Pitch factor

The pitch factor or pitch is a dimensionless recording parameter in spiral computed tomography and describes the ratio of table feed to beam collimation . According to IEC standard 60601 from 1999, it is calculated as follows:

${\ displaystyle P = {\ frac {d} {M \ cdot S}}}$

With

${\ displaystyle P}$ = Pitch factor

${\ displaystyle d}$ = Table feed per 360 ° tube revolution

${\ displaystyle M}$ = Number of simultaneously detected detector lines (beam collimation)

${\ displaystyle S}$ = selected layer thickness

Example: With a fictitious 16 line CT, a table feed of 32 mm per tube revolution is selected. The selected layer thickness is 1 mm. Then the pitch factor is calculated as: 32 mm / (16 * 1 mm) = 2.

In some CTs, the inner detector lines have a higher resolution than the outer ones and detector lines can be grouped. If, in the example mentioned, the slice thickness is set to 0.5 mm instead of 1 mm, the beam collimation is reduced from 16 mm to 8 mm and the resulting pitch factor doubles with the same table feed.

The pitch factor influences the radiation exposure of the patient and the image quality. With the same tube voltage and current, the radiation exposure is lower the higher the pitch factor. The lower the pitch factor, the higher the image quality. Since the circulation speed of the tube is limited (depending on the manufacturer and model to a minimum of approx. 0.3 seconds), the maximum reasonable breath-hold time for the patient is often the decisive criterion for the choice of the pitch factor. The maximum possible radiation duration of the tube is also limited; this can mean that a certain examination protocol can only be run with a high pitch.

Values between 0.5 and 2. Pitch factors less than 1 are used for high-resolution recordings, values greater than 2 must not be set, as otherwise the examination object would be scanned with gaps. The control software of the CT therefore does not offer the selection of pitch factors greater than 2.

source

Calendar W. A .: Computed tomography. Basics, device technology, image quality, applications with multi-slice spiral CT. Publicis MCD advertising agency Munich 2000; ISBN 3-89578-082-0