Slide 7 of 16
The reason why differences are expected to occur between the helical imaging performance of multi-slice and single-slice systems is due to the different ways images are reconstructed. These slides illustrate two methods of image reconstruction used commonly, though not universally, on the two types of scanner.
On a single-slice scanner, the image is normally reconstructed from interpolation of pairs of data points, projected through the same angle, either side of the required slice position.
For this type of interpolation, we expect the amount of image noise to remain constant with increasing pitch at fixed mA, because the same number of photons are reaching the detectors whatever the pitch, and because only two points are ever being used in the interpolation. However, as the pitch increases, the two data points move further apart, and thus the slice profile gets broader, degrading the image.