CTDI

The CTDI is a measured variable in dosimetry and the basis for calculating the radiation exposure during an X-ray exposure with the aid of a computer tomograph . The acronym stands for " C omputed T omography D ose I ndex".

In contrast to other diagnostic radiology procedures , the patient is illuminated from all sides during a computed tomographic examination. In addition, the x-ray beam is not fanned out broadly, but rather collimated to form a very narrow slit in the direction of the axis of rotation of the x-ray tube . There are also not inconsiderable dose proportions outside the actual shift. The CTDI corresponds to the absorbed dose that would have been absorbed in an assumed, rectangular profile of the slice thickness of the CT specified by the manufacturer. Dose proportions lying outside the shift are added to the shift dose.

The CTDI was first defined by the FDA in 1981 as:

${\displaystyle CTDI={\frac {1}{nT}}\int _{-7T}^{7T}{D(z){\text{d}}z}}$

With:

T = layer thickness

n = number of layers

D = measured dose

The z-axis is perpendicular to the image plane and runs in the direction of the patient's longitudinal axis.

The CTDI is determined with the help of water phantoms , which simulate the conditions within the human body as ideally as possible. If the CTDI is determined in air without using a phantom, it is called the CTDI _air . The unit of measurement of the CTDI is the mGy (milli- gray ). The dose-length product (DLP) is obtained by multiplying the CTDI by the length of the examination volume; it quantifies the patient's total radiation exposure during a CT examination.

CTDI in spiral CT

For decades, all clinical CTs have been spiral CTs. With these devices, a translational movement of the patient table takes place simultaneously with the rotation of the tube. The ratio between the distance covered d _{360 °} for a full tube rotation to the steel collimation C is called the pitch factor and must be taken into account when calculating the CTDI.

The following applies:

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

The beam collimation C is the total thickness of all simultaneously recorded slices.

The so-called CTDI _{vol is} thus calculated as:

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

swell

• Jürgen Freyschmidt, Theodor Schmidt: Manual of diagnostic radiology. Volume 1: Radiation Physics, Radiation Biology, Radiation Protection. Springer, Berlin 2003, ISBN 3-540-41419-3 , p. 27 ff. ( Limited preview in the Google book search).

• Shope, TB; Gagne, RM; Johnson, GC: A method for describing the doses delivered by x-ray computed tomography. In: Med. Phys. 1981; 8: 488-495