This chapter presents a rigorous theoretical description of the physics from which the dependence on the various scan parameters can be deduced. This forms a basis for a fundamental physical understanding by medical physicists, and which may be useful in aiding manufacturers in the design of shift-variant protocols with regard to minimizing patient dose. The chapter discusses flaws in scanner-reported values of computed tomography dose index (CTDIvol) and recommended updates to increase their validity. Shift-invariance is a necessary condition for application of the CTDI-paradigm and formula for which no parameters change along z during the scan. Rigorous convolution equations were derived for shift-variant tube current modulation (TCM), pitch modulation (PM), and combined TCM/PM techniques as well as for the shift-invariant technique – the latter providing the basis for the CTDI-paradigm.