Computed tomography dose index

Definitions

It is defined 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:

C T D I = 1 n T ∫ − 7 T 7 T D ( z ) d z

where n is the number of slices acquired, T is the slice thickness and D ( z ) is the radiation dose measured at position z 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:

C T D I 100 = 1 n T ∫ − 50 m m 50 m m D ( z ) 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:

C T D I w = 1 3 C T D I 100 c e n t r a l + 2 3 C T D I 100 p e r i p h e r a l

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 = d 360 ∘ C

where d 360 ∘ 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:

C T D I v o l = C T D I 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

D L P = C T D I v o l . n T

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