What are the advantages and disadvantages of using aec and anatomically programmed techniques?

All 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. 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.

The difference in AEC systems lies in the type of device that is used to convert radiation into electricity. Two types of AEC systems have been used: phototimers and ionization chambers. 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. Phototiming specifically refers to the use of an AEC device that uses photomultiplier tubes or photodiodes, even though these systems are uncommon today. Therefore, the use of the term phototiming is usually incorrect. The more common type of AEC system uses 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 (Figure 8-1). The radiographer selects the configuration of these devices, determining which one (or more) of the three actually measures radiation exposure reaching the IR. These devices are variously referred to as sensors, chambers, cells, or detectors. These radiation-measuring devices are referred to here for the remainder of the discussion as detectors

AEC controls only the quantity of radiation reaching the IR and has no effect on other image characteristics, such as contrast. The kVp for a particular examination should be selected as it would be for that examination, regardless of whether an AEC device is used. The radiographer must select the kVp level that provides an appropriate level of contrast and is at least the minimum kVp to penetrate the part. Although in digital imaging contrast can be computer manipulated, the kVp should still be selected to visualize best the area of interest. In addition, the higher the kVp value used, the shorter the exposure time needed by the AEC device. Because high kVp radiation is more penetrating (reducing the total amount of x-ray exposure to the patient because more x-ray photons exit the patient) and the detectors are measuring quantity of radiation, the preset amount of radiation exposure is reached sooner with a high kVp.

When the radiographer uses a control panel that allows the mA and time to be set independently, he or she should select the mA value as it would be for that particular examination, regardless of whether an AEC device is used. The mA value selected will affect the exposure time needed by the AEC device. Therefore, if the radiographer wants to decrease exposure time for a particular examination, he or she may easily do so by increasing the mA value. For a given procedure, increasing the mA on the control panel decreases the exposure time, and decreasing the mA selected on the control panel increases the exposure time.

Backup time refers to the maximum length of time the x-ray exposure will continue when using an AEC system. The backup time may be set by the radiographer or controlled automatically by the radiographic unit. It may be set as backup exposure time or as backup mAs (the product of mA and exposure time). The backup time acts as a safety mechanism when an AEC system fails or the equipment is not used properly. In either case, the backup time protects the patient from receiving unnecessary exposure and protects the x-ray tube from reaching or exceeding its heat-loading capacity. If the backup time is controlled automatically, it should terminate at a maximum of 600 mAs.

The backup time might be reached as the result of operator oversight when an AEC examination, such as a chest x-ray, is done at the upright Bucky and the radiographer has set the control panel for table Bucky. The table detectors are forced to wait an excessively long time to measure enough radiation to terminate the exposure. The backup time is reached and the exposure is terminated, limiting the patient’s exposure and keeping the tube from overloading. However, newer x-ray units with AEC include a sensor in the Bucky tray for the IR and do not allow an exposure to activate if the table Bucky detectors were selected but the x-ray tube centered to the upright Bucky.

When controlled by the radiographer, the backup time should be set high enough to be greater than the exposure needed but low enough to protect the patient from excessive exposure in case of a problem. Setting the backup time at 150% of the expected exposure time is appropriate. If the backup timer periodically or routinely terminates the exposure, higher mA values should be used to shorten the exposure time.

Selection of the detectors to be used for a particular examination is critical when using an AEC system. AEC systems with multiple detectors typically allow the radiographer to select any combination of one, two, or all 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 impact on the total exposure.

Measuring radiation that passes through the anatomic area of interest is important. The general guideline is to select the detectors that would be superimposed by the anatomic structures that are of greatest interest and need to be visualized on the radiograph. Failure to use the proper detectors could result in either underexposure or overexposure to the IR. In the case of a posteroanterior (PA) chest radiograph, the area of radiographic interest includes the lungs and heart; therefore, one or two outside detectors should be selected to place the detectors directly beneath the critical anatomic area. If the center detector were mistakenly selected, the anatomy superimposing this detector includes the thoracic spine. If the exposure is made, the resultant image shows sufficient exposure in the spine, with the lungs overexposed (Figure 8-4). In the manual that accompanies a radiographic unit, the AEC device manufacturer provides recommendations for which detectors to use for specific examinations. Recommendations for detector combination also can be found in many radiographic procedures textbooks.

Many radiographic units have AEC devices in both the table Bucky and an upright Bucky. If more than one Bucky per radiographic unit uses AEC, the radiographer must be certain to select the correct Bucky before making an exposure. Failure to do so may result in the patient and IR being exposed to excessive radiation. The backup time is reached, the exposure is terminated prematurely, and a repeat radiographic study may need to be done, increasing the patient’s dose.

A similar problem can occur in some systems when not using a Bucky, such as with cross-table, tabletop, or stretcher or wheelchair studies. If the AEC system is activated with these types of examinations, an unusually long exposure results because the detectors are not being exposed to radiation. Again, the backup time will likely be reached, and the patient’s dose will be excessive. Some radiographic units are designed so that an exposure does not occur if the AEC device has been selected and there is no IR detected in the Bucky.