English: Measurement of a kidney stone in a CT window for soft tissue (width of 400 HU, center at 50 HU) and bone window (W1800,C400). The kidney stone was directly measured outside the body to be 5.6 mm, and is here seen after placement in a potato (whose CT attenuation is regarded as comparable to that of a kidney¹), in turn placed in a human-shaped water tank. The lines in the images show the same measurement of 5.6 mm. Even without knowing this reference point, the mean maximal value made by 6 investigators in the CT image (5.4 mm) was only 0.2 mm off the direct measure outside the body when using a HU cutoff of 150.

Reference

1. (2009). "Computerized Tomography Magnified Bone Windows are Superior to Standard Soft Tissue Windows for Accurate Measurement of Stone Size: An In Vitro and Clinical Study". The Journal of Urology 181 (4): 1710–1715. DOI:10.1016/j.juro.2008.11.116. ISSN 00225347.