This light is transmitted to remote photomultiplier tubes or photodiodes that convert this light into electricity. Light paddles, coated with a fluorescent material, serve as the detectors, and the radiation interacts with the paddles, producing visible light. Phototimer AEC devices are considered exit-type devices because the detectors are positioned behind the IR (Figure 8-2) so that radiation must exit the IR before it is measured by the detectors. A photodiode is a solid-state device that performs the same function. A photomultiplier tube is an electronic device that converts visible light energy into electrical energy. Phototimers use a fluorescent (light-producing) screen and a device that converts the light to electricity. These radiation-measuring devices are referred to here for the remainder of the discussion as detectors These devices are variously referred to as sensors, chambers, cells, or detectors. The radiographer selects the configuration of these devices, determining which one (or more) of the three actually measures radiation exposure reaching the IR. Regardless of the specific type of AEC system used, almost all systems use a set of three radiation-measuring detectors, arranged in some specific manner (Figure 8-1). The more common type of AEC system uses ionization chambers. Therefore, the use of the term phototiming is usually incorrect. Phototiming specifically refers to the use of an AEC device that uses photomultiplier tubes or photodiodes, even though these systems are uncommon today. Phototimers represent the first generation of AEC systems used in radiography, and it is from this type of system that the term phototiming has evolved. Two types of AEC systems have been used: phototimers and ionization chambers. The difference in AEC systems lies in the type of device that is used to convert radiation into electricity. The predetermined level of radiation that must be reached before exposure termination is calibrated by service personnel to meet the departmental standards of image quality. Knowledge about the performance of these tools and their operation assists the radiographer in producing quality radiographic images.Īll AEC devices work by the same principle of operation: Radiation is transmitted through the patient and converted into an electrical signal, terminating the exposure time this occurs when a predetermined amount of radiation has been detected, as indicated by the level of electrical signal that has been produced. Tools are available to assist the radiographer in selecting appropriate exposure techniques: automatic exposure control (AEC) devices, anatomically programmed radiography, and exposure technique charts. When combined with patients of various sizes and with various pathologic conditions, the selection of proper exposure factors becomes a formidable task. There are many thousands of possible combinations of kVp, mA, SID, exposure time, image receptors (IRs), and grid ratios. The radiographer is responsible for selecting exposure factor techniques to produce quality radiographs for a wide variety of equipment and patients. Differentiate between the types of exposure technique charts.Īnatomically programmed radiography (APR) Define anatomically programmed radiography (APR).ġ3. State the importance of calibration of the AEC system to the type of image receptor used.ġ2. Describe patient protection issues associated with AEC.ġ1. Recognize the effect of the type of image receptor on AEC calibration, its use, and image quality.ġ0. Analyze unacceptable images produced using AEC, and identify possible causes.ĩ. Discuss patient and exposure technique factors and their effect on the response of the AEC device.Ĩ. Explain how alignment and positioning affect the response of the AEC device.ħ. Recognize how the detector size and configuration affect the response of the AEC device.Ħ. Differentiate among the types of radiation detectors used in AEC systems.ĥ. State the purpose of automatic exposure control (AEC) in radiography.Ĥ. State all the important relationships in this chapter.ģ. Define all the key terms in this chapter.Ģ. After completing this chapter, the reader will be able to perform the following:ġ.
0 kommentar(er)
