Variations in measured computed tomography number values.

Computed Tomography (CT) scans of a water phantom have been performed on a number of different CT scanners, and variations in the measured mean pixel value for a central region have been observed. In addition, a positional variation of mean CT number was noted. In particular, the head rest on an EMI CT1010 scanner had a significant effect on the pixel values. To stimulate head scanning, a phantom with a Teflon annulus surrounding a water bath was used. This annulus affected both the mean and standard deviation of the pixels in a central area of the scan. Both an mAS and beam width dependence on CT numbers was noted. The effect of calibration on the measured CT numbers for each reconstruction filter was examined on one machine. The variation of CT number with reconstruction matrix and compensation filter was also investigated. These measurements suggest caution must be exercised when interpreting actual CT number values. This will be particularly relevant when the numbers are used to differentiate between tissue types for diagnostic or therapeutic purposes.