Radiology Lecture 4.docx

Lecture 4 Chapter 6 – Dental X-ray Image Characteristics Radiolucent refers to that portion of an image that is dark or black. A structure that appears radiolucent lacks density and permits the passage of the x-ray beam with little or no resistance. For example, air space freely permits the pass...

Lecture 4 Chapter 6 – Dental X-ray Image Characteristics Radiolucent refers to that portion of an image that is dark or black. A structure that appears radiolucent lacks density and permits the passage of the x-ray beam with little or no resistance. For example, air space freely permits the passage of dental x-rays and appears. Radiopaque refers to that portion of an image that appears light or white. Radiopaque structures are dense and absorb or resist the passage of the x-ray beam. For example, structures that resist the passage of the x-ray beam include enamel, dentin, and bone and appear radiopaque on a dental image. Fillings, tooth structure Two visual characteristics – density and contrast – directly influence the diagnostic quality of a dental image. The overall blackness or darkness of a dental image is termed density. Density is affected by exposure time, kV, and mA. An increase in kV increases density by producing x-rays of higher energy. If the kilovoltage is increased, the density increases, and the image appears darker. Conversely, if kilovoltage is decreased, the density decreases, and the image appears lighter. If the operating kV is increased, the receptor density increases because the average energy of the x-rays is increased. If the milliamperage is increased, the density increases, and the image appears darker. Conversely, if the milliamperage is decreased, the density decreases, and the image appears lighter. If the exposure time is increased, more x-rays reach the receptor, the density increases, and the image appears dark. Conversely, if the exposure time is decreased, the density decreases, and the image appears lighter. Fewer x-rays reach the receptor in a patient with an increased amount of soft tissue, muscle, or thick, dense bones. As a result, the image has less density and appears lighter. The difference in the degrees of blackness (densities) between adjacent areas on a dental image is termed contrast. A dental image that has very dark areas and very light areas demonstrates high contrast; the dark and light areas are strikingly different. An image that does not have very dark and very light areas but instead has many shades of gray demonstrates low contrast. Development time or the temperature of the developer solution affects the contrast of a dental radiograph. An increase in development time or developer temperature results in a film with high contrast. Only one exposure factor has a direct influence on the contrast of a dental image: KILOVOLTAGE When a high operating kV is used, low subject contrast and many shades of gray are seen on the dental image. When a low operating kV is used, high subject contrast and areas of black and white are seen on the dental image. The range of useful densities seen on a dental image is termed the scale of contrast. In dental radiography, the terms short-scale contrast and long-scale contrast may be used to describe the appearance of an image. A dental image that shows only two densities, areas of black and areas of white, has a short contrast scale. A lower kilovoltage range results in an image with a short-scale contrast; many areas of black and white, rather than shades of gray, are seen. An image that exhibits a short contrast scale can also be described as having high contrast, in which the black and white areas are easily distinguished from each other A dental image that exhibits many densities, or many shades of gray, has a long contrast scale. A higher kilovoltage range results in an image with a long-scale contrast; many shades of gray, rather than areas of black and white, are present. An image that exhibits a long contrast scale can also be described as having low contrast, in which areas of gray are not easily distinguished from each other Note the long gray scale (low contrast) image with high kV and the short gray scale (high contrast) image when using low kV. So, which type of contrast is preferred in dental images? If the operator is suspicious of finding dental caries on the patient’s images, decreasing the kilovoltage to produce short-scale contrast may be beneficial. The carious lesion appears radiolucent as a result of the demineralization of the hard tooth structure. A short-scale contrast may help allow the radiolucent caries “stand out” more on the image because it demonstrates more black and white areas. On the other hand, if the operator is treating a patient with periodontal issues, an increase in the kilovoltage may be recommended to produce long-scale contrast. The long-scale contrast demonstrates many shades of gray; this may be beneficial to see the subtle changes in bone loss associated with periodontal disease A stepwedge consists of uniform-layered thicknesses of an x-ray absorbing material, usually aluminum. The typical stepwedge is constructed of aluminum steps in 2-mm increments Application: A step wedge will reveal that images taken at a lower kV will have higher or lower contrast than an image taken at a higher kV? HIGHER Step wedge images taken at a higher kV will have long-scale contrast compared to those taken at lower kV. Three geometric characteristics—sharpness, magnification, and distortion—influence the diagnostic quality of a dental image. These geometric characteristics must be controlled to produce an accurate (DETAILED, DEFINED) radiographic image. A certain lack of image sharpness, or unsharpness, is present in every dental image. Sharpness (also known as detail, resolution, or definition) refers to the capability of the receptor to reproduce the distinct outlines of an object—in other words, how well the smallest details of an object are reproduced on a dental image. The fuzzy, unclear area that surrounds a structure (e.g., a tooth) on an image is termed the penumbra. Penumbra can be defined as the unsharpness, or blurring, of the edges. The sharpness of an image is influenced by the following three factors Focal spot size Film composition Movement The focal spot size influences sharpness. the tungsten target of the anode serves as a focal spot; this small area converts bombarding electrons into x-ray photons. To limit the amount of heat produced and to prevent damage to the x-ray tube, the size of the focal spot is limited. The smaller the focal spot area, the sharper the image; the larger the focal spot area, the greater the loss of image sharpness (more blurry the inage). The composition of the film emulsion influences sharpness. Sharpness is relative to the size of the crystals found in the film emulsion. The emulsion of faster film contains larger crystals that produce less image sharpness, whereas slower film contains smaller crystals that produce more image sharpness. Unsharpness occurs because larger crystals do not produce object outlines as well as smaller crystals do. Movement influences image sharpness. A loss of image sharpness occurs if the tubehead, the receptor, or the patient moves during x-ray exposure. Even slight amounts of movement result in unsharpness, which may cause the image to be nondiagnostic Image magnification is a geometric characteristic that refers to a radiographic image that appears larger than the actual size of the object it represents. The magnification on a dental image is influenced by the following Target-receptor distance Object-receptor distance The target-receptor distance is determined by the length of the position-indicating device (PID). As a result, a longer PID and target-receptor distance result in less image magnification, and a shorter PID and target-receptor distance result in more image magnification The object-receptor distance is the distance between the object being radiographed (the tooth) and the image receptor. The tooth and the receptor should always be placed as close together as possible. The closer the tooth is to the receptor, the less the image is enlarged. Dimensional distortion of a radiographic image is a variation in the true size and shape of the object being radiographed. A distorted image results from the unequal magnification of different parts of the same object. Distortion results from improper receptor alignment or beam angulation. Foreshortened and elongated images are examples of distortion. The dimensional distortion of a radiographic image is influenced by the following Object-receptor alignment X-ray beam angulation To minimize dimensional distortion, the object and receptor must be parallel to each other. If the object (tooth) and receptor are not parallel, an angular relationship results. To minimize dimensional distortion, the x-ray beam must be directed perpendicular to the tooth and the receptor. Chapter 8 – Digital Imaging Digital imaging uses an electronic sensor and specialized computer software that produces images almost instantly on a computer monitor. The digital image is an array of picture elements, called pixels, with discrete gray values for each pixel. In dental imaging, the term digital image (not radiograph or x-ray film) is used to describe the pictures that are produced Depending on the speed of film that is being used, exposure times for digital imaging are 50% to 90% less than those required for conventional radiography. Less radiation exposure supports the ALARA principle, and therefore the use of digital imaging is highly recommended. A conventional intraoral dental x-ray unit may be used in both film-based and digital imaging. If the dental x-ray unit has a timer that uses impulses (allowing for 1/60 of a second), the timer must be adapted to allow for exposures in 1/100 of a second. The term wired refers to the fact that the imaging sensor is physically linked by a fiber optic cable to a computer that records the generated signal A pixel is a small box, or “well,” into which the electrons produced by the x-ray exposure are deposited. A pixel is the digital equivalent of a silver crystal used in conventional radiography. As opposed to a film emulsion that contains a random arrangement of silver crystals, a pixel is structured in an ordered arrangement. Component parts of a sensor. Dexis Platinum Sensor (1) front casing—forms watertight and light-tight barrier, (2) scintillator screen—converts x-ray beam into visible light, (3) fiber optic face plate—transmits light to sensor surface, (4) CMOS imaging chip— captures light from screen and creates a charge in each pixel, (5) sensor electronics— reads the charge in each pixel and transmits to computer, and (6) back casing /protective shield & cable—forms watertight and light-tight barrier and prevents backscatter.Digital sensors contain distinct units of programmable gray values called pixels that are sensitive to x-rays. The imaging software is responsible for converting the electronic signal from the sensor into a shade of gray that is viewed on the computer monitor. Each pixel is represented numerically in the computer by location and the color level of the gray. The range of numbers for a pixel varies from 0 (black) to 255 (white), which creates 256 shades of gray, referred to as a pixel’s gray-scale resolution. In comparison, the human eye can perceive only 32 shades of gray. The image may be stored permanently in the computer, printed out as a hard copy for the patient record, or transmitted electronically to insurance companies or referring dental specialists. Two methods of obtaining a digital image currently exist: (1) direct digital imaging and (2) indirect digital imaging. Direct digital imaging is a method of obtaining a digital image using an intraoral sensor that is exposed to x-radiation to transfer information directly to a computer with imaging software. Indirect digital imaging is a method of obtaining a digital image from a sensor following exposure to x-radiation by using a scanner to convert information into a digital form so that it can then be viewed on a computer monitor. A common type of indirect digital imaging is storage phosphor imaging. Storage phosphor imaging is also referred to as photo-stimulable phosphor imaging (PSP). In this system, a reusable imaging plate coated with phosphors known as a PSP plate is used. The PSP plate is flexible like film and similar in size, shape, and thickness. The intraoral PSP plate is reusable and is placed into the mouth in the same way as an intraoral film is positioned. Unlike direct digital imaging, there is an extra step—the scanning. Images remains on the reusable plate until it is erased after the scanning process. Manufacturers use technology that scans and retrieves the digital image, followed by a clearing step for reuse of the PSP plate. Once the image is erased, the plate may be disinfected and then inserted into a disposable barrier envelope for reuse. Extreme care must be taken when exposing, handling, and wrapping these receptors because bending or scratching can damage PSP plates. The dental radiographer must make certain to orient the PSP plate in the patient’s mouth so that the correct side faces the beam. To aid in placement, “opposite side toward tube” is printed on the PSP plate. Whether using indirect or direct digital imaging, it is important that the individual sensors be protected from oral fluids. A PSP plate is placed in a disposable barrier sleeve that is waterproof Rigid digital sensors, wired or wireless, must be covered with a disposable barrier sleeve, between each patient. Digital sensors cannot withstand heat sterilization. As with conventional intraoral film, the sensor is centered over the area of interest. Advantages of Digital Imaging Superior gray-scale resolution. A primary advantage of digital imaging is the superior gray- scale resolution that results. Digital imaging uses up to 256 shades of gray compared with the 16 to 25 shades of gray differentiated on conventional film. Reduced exposure to x-radiation. Another primary advantage of the digital imaging system is the reduction in patient exposure to x-radiation. Increased speed of image viewing. Dental professionals as well as patients are able to view the digital images instantaneously, which allows for immediate interpretation and evaluation. Lower equipment and film cost. (long-term) Increased efficiency Enhancement of diagnostic image. One feature that can be used to enhance a diagnostic image is digital subtraction. With digital subtraction, the gray-scale is reversed so that radiolucent images (normally black) appear white and radiopaque images (normally white) appear black Effective patient education tool. Eco-friendly alternative. Disadvantages of Digital Imaging Initial setup costs. Image quality. At one time, image quality was a source of debate. The spatial resolution of an image is defined as the number of line pairs per millimeter (lp/mm). Conventional dental x- ray film has a resolution of 12 to 20 lp/mm. A digital imaging system using a CCD sensor has a resolution closer to 10 lp/mm. Considering that the human eye can only perceive 8 to 10 lp/mm, a CCD system is significantly more effective in the recognition of dental disease. The majority of this research has shown that digital imaging performs at least as well as, and at times even better than, traditional radiography. Sensor size and thickness. Infection control. Digital sensors cannot withstand heat sterilization. Therefore, these sensors require complete coverage with disposable plastic barrier sleeves that must be changed between patients to prevent cross-contamination. Wear and tear. Legal issues. Because the original digital image can be enhanced, it is questionable whether digital images can be used as evidence in lawsuits.