Automatic exposure control

Operation

Projectional radiography

In projectional radiography, an AEC system uses one or more physically thin radiation ionization chambers (the "AEC detector") positioned between the X-ray source and X-ray receptor. Where low energy X-rays are used, such as in mammography, the AEC detector is placed behind the image receptor to avoid creating a shadow.

In early radiographic AEC systems, a large paddle (17" x 17") of transparent Lucite was sandwiched between rare earth screens, which emitted photons when excited by X-rays. The individual Lucite sections were open on one end, and a solenoid was used to select one of three, or a combination of shutters that allowed the generated light into a photomultiplier tube (PMT). The output of the PMT was then converted to a signal, which ramped in the positive direction until a preprogrammed threshold was reached. At this point, the X-ray generator terminated the exposure. This method is no longer used, and has been replaced by the iontomat. In an iontomat, a weak ionization signal resulting from the radiographic X-rays passing through it are 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 by the X-ray generator. AEC devices are calibrated to ensure that similar exams have linearity in optical density.

Computed tomography

Modern computed tomography (CT) scanners have AEC systems which aim to maintain image quality for patients of varying sizes, whilst keeping doses as low as reasonably practicable. The systems are also designed to maintain quality with the varying size and attenuation of an individual patient over their length. Implementations vary between manufacturers; some systems are based on a desired noise level in the image, while others are based on a specified reference output (milliampere second, mAs).

CT AEC systems use the initial "scanogram", a fixed angle planning view, to determine the relative size of the patient, and variation over their length. The tube output is then adjusted for overall size. The output is also typically modulated for each rotation in response to changes in attenuation over patient length. Some systems adjust output during each rotation, which is known as rotational modulation, based on measured attenuation in the previous rotation.

Advantages

Because patients vary in size and shape, an AEC device is very useful in achieving consistent X-ray film densities, which can be difficult when manually setting exposure factors without AEC.

Disadvantages

AEC devices are susceptible to operator error (usually due to mispositioned anatomy or having the incorrect AEC chamber selected). 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.

References

References

1. (1988). "Automatic exposure control: a primer.". Radiologic Technology.
2. "Automatic exposure control devices".
3. (2014). "Diagnostic radiology physics : a handbook for teachers and students.". International Atomic Energy Agency.
4. link
5. (2009). "The physics of medical imaging". Taylor & Francis.
6. (7 November 2006). "Calibrating automatic exposure control devices for digital radiography". Physics in Medicine and Biology.
7. (July 2010). "Automatic exposure control in computed tomography – an evaluation of systems from different manufacturers". Acta Radiologica.
8. (2012). "Radiation Dose from Multidetector CT". Springer.
9. (2005). "CT scanner automatic exposure control systems". Department of Health.
10. (November 2004). "Exposure variations under error conditions in automatic exposure controlled film–screen projection radiography". The British Journal of Radiology.
11. (2014). "Radiography in the digital age: physics, exposure, radiation biology". Charles C Thomas.