[Single-slice and multi-slice computerized tomography: dosimetric comparison with diagnostic reference dose levels]

The absorbed dose during clinical examinations of the head, thorax, abdomen and pelvis performed with a single-slice CT scanner and a new multi-slice CT system was measured and compared. Technical parameters, defined at installation and memorized on the two CT machines relate to a standard-sized patient and were considered the reference standard. Our experimental data were also been compared with the Diagnostic Reference Levels (D.L. 26/5/2000 n.187, Annex V).

We compared the performance of a multi-slice GE LightSpeed QX/i ADVANTAGE to that of a single-slice GE ProSpeed SX. The radiation beam profiles were measured at isocenter using a phosphor plate. Dose measurements were performed, according to the EUR 16262 EN Guidelines, with a 10-cm long CT pencil ionisation chamber and two PMMA phantoms (CEI EN 61223-2-6) for head and body respectively.

The obtained (normalised and weighted) computed tomographic dose index (nCTDIW) values were systematically higher for the multi-slice system (up to 36%) and the dose-length product (DLP) values on the multi-slice scanner exceeded the equivalent single-slice DLP values. The values were, however, always lower than DRLs, except in the case of the head multi-slice protocol, the technical parameters of which need to be improved. Our results allowed moreover to calibrate the automatic dose evaluation system of the multi-slice system, which systematically underestimated DLP values.

The comparison showed that the multi-slice scanner delivers a higher dose compared to the single-slice scanner. This is due to the radiation beam profile which is wider than the total active detector width, to the shorter focal spot-to-isocenter distance and to the effective scan length, which is longer than the nominal irradiated volume because the reconstruction algorithm of a multi-slice helical CT image requires the projection data from all detector rows. Nevertheless, the technology of new CT systems equipped with a multiple row detectors array can improve the protection of the patient thanks to very short irradiation time (less than 1 s) and reduced current values. In order to optimize the dose to the patient some acquisition parameters have been adjusted for head examinations.