Computed tomography dose index
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The computed tomography dose index (CTDI) is a commonly used radiation exposure index in X-ray computed tomography and is reported by the CT manufacturers to scan personnel for each exam. The CTDI can be used in conjunction with patient size to determine the absorbed dose. The CTDI and absorbed dose may differ by more than a factor of two for small patients such as children.
It is defined by the US Department of Health and Human Services (HHS) as the average dose imparted by a single axial acquisition to a standard 100-mm pencil chamber dosimeter inside a PMMA phantom over the width of 14 CT slices:
where is the number of slices acquired, is the slice thickness and is the radiation dose measured at position along the scanner's main axis.
This equation is often used in a modified form
The dose absorbed in air can be conveniently assessed with a 100-mm standard pencil dose chamber. Typically, the dose distribution within the body cross section imparted by a CT scan is much more homogeneous than that imparted by radiography, but is still somewhat larger near the skin than in the body center. Therefore, a third measure, the weighted CTDI was introduced:
Similar measures with yet wider chambers are useful for CT systems with large numbers of detector rows.
CTDI can also be measured with polymer gel dosimeters. 
- McCollough, C. H.; Leng, S.; Yu, L.; Cody, D. D.; Boone, J. M.; McNitt-Gray, M. F. (18 April 2011). "CT Dose Index and Patient Dose: They Are Not the Same Thing". Radiology 259 (2): 311–316. doi:10.1148/radiol.11101800. Retrieved 22 June 2012.
- Geleijns 2009
- Hill 2005
- Rothenberg, LN and Pentlow, KS (2000). "CT dosimetry and radiation safety", Categorical Course in Diagnostic Radiology Physics: CT and US Cross-sectional Imaging, LW Goldman and JB Fowlkes, eds., pp171-188, RSNA, Oak Brook, IL
- J Geleijns, M Salvadó Artells, P W de Bruin, R Mather, Y Muramatsu and M F McNitt-Gray (2009). "Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner", Phys. Med. Biol. 54, pp3141-3159
- Hill B, Venning AJ, Baldock C, 2005. A preliminary study of the novel application of normoxic polymer gel dosimeters for the measurement of CTDI on diagnostic x-ray CT scanners. Med.Phys. 32 1589-1597.