Automatic Exposure Control | HTM Wiki - Fandom

Contents

• 1 About
• 2 Operation
• 3 Advantages
• 4 Disadvantages
• 5 Reference
• 6 Links

About[]

Automatic Exposure Control (AEC) is an X-ray exposure termination device. An medical radiographic x-ray exposure is always initiated by an x-ray technician. Next, the x-ray termination can be performed either by the x-ray technician or an exposure control device such as an phototimer or ion chamber detectors, which ever comes first. The purpose of AEC is to deliver consistent, reproducible exposures across a wide range of anatomical thicknesses, kVp, and users. All automatic exposure control devices work on the same physical principle,based upon the ability of radiation detection devices to convert radiant energy into an electrical current.

Operation[]

An AEC system uses a physically thin radiation ionization detector "AEC detector". AEC detectors used in AEC systems include fluorescent screens with Photo Multiplier Tubes (PMT)/photodiodes, ionization chambers, and possibly solid state detectors. These AEC detectors are positioned on x-ray film cassette. They can be adjusted in three ways depending on OEM design: 1). software adjustment or 2). pots located on the detector or x-ray cassette.3). x-ray generator.  X-rays passing through the patient pass through this "AEC detector" before they strike the x-ray film. AECs only control the exposure time and the total mAs. kVp and mA must still be manually set by the x-ray technician when using the AEC function.

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 ensure that similar exams have linearity in exam densities. This is due to the fact that a milliampere station is no longer selected and instead relies upon the ionization within the selected chambers.

Advantages[]

Because patients vary in size and shape, an AEC device is very useful in achieving three factors:

1)more consistent film densities

2)reducing repeat rates

3)ultimately reducing repeat dose.

compared to non AEC x-ray exposures.[1]

In the case of a system malfunction or technical error, the exposure must be terminated after a certain period of time or delivered mAs:

• 600 mAs ≥ 51 kVp

„* 2,000 mAs < 51 kVp „* IAW 21CFR 1020

Disadvantages[]

AEC devices are susceptible to operator error usually due to mispositioned anatomy or having selected the incorrect AEC chamber. Prosthetic devices such as total hip hardware can also cause the selected ionization chamber to overexpose the image receptor. This is due to the absorption of the x-ray beam into the metal of the hardware as opposed to exposing the ionization chamber.

Reference[]

1. ↑ A. Kyle Jones, Ph.D. "Using Automatic Exposure Control in Digital Radiography." AAPM. 2008. http://www.aapm.org/meetings/amos2/pdf/35-9964-61632-988.pdf

Links[]