Computed tomography dose index

The computed tomography dose index (CTDI) is a commonly used radiation exposure index in

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

${\displaystyle z}$

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

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