Computed tomography dose index

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:

CTDI=\frac{1}{nT}\int_{-z}^{+z}{D(z)\text{d}z}

where n is the number of slices acquired per single axial rotation, T is the width of a single acquired slice (and thus nT is the nominal beam width) and D(z) is the radiation dose measured at position 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:

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

The absorbed dose to water 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.

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:

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

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

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

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

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 dosimetry.

Relation to DLP

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

DLP = CTDI_{vol}.nT

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

References

References

1. (July 1981). "A method for describing the doses delivered by transmission x-ray computed tomography". Medical Physics.
2. (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.
3. McCollough, C. H.. (18 April 2011). "CT Dose Index and Patient Dose: They Are Not the Same Thing". Radiology.
4. (2006). "The Physics of Diagnostic Imaging". Hodder Education.
5. "AAPM REPORT NO. 96 The Measurement, Reporting, and Management of Radiation Dose in CT". AAPM.
6. (2015). "Practical Radiation Protection in Healthcare". Oxford University Press.
7. (21 May 2009). "Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner". Physics in Medicine and Biology.
8. (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.