Slide 12 of 16
Here, the noise measurements for all three multi-slice scanners have been plotted against pitch for the situation which would apply in practice, i.e. constant dose at all pitches.
For the scanners for which we were able to make measurements at the full range of pitches – the Volume Zoom and the Mx8000 – noise remains pretty constant with pitch. The results are very similar for these two scanners, as we would expect since they share the same basic design.
For the LightSpeed, we could only make measurements at the two pitches available on this scanner. Interestingly, there is more noise in the “High Quality” mode, i.e. pitch 0.75 than in the “High Speed” mode.
Going back to the multi-slice plots on the right, it can be seen that, apart from one point on the LightSpeed, all the noise values are lower than the noise obtained in axial mode at the same dose. This is because, for each scanner, an imaged slice width about 30% wider than the collimated slice width has been used in this study, to reflect what is commonly recommended in the scan protocols. For example, on the Mx8000, a collimated slice width of 2.5 mm has been used to achieve an imaged slice width of 3.2 mm.
It is for this reason that we cannot look at noise in isolation, but must also consider measured slice width and its effect on noise.