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Technique Chart Construction
Fixed-kVp charts hold kVp and change mAs with thickness; variable-kVp charts add about 2 kVp per cm.
The rule
A technique chart lists exposure factors for each exam based on the measured thickness of the part. A fixed-kVp chart sets one optimal kVp for each body part and changes mAs as thickness changes. A variable-kVp chart keeps mAs constant for a part and adds about 2 kVp for every centimeter of added thickness.