Exposure Timer in X-ray Imaging Equipment - PDF

X-ray Imaging Equipment RAD 232 [Exposure timer] V2025 Exposure timer Exposure timer is a device used to control the length of X-ray exposure....

X-ray Imaging Equipment RAD 232 [Exposure timer] V2025 Exposure timer Exposure timer is a device used to control the length of X-ray exposure. The exposure timer is located in the primary circuit (of the X-ray circuit) ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- -------------------------------------------------- ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 1 Exposure timer There are several variations of the exposure timer: 1. Synchronous timer 2. The electronic timer “manually-set” techniques 3. The mAs timer 4. Automatic exposure control “automatically-set” techniques ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 2 Exposure timer 1. Synchronous timer It is based on a synchronous motor. The motor is designed to turn a shaft at precisely 60 revolutions per second (it depends on the frequency of the current). This shaft turns a disk, which is connected to on–off switches, through reduction gears. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 3 Exposure timer The exposure time selected determines the reduction gear used and therefore the time it takes the disk to move from the on switch to the off switch. Such units have a minimum exposure time of 1/60 second. The available exposure times are multiples of 1/60. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 4 Exposure timer 2. The electronic timer It is a very sophisticated and accurate timer that is the most widely used today. This timer is based on the time it takes to charge a capacitor through a variable resistor. Once the capacitor receives its preprogrammed charge, it terminates the exposure. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 5 Exposure timer Examples: To achieve a 1-second exposure, the resistance is increased so that it takes longer to charge the capacitor. To achieve a 1-millisecond exposure, there is virtually no resistance; the capacitor charges very quickly and the timer terminates the exposure. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 6 Exposure timer 3. The mAs timer It is a variation of the electronic timer, but it monitors the current passing through the x-ray tube and terminates the exposure when the desired mAs is reached. Because of the way it functions (i.e., The mAs timer), this timer is located in the secondary circuit instead of the primary circuit. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 7 Exposure timer 4. Automatic exposure control An automatic exposure control (AEC) system is a tool available on most modern radiographic units to assist the radiographer. AEC is a system used to consistently control the amount of radiation reaching the image receptor by terminating the length of exposure. AEC systems also are called automatic exposure devices, and sometimes they are erroneously referred to as phototiming. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 8 Exposure timer Technique charts make setting technical factors much more manageable, but there are always patient factors that require the radiographer’s assessment and judgment. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 9 Exposure timer The single function of an AEC is to eliminate the need for the radiographer to set an exposure time. The radiographer loses control over time, and as a result mAs, when using an AEC. All other factors are preprogrammed by the anatomically programmed radiography system. These systems can be overridden and the mA and kVp can be set manually when the radiographer determines adjustments to the technical factors may improve the image. This is especially true when kVp needs to be adjusted to increase or decrease the quantity of scatter radiation produced. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 10 AEC radiation detectors 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. Service personnel calibrate the predetermined level of radiation to meet the departmental standards of image quality. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 11 AEC radiation detectors Two types of AEC systems have been used: I. Phototimers II. Ionization chambers. 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 ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 12 AEC radiation detectors Arrangement of three automatic exposure control detectors on an upright Bucky unit ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 13 AEC radiation detectors I. Phototimers Phototimers use a fluorescent (light-producing) screen and a device that converts the light to electricity. A photomultiplier (PM) tube is an electronic device that converts visible light energy into electrical energy. A photodiode is a solid-state device that performs the same function. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 14 AEC radiation detectors Phototimer AEC devices were considered exit-type devices. ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- -------------------------------------------------- ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 15 AEC radiation detectors Light paddles, coated with a fluorescent material, served as the detectors, and the radiation interacted with the paddles, producing visible light. This light was then transmitted to remote PM tubes or photodiodes that convert this light into electricity. The electrical charge was in proportion to the radiation to which the light paddles have been exposed. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 16 AEC radiation detectors The timer is tripped, and the radiographic exposure was terminated when a sufficiently large charge had been received. Phototimers have largely been replaced with ionization chamber systems. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 17 AEC radiation detectors II. Ionization chamber systems An ionization or ion chamber is a hollow cell that contains air and is connected to the timer circuit via an electrical wire. Ionization-chamber AEC devices are considered entrance-type devices. ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- -------------------------------------------------- ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 18 AEC radiation detectors When the ionization chamber is exposed to radiation from a radiographic exposure, the air inside the chamber becomes ionized, creating an electrical charge. This charge travels along the wire to the timer circuit. The electrical charge is in proportion to the radiation to which the ionization chamber has been exposed. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 19 AEC radiation detectors The timer is tripped, and the radiographic exposure is terminated when a sufficiently large charge has been received. Compared with phototimers, ion chambers are less sophisticated and less accurate, but they are less prone to failure. Most of today’s AEC systems use ionization chambers. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 20 What happens when AEC is used? The total amount of radiation ( mAs) required to produce the appropriate exposure to the image receptor is determined by the system. AEC has no effect on other image characteristics such as contrast. Therefore, the kVp level that provides an appropriate subject contrast and is at least the minimum kVp to penetrate the part must be selected. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 21 What happens when AEC is used? The higher the kVp value used, the shorter the exposure time needed by the AEC device. Using higher kVp with AEC decreases the exposure time and overall mAs needed to produce a diagnostic image, significantly reducing the patient’s exposure. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 22 Alignment and positioning considerations when using AEC I. Detector selection AEC systems with multiple detectors typically allow the radiographer to select any combination of one, two, or three detectors. The selected detectors actively measure radiation during exposure, and the electrical signals are averaged. Typically, the detector that receives the greatest amount of exposure has a greater effect on the total exposure. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 23 Alignment and positioning considerations when using AEC The general guideline is to select the detectors that will be superimposed by the anatomic structures of greatest interest that need to be visualized on the radiographic image. Failure to use the proper detectors could result in either underexposure or overexposure to the image receptor. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 24 Alignment and positioning considerations when using AEC II. Patient centering Proper centering of the part being examined is crucial when using an AEC system. The anatomic area of interest must be centered properly over the detectors that the radiographer has selected. Improper centering of the part over the selected detectors may either underexpose or overexpose the image receptor. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 25 Alignment and positioning considerations when using AEC III. Detector size The size of the detectors manufactured within an AEC system is fixed and cannot be adjusted. It is important for the radiographer to determine whether AEC should be used during the radiographic procedure. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 26 AEC system: Compensating issues I. Patient considerations The AEC system is designed to compensate for changes in patient thickness. If the area of interest is thicker because of the patient’s size, the exposure time will lengthen to reach the preset exposure to the detectors. AEC systems that do not adequately compensate for changes in patient thickness may need to be adjusted. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 27 AEC system: Compensating issues Examples: Abdominal examinations using AEC can be compromised if a patient has an excessive amount of bowel gas. If a detector is superimposed by an area of the abdomen with excessive gas, the timer will terminate the exposure prematurely, resulting in an underexposed image. Likewise, destructive pathologic conditions can cause underexposure of the area of radiographic interest. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 28 AEC system: Compensating issues The presence of positive contrast media, an additive pathologic condition, or a prosthetic device that superimposes the detector can overexposure the area of interest. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 29 AEC system: Compensating issues II. Collimation Failure to accurately restrict the beam may underexposure the area of interest. ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- -------------------------------------------------- If the x-ray field size is collimated too closely, the detector does not receive sufficient exposure initially and may prolong the exposure time, which could result in overexposure the area of interest. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 30 AEC system: Timing considerations The minimum response time is the length of time necessary for the AEC to respond to the ionization and send a signal to terminate the exposure. Modern AECs have a minimum response time in the region of 0.001 second. The use of extremely high-speed systems for smaller part sizes can cause problems when AECs need less than 0.001 second to produce a diagnostic-quality exposure. In these instances, mA should be decreased to permit longer AEC time. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 31 AEC system: Timing considerations The backup time establishes the maximum exposure time for the system in order to prevent overexposure. It should be set at 150 percent of the anticipated manual exposure time. U.S. public law requires that generators automatically terminate AEC exposures at 600 mAs above 50 kVp, and 2,000 mAs below 50 kVp. When the backup time is too short, it will terminate the exposure before the AEC signal, thus producing an underexposed image. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 32 Computerized AEC Anatomically programmed This choice results in the radiography (APR) units computer entering the combine an AEC system suggested average with an exposure system technique (e.g., 120 kVp, that is computerized to 600 mA, with the right correspond to anatomical and left ionization procedures. chambers activated). The control console The radiographer may permits the choice of an override the suggested anatomical region (e.g., technique when patient the chest) and the condition, pathology, or projection (e.g., PA). other factors make it desirable to do so. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 33 Quality control When a radiographic unit with AEC is first installed, the AEC device is calibrated, and it is recalibrated at intervals thereafter. Failure to maintain regular calibration of the AEC results in: The lack of consistent exposures. Could affect image quality. May leads to overexposure of the patient. Consequently, this could lead to Poor efficiency of the imaging department, Possibility of improper interpretation of radiographic images. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 34 Quality control The AEC device should provide consistent exposures to the image receptor for variations in technique factors, patient thicknesses, and detector selection. Several aspects of the AEC performance can be monitored by imaging a homogeneous patient equivalent phantom plus additional thickness plates. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 35