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[Dose distribution in conventional CT and spiral CT and on the topic of dose reduction with spiral CT].

OBJECTIVE
To compare spiral CT with conventional CT with regard to the dose distribution in the irradiated body and to examine when an increase in the table feed in spiral CT leads to a dose reduction compared with conventional CT.


METHODS
In order to show the dose distribution, films were exposed in an acrylic phantom. The nominal slice thickness was 5 mm; the slice distance in conventional CT was 5 and 10 mm; the table feed in spiral CT was 5 and 10 mm per rotation (pitch 1 and 2). The integral doses delivered in spiral CT and conventional CT were related to the table feed per rotation or slice distance, respectively, and were compared arithmetically with each other.


RESULTS
Spiral CT showed reduced dose fluctuations in axial direction compared with conventional CT. Only in conventional CT with a slice distance of 5 mm did the axial dose distribution show relative dose peaks near the rotation axis. The longer the body region to be examined is, the less table feed is necessary in spiral CT for a dose reduction. From a length of 15 times the slice thickness, a pitch of > 1.1 leads to a reduction of the integral dose compared with conventional CT.


CONCLUSIONS
The continuous table feed in spiral CT results in evenly distributed axial dose distributions compared with the discontinuous scanning in conventional CT. The spiral technique can give an important contribution to dose reduction in CT.