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CTDI_100 CTDI_{100} = \frac{1}{C}\int\limits_{-50 mm}^{+50 mm}dzD(z)
CTDI_100w CTDI_{100w} = \frac{1}{3}CTDI_{100}^{central} + \frac{2}{3}CTDI_{100}^{peripheral}
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where C is the radiographic exposure (mAs) and D(z) is the radiation dose measured at position z along the scanner's main axis.
Since doses at center of body are approximately two times of those at shallow depths in typical CT scans, improved indices are proposed and used. One of them is the weighted CTDI, CTDI100w<math>CTDI_{100w}</math>, defined as:
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where <math>CTDI_{100}^{central}</math> and <mtahmath>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.
''[[ICRP Glossary]] entry - May 2019''
