Differences in noise and dose production between a 16 and 64 slice CT scanner at various ATCM settings

Objectives There is peer-reviewed evidence in recent years detailing that manipulating certain CT parameters, such as, spin or pitch, would lead to noisier images or an undesirable increase in dose for a given automated tube current modulation (ATCM)setting. GE calls this setting noise index (NI). This effort aims to delineate the differences in modulation or dose delivery when using different slice scanners for brain acquisitions at various noise indexes. Comparison between a GE 16 slice lightspeed vs. a 64 slice GE VCT which utilizes X, Y and z-axis modulation techniques (Smart mA program), is tested.

Methods An anthropomorphic head phantom was positioned in a head holder and iso-centered on both the 16 and 64 slice scanner. Lateral scouts were utilized for landmark purposes and to serve as a reference for the z-axis portion of the ATCM. A CT was acquired from the vertex to the skull base. Scans were obtained with NIs of 5, 7.5, 10, 12, 14, 16 and 18. These scans were accomplished with a pitch of 1.38 and 0.938. Spin and kV were maintained at a constant 0.5 sec and 120. A dose report was generated to compare changes in CTDIvol and DLP.

Results Noise production, using a pitch of 1.38, averaged 8.8 for the 16 slice and 9.0 for the 64 slice. A 3% higher noise production was seen in the 64 slice machine. The 64 slice system delivered a 2% lower dose on average, with a maximum of 17% lower dose. Using a pitch of 0.938 resulted in an averaged 14% increase in noise and a 34% decrease in dose on the 64 slice system vs. the 16 slice system.

Conclusions There is strong correlation between the 16 and 64 slice machines when using a pitch of 1.38. This agreement is not seen when a pitch of 0.98 is used. This suggests that a 64 slice machine may need a lower ATCM setting than a 16 slice machine. Further work needs to be done to assess the change that would be distinguished in the region of the torso of a patient as the density change (e.g. lung to liver) would be greater than that witnessed in the human brain