Changes

where <math>CTDI_{100}^{central}</math> and <math>CTDI_{100}^{peripheral}</math> are the CTDIs at the center and peripheral, respectively. The standard CT phantoms are PMMA cylinders having defined diameters (32 cm for torso and 16 cm for head-and-neck) and appropriate length enough to cover the 100 mm (usually 15 cm is employed). There are holes to place the detectors: one at the center and a few on a peripheral circle. Traditionally pencil ion chambers of 100mm long were used as the detectors but detectors of other types may be used as calibrated appropriately.