The Basic Pencil chamber measurement concept was introduced by Bob Jucius and George Kambic of Ohio Nuclear/Technicare, and the computed tomography dose index (CTDI)-paradigm was formally developed by a group from the US FDA. The CTDI can be a confusing concept since it is not an actual dose, but an expectation. The actual measured quantity is the dose integral, and the divisor nT has nothing to do with the dose, but merely acts as a place-keeper. In the case of axial scans, a single rotation is made at each of a series of equally spaced locations along the z-axis, with no phantom motion during the beam-on time. The dose profile at the phantom entrant surface is narrower than that at isocenter due to beam divergence from the x-ray source.