Vin screen shade5/11/2023 In DR systems the X-ray photons are converted directly into an electrical signal. After scanning, the detector is erased by a bright light and the cassette is reloaded. The electrical signal is then used to create the image. This light is detected by a photomultiplier tube (detector) and converted into an electrical signal. The energy is released as light, with the intensity of light emitted proportional to the number of photons reaching the detector. Electrons within the plate are stimulated to release their energy by a laser scanner. ![]() ![]() To read the latent image the imaging plate is loaded into a reader (this is normally automatic, but in some systems this has to be done manually). When the X-ray photon hits the imaging plate there is transfer of energy to electrons within the phosphor. These differ in how the X-ray pattern is converted to an electrical digital signal:ĬR systems use a cassette containing a phosphor screen that can store the latent image for a period of time. There are two main types of digital radiography. This means it is possible to view simultaneously areas on the radiograph with marked differences in tissue thickness, or to look at the soft tissues and bones without having to take multiple exposures. This means that less information is lost (e.g., once film is black extra photons do not result in new information whereas in a digital system the extra photons carry useful information). With digital radiographic systems there is a direct linear relationship between the exposure and signal. By comparison an analogue system can measure an almost infinite (continuous) range of values. In digital radiography the number of photons reaching the detector is converted into a range of discrete values. The term digital comes from the same source as digit (finger) and a useful analogy is that digital information is similar to counting on fingers. If mistakes are made in the choice of exposure or if there are marked differences in thickness of tissue being radiographed then information may be lost.ĭigital radiographic systems were developed to overcome some of these problems. There is a limited range of useful exposures that make a diagnostic radiograph. ![]() Information stored on the film is fixed and cannot be manipulated following the exposure. The main disadvantages of conventional film based systems are: Once the film is processed the brightness and contrast are fixed. Although there are potentially hundreds of shades of grey on the image, the human eye can only differentiate about 60 shades. When taking conventional radiographs the exposure is set to use the straight part of the characteristic curve. Within the useful (straight) part of the characteristic curve there is a continuous (but logarithmic) linear relationship between the number of photons and the density of the film. If there are too few (underexposure) or too many photons (overexposure) the information given by the pattern of photons is lost. The relationship between the radiographic exposure (number of photons) and optical density of the film is known as the characteristic curve and is sigmoidal (S shaped). The shade of grey on any part of the film is determined by the number of photons reaching that part. Conventional film-screen-based radiography is an analogue system which means there is a continuous range of shades of grey from white to black on the radiograph. With conventional film-screen systems, the radiographic image is created by the pattern of photons or light reaching the film. ![]() Whilst digital systems have some advantages, the images they produce are often no better than good-quality conventional radiographs. Digital systems convert the pattern of photons reaching the detector into an electrical signal. The difference between the systems is in the detection of the X-ray photons and the production of the image. The production of the X-ray beam and interaction of the X-ray photons with the tissues is the same with digital radiography as conventional radiography. Books & VINcyclopedia of Diseases (Formerly Associate).VINcyclopedia of Diseases (Formerly Associate).
0 Comments
Leave a Reply. |