Pitch and patient dose.

OBJECTIVE. With single-slice helical CT, an increased pitch can decrease the radiation dose to the patient if all other parameters are constant. The purpose of this study was to determine whether the same relationship holds for a particular multislice helical CT system (Somatom Plus 4 VZ multislice helical CT scanner, version A11A) in our department.
CONCLUSION. The measured radiation dose to the phantom was identical for all pitch selections on the multislice helical CT system we tested. This unexpected result was because of an automatic proportionate increase in the tube current when the pitch selection was increased. Radiologists and physicists should exercise caution when extrapolating dose reduction strategies from single-slice to multislice helical CT systems, and they must acquire a detailed understanding of the multislice helical CT scanner of their chosen manufacturer.

For multislice helical CT scanners, manufacturers use different definitions of pitch, which has resulted in much confusion [9]. For the multislice CT scanner we described, the manufacturer defines "pitch" as the ratio of table movement per 360° rotation to single section thickness (P). We chose to use the definition of pitch [9] as table increment per 360° rotation divided by the total beam width (P'). This definition is applicable to both single- and multislice helical CT scanners, as shown in Table 1. Using slice combinations of 4 x 1 mm and 4 x 2.5 mm, the test volume was scanned on the multislice helical CT scanner at the manufacturer's defined pitch selections of 2, 4, and 8 (P' = 0.5, 1, 2). At a slice width of 3 mm, the same volume was scanned at pitch selections of 0.5, 1, and 2, respectively, on the single-slice helical CT system for comparison. Three dose measurements were recorded and averaged for each pitch setting we tested.

http://www.ajronline.org/cgi/content/full/177/6/1273