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A photodiode is a solid-state device that performs the same function. A photomultiplier (PM) 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 of the three individually or in combination actually measures radiation exposure reaching the image receptor. 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 13-1). The more common type of AEC system uses ionization chambers.
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Therefore the use of the term phototiming is usually in error. Phototiming specifically refers to the use of an AEC device that uses photomultiplier tubes or photodiodes, and these systems are not common 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 used to convert radiation into electricity.
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The predetermined level of radiation is calibrated by service personnel to meet the departmental standards of image quality. This occurs when a predetermined amount of radiation has been detected, as indicated by the level of electrical signal that has been produced. However, the AEC device determines the exposure time (and therefore total exposure).Īll AEC devices work by the same principle: Radiation is transmitted through the patient and converted into an electrical signal, terminating the exposure time. When using AEC systems, the radiographer must still use individual discretion to select an appropriate kVp, mA, image receptor, and grid. Technique charts make setting technical factors much more manageable, but there are always patient factors that require the radiographer’s assessment and judgment. AEC systems also are called automatic exposure devices, and sometimes they are erroneously referred to as phototiming. An automatic exposure control (AEC) system is a tool available on most modern radiographic units to assist the radiographer.ĪEC is a system used to consistently control the amount of radiation reaching the image receptor by terminating the length of exposure. When combined with patients of various sizes and with various pathologic conditions, the selection of proper exposure factors becomes a formidable task.
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There are many thousands of possible combinations of kilovoltage peak (kVp), milliamperage (mA), source-to-image receptor distance (SID), exposure time, image receptors, and grid ratios. The radiographer is tasked with selecting exposure factor techniques to produce quality radiographs for a wide variety of equipment and patients.