Radiation dose in spiral CT: The relative effects of collimation and pitch

The proliferation of helical CT and thin slice protocols has motivated a reevaluation of methods for determining radiation dose. Traditional measurements may be insufficient in that computed tomography dose index (CTDI) is undefined for the case of continuous table motion which characterizes helical CT. We have measured the radiation dose using thermoluminescent dosimeters (TLDs) under contiguous axial, contiguous helical, and noncontiguous helical scans. A device that holds tightly spaced TLDs was inserted into a 32-cm-diam Plexiglas(TM) (CTDI) phantom so that (1) TLDs were exposed to contiguous axial scans; (2) TLDs were exposed to a contiguous (pitch 1) helical scans, and (3) TLDs were exposed to noncontiguous (pitch > 1) helical scans. The TLDs integrated exposure from both primary and scattered radiation resulting from scanning a volume of the phantom. The TLD measurements were repeated at several slice thicknesses and for pitches of 1, 1.5, and 2. For a direct comparison, conventional pencil ionization chamber measurements were made at the same slice thicknesses. Our results show that contiguous helical CT scans (pitch of 1) give approximately the same radiation dose as contiguous axial scans acquired with the same technical factors. For noncontiguous scans (pitch >1) at a given collimation, radiation dose decreases as pitch increases; specifically as 1/pitch. However, the dose remained relatively constant across collimations (±20% of the 10 mm slice dose) for both axial and helical scans. At smaller slice thicknesses, the radiation profile width (full width at half-maximum) is greater than the nominal slice thickness, which results in extended radiation overlap between slices and no net change in radiation dose compared to thicker slices. Finally, for a given table speed, radiation dose decreases when a higher pitch is used with a thinner collimation.