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 estimate 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 measurement is most often made using a 100-mm standard pencil dose chamber as this is representative of a typical scan length:
The absorbed dose to water (used to refer back to patient dose) is typically measured in a cylindrical head (16 cm diameter) or body (32 cm diameter) phantom of length approximately 14–15 cm.
The dose distribution 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 centre of the body. The weighted CTDI was introduced to account for this:
using measurements acquired at central and peripheral positions in the head or body phantoms described above.
CTDI in helical CT
In helical CT, the pitch of the machine - a factor of the speed at which the couch travels through the gantry and the tube rotation frequency - also impacts on patient dose. The pitch factor, P, is defined as
where is the distance travelled by the couch during one full gantry rotation and is the beam collimation (single-slice CT) or the total thickness of all simultaneously acquired slices (multislice CT). The following quantity is therefore used to take account of pitch:
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. 
Relation to DLP
The dose-length product (DLP) is a quantity defined for use in CT as
for and as described above ( is therefore the total scan length). This quantity is analogous to the dose-area product (DAP) used in planar radiography.
- 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.