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AEC chamber selection
AEC measures only the tissue over the chambers you select, so the anatomy of interest must cover them.
The rule
Automatic exposure control ends the exposure once a set amount of radiation has reached the selected detectors, usually ionization chambers placed between the patient and the image receptor. Most systems have three: a center chamber and two outer chambers. The radiographer chooses which ones are active. The AEC only knows about the tissue lying over the active chambers, so the anatomy of interest must completely cover them.