Primary radiation outside the imaged volume of a multislice helical CT scan

Multislice helical CT scanning has advantages of speed and X-ray tube loading, making it possible to image larger volumes in a single exposure. Our aim is to investigate dose implications for short scans from the additional X-ray tube rotations required to reconstruct a given volume in helical scanning. To this end a multislice scanner was compared with a single slice scanner. Two independent methods were used. The first was based on optical density measurements taken from a film exposed free-in-air as it moves with the CT bed along the scan axis. The second used measurements from a pencil ionization chamber supported free-in-air at the centre of the CT aperture for the duration of both a long scan and a short scan. This method assumes the same excess primary radiation at the extremes of both scans and the measurements are incorporated into two simultaneous equations. The dose-length product outside the imaged volume has been compared with the dose-length product inside the imaged volume using both methods. For 4 x 5 mm multislice collimation with a 360 degrees interpolation and a pitch of 0.875, the film and simultaneous equations methods show an excess dose-length product at the extremes of the scan volume equivalent to 3.3 cm and 3.5 cm extra scan length, respectively. This represents a large percentage of a short scan and is substantially greater than for a helical scan using the single slice scanner with 5 mm collimation, a 360 degrees interpolation and a pitch of 1. The latter showed an excess dose-length product at the extremes which was equivalent to 0.35 cm scan length by the film method and 0.25 cm using simultaneous equations. Taking the abdominal protocols recommended by the respective manufacturers, however, the multislice scanner could cover a 45 cm scan length in a single exposure, while the single slice scanner needed six exposures to image the same volume. With the multislice scanner set at 4 x 2.5 mm collimation, 360 degrees interpolation and a pitch of 0.875, the dose-length product outside the volume of interest was equivalent to 1.9 cm scan length by the first method and 1.8 cm using the second method. With 4 x 1 mm collimation it was equivalent to 1.0 cm using both methods. Changing the interpolation algorithm from 360 degrees to 180 degrees had no effect on the additional equivalent scan length while doubling the pitch resulted in a 25% increase. We conclude from this study that with the multislice scanner, the axial mode is to be preferred for short CT scans such as those used in patient biopsies and drainage. For paediatric helical scans shorter than 13 cm, dose length product is reduced by using 4 x 2.5 mm rather than 4 x 5 mm collimation. For longer scans, however, the increased CT dose index associated with narrower collimation in the multislice mode offsets the dose reduction at the extremes.