I'm sorry I don't know what the term is in French, but I do believe the "pitch" referred to here is as defined below:

In helical CT, the patient is continuously moved through the scanner as the source and receptor rotate around the patient. To achieve such a detailed and expansive feat, it is necessary to expose the entire thickness of the patient to a greater quantity of x-ray photons. ***The pitch is the ratio of distance the patient has moved through the scanner per rotation per slice thickness.*** Usually, the number is between one and two. Higher pitch lowers radiation dose, but at the expense of partial volume effect (loss of detail). Larger patients receive a relatively higher skin dose in order to penetrate their bodies and smaller patients have a relatively higher dose to internal organs. (Since internal organs are more radiosensitive than skin, smaller patients generally incur a higher effective dose for the same CT settings and, hence, higher long-term cancer risk.)


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Note added at 13 hrs 9 mins (2005-09-25 07:49:18 GMT)
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Two technical factors -- tube current and pitch -- "can be adjusted easily to reduce radiation exposure in the pediatric population and have a profound affect," says Dr. Donnelly. Tube current refers to the amount of electrons used to generate the x-ray source. ***Pitch refers to the ratio between the rate at which the table the patient lies on moves through the scanner and the rate at which the scanner spins.***

http://www.cincinnatichildrens.org/about/news/release/2001/1...