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Focal Spot Blooming
High mA spreads the electron stream and enlarges the effective focal spot, adding blur to fine detail.
The rule
Focal spot blooming is an increase in the actual size of the focal spot as tube current, mA, increases. The nominal focal spot size listed on the control panel, such as 0.6 mm or 1.2 mm, is measured under standard conditions. At high mA the real spot can be larger than that, and a larger effective focal spot means more blur at the edges of structures.