The computed tomography dose index (CTDI)-Paradigm and associated equations are subject to strict physical constraints which are not widely appreciated — the most important being the requirement for shift-invariance along the direction of table translation. The best way to unequivocally demonstrate the nature of the CTDI-paradigm, and to illustrate how it is affected by loss of shift-invariance, is to first derive the associated equations based on a shift-invariant fixed tube current i0 technique; and then repeat the derivation using a shift-variant i(z) tube current modulation technique. The chapter aim to provide a physical understanding of the dose distribution for tube current modulation.