoramic units may differ with regard to the size and shape of the focal trough and the type of receptor transport mechanism used. WHAT IS THE PURPOSE OF THE COLLIMATOR IN AN INTRAORAL X-RAY MACHINE? The collimator used in the panoramic x-ray tubehead, however, differs from the collimator used in the intraoral x-ray tubehead. The function of the collimator is to restrict the size and shape of the x-ray beam. The collimator used in the panoramic x-ray machine is a lead plate with an opening in the shape of a narrow vertical slit The vertical angulation of the panoramic tubehead does not vary as in the case of the intraoral tubehead. The tubehead of the panoramic unit is fixed in position so that the x-ray beam is directed slightly upward (approximately −10 degrees). In addition, the tubehead of the panoramic unit always rotates behind the patient's head, while the receptor rotates in front of the patient. Although predetermined exposure settings for panoramic imaging are available, the milliamperage and kilovoltage settings are adjustable and can be varied to accommodate patients of different sizes. Image Receptors WHAT EXPOSES SCREEN FILM? Extraoral screen film is used in film-based panoramic imaging; this film is sensitive to the light emitted from intensifying screens. A screen film is placed between two intensifying screens in a cassette holder. When the cassette holder is exposed to x-rays, the screens convert the x-ray energy into light, which, in turn, exposes the screen film. WHAT 2 COLORS ARE ASSOCIATED WITH SCREEN FILMS? DO THE COLORS HAVE TO BE PAIRED TOGETHER? Some screen films are sensitive to green light (T-Mat film), whereas others are sensitive to blue light (X-Omat DBF films). Blue-sensitive film must be paired with screens that produce blue light, and green- sensitive film must be paired with screens that produce green light. WHAT ARE THE TWO TYPES OF SCREEN FILMS? WHAT COLOR LIGHT DOES CALCIUM TUNGSTATE EMIT? WHAT COLOR DOES RARE EARTH EMIT? Two basic types of intensifying screens are used: calcium tungstate and rare earth. Calcium tungstate screens emit blue light, and the rare earth screens emit green light. Rare earth screens require less x-ray exposure than do calcium tungstate screens and are considered “faster.” Consequently, rare earth screens are recommended in panoramic imaging because of less radiation exposure to the patient. The cassette is a device that is used to hold the extraoral film and intensifying screens One intensifying screen is placed on each side of the film and held in place when the cassette is closed. Anatomic Features Each panoramic image should be assessed to determine if the dentition and bones of the maxillofacial region are representative. Assessment of acceptable dentition features includes the following: anterior teeth are in focus with pulp chambers visible, anterior teeth are not excessively narrow or wide, and, posterior teeth on right side appear similar in size to the posterior teeth on the left side. Assessment of bony anatomic accuracy includes the following: Both condyles appear on the image, the palate appears above the apices of the maxillary teeth and superimposed over the maxillary sinus, and the width of the right ramus is similar to the width of the left ramus. Density and Contrast An overexposed image appears excessively dark with areas of “burnout” An underexposed image appears excessively light with areas of “whiteout” It is important to note that with digital imaging, an overexposed image can be corrected with the use of software, but an underexposed image cannot. Proper contrast on a panoramic image is also critical, especially when multiple anatomic structures appear overlapped. Ideal contrast on a panoramic image should allow for the identification of the junction between enamel and dentin in the molar region Common Errors Ghost Image A ghost image results when an anatomic structure or object is located outside of the focal plane and close to the x-ray source. A ghost image resembles its true image and is found on the opposite side of the receptor; it appears blurred, magnified, and higher than the actual counterpart. A ghost image appears in a different location than the true image; it appears on the opposite side because the receptor was on the opposite side when the x-rays passed through the structure. A ghost image appears blurred and distorted because the structure is far from the focal trough. A ghost image appears higher than the true image as the result of the negative vertical angulation of the x-ray beam. Anatomic structures that are located laterally, such as the ramus of the mandible, or located centrally, such as the hard palate, can produce ghost images. Objects located laterally such as earrings can also produce ghost images. If all metallic or dense objects (e.g., eyeglasses, earrings, necklaces, intraoral and extraoral piercings, hairpins, removable partial dentures, complete dentures, orthodontic retainers, hearing aids, napkin chains) are not removed before the exposure of a panoramic receptor, a ghost image results that may obscure diagnostic information. A ghost image resembles its real counterpart and is found on the opposite side of the image; it appears indistinct, larger, and higher than its actual counterpart. Lead apron artifact If the lead apron is incorrectly placed on the patient, a radiopaque cone-shaped artifact results that obscures diagnostic information. If a lead apron with a thyroid collar is used during the exposure of a panoramic projection, a bilateral radiopaque artifact results that obstructs the mandible To prevent such artifacts, the dental radiographer must always use a lead apron without a thyroid collar when exposing a panoramic projection Positioning of Lips and Tongue If the patient's lips are not closed on the bite-block during the exposure of a panoramic projection, a dark radiolucent shadow results that obscures anterior teeth. The patient should bite on the bite-block. Then the patient must then be instructed to swallow once and to hold the tongue against the hard palate during the exposure of the projection. Chin tipped up If the patient is positioned such that the chin is too high or is tipped up, the Frankfort plane is angled upward, and the following errors result: The condyles may not be visible or may appear near the lateral edge of the image. The hard palate and floor of the nasal cavity appear superimposed over the roots of maxillary teeth. The maxillary incisors appear blurred and magnified. A loss of detail occurs in the maxillary incisor region. A “reverse smile line” (curved downward) is seen on the image. To prevent such an error, the dental radiographer must carefully position the patient such that the Frankfort plane (imaginary plane that passes from the bottom of the eye socket through the top of the ear canal) is parallel to the floor. Chin tipped down If the patient is positioned such that the chin is too low or is tipped down, the Frankfort plane is angled downward, and the following errors result: The condyles are positioned higher on the image. The hyoid bone forms a single widened line. The mandibular incisors appear blurred; roots may appear short. A loss of detail occurs in the anterior apical region. An “exaggerated smile line” or “jack-o'-lantern” appearance (curved upward) is seen on the image. When the chin is tipped down too far, these anatomic features described may be severe, requiring a retake of the image. To prevent such an error, the dental radiographer must carefully position the patient such that the Frankfort plane is parallel to the floor. Teeth anterior to the focal trough If the patient is positioned such that the anterior teeth are not positioned in the focal trough, as indicated by the groove in the bite-block, teeth appear blurred. If the patient's teeth are too far forward on the bite-block or anterior to the focal trough, anterior teeth appear “skinny” and out of focus on the image. In addition, pronounced overlap of the premolars may be seen. To prevent such an error, the dental radiographer must position the patient such that the anterior teeth are in an end-to-end position in the groove on the bite-block. The forehead support must then be adjusted to stabilize the patient's head position and prevent the patient from sliding forward on the bite-block. Teeth Posterior to the focal trough If the patient's anterior teeth are not positioned in the focal trough, as indicated by the groove in the bite-block, the teeth appear blurred. If the patient's anterior teeth are aligned too far back on the bite-block or posterior to the focal trough , the teeth appear “fat” and out of focus on the image. The roots of the anterior teeth may appear to be cut off. To prevent such an error, the dental radiographer must position the patient such that anterior teeth are in an end-to-end position in the groove on the bite-block. Head tilted (turned) If the patient's head is turned slightly to one side and not centered on the bite-block, the structures on one side are closer to the receptor while the structures on the other side are farther away. As a result, the ramus and posterior teeth on one side of the image appear larger than those on the other side of the image. The side farthest from the receptor appears magnified, and the side closest to the receptor appears smaller. For example, if the patient's head is turned to the right, the teeth on the patient's right side are closer to the receptor. The teeth closest to the receptor demonstrate the least amount of magnification. To prevent such an error, the dental radiographer must position the patient's head such that the midsagittal plane (imaginary plane that divides the face into right and left equal sides) is perpendicular to the floor while the midline is centered on the bite-stick. Slumped posture When the patient is slouched, slumped, or not standing with the shoulders back, the x-ray beam passes through more of the cervical spine because the beam is angled upward at a negative vertical angulation (−10 degrees). The cervical spine appears as a radiopacity in the center of the image and obscures diagnostic information To prevent such an error, the dental radiographer must instruct the patient to stand or sit “as tall as possible” with a straight back. An additional instruction to the patient may include “step forward slightly closer to the machine.” This movement has the effect of straightening the cervical spine. Advantages of Panoramic Imaging Field size. Simplicity. Patient cooperation Disadvantages of Panoramic Imaging Image quality. The images seen on a panoramic image are not as sharp as images produced with intraoral projections. As a result, the panoramic image cannot be used to diagnose dental caries, periodontal disease, or periapical lesions. Focal trough limitations. Objects of interest that are located outside the focal trough cannot be seen. Distortion. Certain amounts of magnification, distortion, and overlapping are present on a panoramic image, even when proper technique is used. Equipment cost. The cost of a panoramic x-ray unit is relatively high compared with the cost of an intraoral x-ray unit.