Purpose: In an attempt to improve field uniformity and CT number stability, the software in a CT scanner was replaced with a new version. The purpose of the present study was to investigate the influence of the software upgrading on density measurements. Material and Methods: The upgrading influence was investigated by means of a torso phantom (European spinal). The phantom simulates a torso which contains 3 vertebrae of different densities. Results: Despite the use of a reference standard under the phantom, the software change resulted in: a) improved short-term precision; b) increased density values at 80 and 100 kVp, decreased at 130 kVp; and c) increased influence of the kVp on the values. Conclusion: The results of the present study suggest that conversion factors have to be used in density values for reference populations as well as in the values of patients undergoing follow-up studies
