Automatic exposure control
Encyclopedia
Automatic Exposure Control (AEC) is an X-ray
exposure termination device. A medical radiography x-ray
exposure is always initiated by a human operator. X-ray termination is done by the same human operator or an exposure control device such as an electronic timer
or an AEC device, which ever comes first. The intention of the AEC termination device is to provide consistent x-ray film appearance.
detector "AEC detector" which is positioned between the patient being x-rayed and the x-ray film cassette. X-rays passing through the patient also pass through this "AEC detector" before they strike the x-ray film.
A weak ionization signal from the AEC detector is integrated as a ramp shaped voltage
waveform
. This ramp signal rises until it matches a pre-set threshold. At this point the x-ray exposure is terminated.
AEC devices are calibrated to match good x-ray film appearance.
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
exposure termination device. A medical radiography x-ray
Projectional radiography
Projectional radiography or plain film radiography is the practice of producing two-dimensional images using x-ray radiation. Radiographic exams are typically performed by Radiologic Technologists, highly trained medical professionals who specialize in the usage of radiographic equipment, patient...
exposure is always initiated by a human operator. X-ray termination is done by the same human operator or an exposure control device such as an electronic timer
Timer
A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...
or an AEC device, which ever comes first. The intention of the AEC termination device is to provide consistent x-ray film appearance.
Operation
An AEC system uses a physically thin radiation ionizationIonization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
detector "AEC detector" which is positioned between the patient being x-rayed and the x-ray film cassette. X-rays passing through the patient also pass through this "AEC detector" before they strike the x-ray film.
A weak ionization signal from the AEC detector is integrated as a ramp shaped voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
. This ramp signal rises until it matches a pre-set threshold. At this point the x-ray exposure is terminated.
AEC devices are calibrated to match good x-ray film appearance.