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, first defined in 1981. 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.
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.
Relation to DLP
The dose-length product (DLP) is a quantity defined for use in CT as
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- "AAPM REPORT NO. 96 The Measurement, Reporting, and Management of Radiation Dose in CT" (PDF). AAPM. Retrieved 12 December 2016.
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- Hill, Brendan; Venning, Anthony J.; Baldock, Clive (2005). "A preliminary study of the novel application of normoxic polymer gel dosimeters for the measurement of CTDI on diagnostic x-ray CT scanners". Medical Physics. 32 (6): 1589. doi:10.1118/1.1925181.