Introduction

The radiographer is tasked with selecting exposure factor techniques to produce quality radiographs for a wide variety of equipment and patients. 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. When combined with patients of various sizes and with various pathologic conditions, the selection of proper exposure factors becomes a formidable task. 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. Technique charts make setting technical factors much more manageable, but there are always patient factors that require the radiographer’s assessment and judgment. When using AEC systems, the radiographer must still use individual discretion to select an appropriate kVp, mA, image receptor, and grid. However, the AEC device determines the exposure time (and therefore total exposure).

AEC systems are excellent at producing consistent levels of exposure when used properly, but the radiographer should also be aware of the technical limitations of using an AEC system.

Radiation Detectors

All AEC devices work by the same principle: 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 is calibrated by service personnel to meet the departmental standards of image quality.

The difference in AEC systems lies in the type of device 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, and these systems are not common today. Therefore the use of the term phototiming is usually in error. 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 13-1). 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. 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.

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. Phototimer AEC devices are considered exit-type devices because the detectors are positioned behind the image receptor (Figure 13-2) so that radiation must exit the image receptor before it is measured by the detectors. Light paddles, coated with a fluorescent material, serve as the detectors, and the radiation interacts with the paddles, producing visible light. This light is transmitted to remote PM tubes or photodiodes that convert this light into electricity. The timer is tripped and the radiographic exposure is terminated when a sufficiently large charge has been received. This electrical charge is in proportion to the radiation to which the light paddles have been exposed. Phototimers have largely been replaced with ionization chamber systems.

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 because the detectors are positioned in front of the image receptor (Figure 13-3) so that radiation interacts with the detectors just before interacting with the image receptor. 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 timer is tripped and the radiographic exposure is terminated when a sufficiently large charge has been received. This electrical charge is in proportion to the radiation to which the ionization chamber has been exposed. 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.

Critical Concept 13-3

Function of the Ionization Chamber

The ionization chamber interacts with exit radiation before it reaches the image receptor. Air in the chamber is ionized, and an electric charge that is proportional to the amount of radiation is created.

Milliamperage/Second Readout

When a radiographic study is performed using an AEC device, the total amount of radiation (milliamperage/second [mAs]) required to produce the appropriate exposure to the image receptor is determined by the system. Many radiographic units include a mAs readout display, on which the actual amount of mAs used for that image is displayed immediately after the exposure, sometimes for only a few seconds. It is critical for the radiographer to take note of this information when it is available. Knowledge of the mAs readout has a number of advantages. It allows the radiographer to become more familiar with manual exposure technique factors. If the image is suboptimal, knowing the mAs readout provides a basis from which the radiographer can make exposure adjustments by switching to manual technique. There may be studies with different positions in which AEC and manual techniques are combined because of difficulty with accurate centering. For example, knowing the mAs readout for the anteroposterior lumbar spine gives the radiographer an option to switch to manual technique for the oblique exposures, making technique adjustments based on reliable mAs information.

Kilovoltage Peak And Milliamperage/second Selection

AEC controls only the quantity of radiation reaching the image receptor and therefore has no effect on other image characteristics such as contrast. The kVp for a particular examination should be selected without regard to whether an AEC device is used. The radiographer must select the kVp level that provides an appropriate scale 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 best visualize 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 high kVp. Using higher kVp with AEC decreases the exposure time and overall mAs needed to produce a diagnostic image, significantly reducing the patient’s exposure.

The kVp selected for an examination should produce the desired radiographic contrast for the part examined, and be as high as possible to minimize the patient’s radiation exposure.

When the radiographer uses a control panel that allows the mA and time to be set independently, he or she should select the mA without regard to whether an AEC device is used. The mA selected has a direct effect on 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. For a given procedure, increasing the mA on the control panel shortens the exposure time and decreasing the mA increases the exposure time.

Minimum Response Time

The term minimum response time

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