Computed tomography dose index

The computed tomography dose index (CTDI) is a commonly used radiation exposure index in X-ray computed tomography (CT), first defined in 1981.[1][2] The unit of CTDI is the gray (Gy) and it 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.[3]

Definitions

Because CT scanners typically acquire multiple slices during a single rotation with a single beam, the CTDI is calculated by integrating over the dose profile for a single axial rotation, then dividing by the nominal beam width:[4]

${\displaystyle CTDI={\frac {1}{nT}}\int _{-z}^{+z}{D(z){\text{d}}z}}$

where ${\displaystyle n}$ is the number of slices acquired per single axial rotation, ${\displaystyle T}$ is the width of a single acquired slice (and thus ${\displaystyle n}$${\displaystyle T}$ is the nominal beam width) and ${\displaystyle D(z)}$ is the radiation dose measured at position ${\displaystyle z}$ along the scanner's main axis - the dose profile.

This measurement is most often made using a 100-mm standard pencil dose chamber as this is representative of a typical scan length:

${\displaystyle CTDI_{100}={\frac {1}{nT}}\int _{-50mm}^{50mm}{D(z){\text{d}}z}}$.

The absorbed dose to water ${\displaystyle D_{w}(z)}$ (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.[2]

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:[5]

${\displaystyle CTDI_{w}={\frac {1}{3}}CTDI_{100}^{central}+{\frac {2}{3}}CTDI_{100}^{peripheral}}$

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[6]

${\displaystyle P={\frac {d_{360^{\circ }}}{C}}}$

where ${\displaystyle d_{360^{\circ }}}$ is the distance travelled by the couch during one full gantry rotation and ${\displaystyle C}$ 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:

${\displaystyle CTDI_{vol}={\frac {CTDI_{w}}{P}}}$

Similar measures with yet wider chambers are useful for CT systems with large numbers of detector rows.[7]

CTDI can also be measured with polymer gel dosimetry.[8]

Relation to DLP

The dose-length product (DLP) is a quantity defined for use in CT as

${\displaystyle DLP=CTDI_{vol}.nT}$

for ${\displaystyle n}$ and ${\displaystyle T}$ as described above (${\displaystyle nT}$ is therefore the total scan length). This quantity is analogous to the dose-area product (DAP) used in planar radiography.

References

1. Shope, Thomas B.; Gagne, Robert M.; Johnson, Gordon C. (July 1981). "A method for describing the doses delivered by transmission x-ray computed tomography". Medical Physics. 8 (4): 488–495. Bibcode:1981MedPh...8..488S. doi:10.1118/1.594995. PMID 7322067.
2. Platten, D J; Castellano, I A; Chapple, C-L; Edyvean, S; Jansen, J T M; Johnson, B; Lewis, M A (July 2013). "Radiation dosimetry for wide-beam CT scanners: recommendations of a working party of the Institute of Physics and Engineering in Medicine". The British Journal of Radiology. 86 (1027): 20130089. doi:10.1259/bjr.20130089. PMC 3922175. PMID 23690435.
3. 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. PMC 3079120. PMID 21502387.
4. Dowsett, David J.; Kenny, Patrick A.; Johnston, R. Eugene (2006). The Physics of Diagnostic Imaging (2nd ed.). London: Hodder Education. p. 430. ISBN 9781444113396.
5. "AAPM REPORT NO. 96 The Measurement, Reporting, and Management of Radiation Dose in CT" (PDF). AAPM. Retrieved 12 December 2016.
6. Martin, Colin J.; Sutton, David G. (2015). Practical Radiation Protection in Healthcare. Oxford: Oxford University Press. p. 288. ISBN 9780199655212.
7. Geleijns, J; Salvadó Artells, M; de Bruin, P W; Matter, R; Muramatsu, Y; McNitt-Gray, M F (21 May 2009). "Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner". Physics in Medicine and Biology. 54 (10): 3141–3159. Bibcode:2009PMB....54.3141G. doi:10.1088/0031-9155/54/10/012. PMC 2948862. PMID 19420423.
8. 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): 1589–97. Bibcode:2005MedPh..32.1589H. doi:10.1118/1.1925181. PMID 16013718.