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Fig. 3 —Functions of automatic exposure control. A, Exposure
adjustment to overall size of patient's body. B, Modulation of
z-axis: tube current occurs between rotations as patient couch moves.
C, In angular modulation, tube current is modulated during one tube
rotation to account for attenuation inconsistencies at different angles of
gantry rotation. 
= location of X-ray tube in gantry measured as angle
from arbitrary place.
