computed tomography dose index. Integral along a line parallel to the axis of rotation (z) of the dose profile (D(z)), measured free-in-air or in a CT dosimetry phantom for a single slice, divided by the nominal slice thickness(mGy) In practice, it is convenient to use a pencil ionisation chamber with an active length of 100 mm so as to provide a measurement of CTDI100 (mGy to air).