The image uniformity module is cast from a uniform material. The material’s CT number is designed to be within 2% (20H) of water’s density at standard scanning protocols. The typically recorded CT numbers range from 5H to 18H. This module is used for measurements of spatial uniformity, mean CT number and noise value.
The precision of a CT system is evaluated by the measurement of the mean value and the corresponding standard deviations in CT numbers within a region of interest (ROI). These measurements are taken from different locations within the scan field.
The Image Owl algorithm provides vertical and horizontal line profiles across the uniformity module. The points on the plot are an average over 5 columns of pixels. The plot is then smoothed further to reduce noise before the fitted curve is completed. To calculate the uniformity index for the horizontal and vertical profiles, the maximum and minimum y-axis CT values (HU) from the fitted curve are entered into the following equation: 1 - (CTmax-CTmin) / (CTmax+CTmin)
The closer a value is to "1", the more uniform the image.
The phenomenon of “cupping” or “capping” of the CT number may indicate the need for calibration.