In the English system, pitch is expressed by the number of teeth per inch (25.4 mm).
The appearance of spiral CT produced a new concept, pitch (P). For the early single-layer spiral, the definition of this is unified by all manufacturers, that is, pitch = feed distance/collimation width of tube rotating 360 degrees. The pitch concept of multi-slice spiral CT is a bit complicated, and a collimation width of multi-slice CT contains multiple adjacent images. In this way, the manufacturer's non-negotiation (or non-compromise) leads to confusion in the definition of denominator in the multi-layer spiral pitch formula: collimation width.
Multi-slice CT such as Marconi takes the whole collimation width as the denominator of the formula (the number of layers x the width of a single collimator), while GE takes the collimation width of each image as the denominator. Because of the confusion of the basic definition, the calculation formula results are confused. In the former, when the feeding distance is equal to the whole collimation width, the calculated pitch is equal to 1 regardless of the 4, 8 or 16 layers, while in the latter, the calculated pitch is equal to 4, 8 and 16 respectively.
References:
Baidu Encyclopedia-Introduction