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Automatic exposure control devices

Introduction

Automatic exposure control (AEC) systems are designed to adjust the kilovoltage, milliamperage, or exposure time in order to obtain an image of diagnostic quality, be it for radiography or fluoroscopy. These systems sense the amount of radiation immediately in front of the image receptor and adjust the dose or dose rate to the patient in order to assure sufficient photons are reaching the image receptor. However, these systems can also result in high patient doses, especially with digital image receptors, without the knowledge of the imaging staff.

AEC systems may also be known by other names including automatic dose control (ADC), automatic dose rate control (ADRC), and automatic brightness control (ABC).


Important Principles

AEC systems work on different principles, primarily based on the design goals of the manufacturer. Some systems may preferentially adjust the exposure time or tube current, while others may preferentially adjust the kilovoltage. Some systems insert extra filtration (typically a copper filter) into the beam to filter out additional soft radiation thereby reducing patient dose.

Most AEC systems have some practical limit beyond which they do not perform optimally. For example, fluoroscopic imaging systems are limited to 100 mGy/min as a maximum fluoroscopic exposure rate. If the system requires a dose rate greater than 100 mGy/min, the AEC cannot deliver the higher dose rate and the image quality degrades significantly. This occurs for larger patients or for oblique image angles.

It would be ideal to use the maximum tube current available for radiographic imaging at the shortest exposure time and with the smallest focal spot. However, these three factors must be traded off. For example, most small focal spots are limited to a tube current of approximately 300 mA. Consequently, a large focal spot may be required for large patients or oblique views.

AEC systems may not produce the same image quality, e.g., film density, over the typical range of thicknesses encountered in the clinical setting. Consequently, a part of a good quality control program will be to test image quality and patient dose over a typical range of patient thicknesses.


Introduction to References

The references include information on AEC systems applicable to radiographic and fluoroscopic imaging systems, and AEC system quality control.