Dependence of substance CT number on scanning technique and position within scanner.

A body tissue will vary in its CT value from one study to another not only because of changes in its physiology, but also because of technical factors related to the scanning procedure itself. Changing the mAs of a scan and/or position of a substance within the CT scanner was shown by the use of a phantom study to give a small relative but readily measurable change in the CT value of the substance. Thus, small relative changes in CT numbers should be regarded with caution when comparing one substance or area with another or one study with another.