Figure 1:

Equipment typically used to measure CTDI100 includes an integrating electrometer (black arrow), a 100-mm-long CTDI ionization chamber (white arrow), and a CTDI phantom made of polymethylmethacrylate (arrowhead). The phantom is placed with its long axis perpendicular to the plane of the transverse CT scan and the ion chamber placed in one of the holes through the phantom. CTDI100 is obtained by integrating the dose over 100 mm from a single transverse scan and dividing it by the nominal beam width. (Reprinted, with permission, from reference 7.)