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Factors Affecting the Measurement of Size and CT Number in Computed Tomography
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1981
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Computed tomography size measurements are affected by viewer window center settings, which can alter the apparent size of structures. A custom phantom was designed to quantify how window center adjustments influence the apparent size of long cylinders and spheres in CT images. The phantom study showed that cylindrical objects larger than one transverse resolution element can have accurate CT numbers but their apparent diameters change with window center, while spherical objects yield inaccurate CT numbers and diameters when their size is comparable to or less than the slice thickness, unless the sphere is centered and the full‑width‑half‑maximum profile is used, enabling ±1 mm accuracy.
Computed tomography scanners may be used to make anatomic size measurements; however, manipulating the viewer controls (especially the window center) can have a significant influence on the apparent size of structures in the image. A special phantom was constructed to study the effect of window center adjustments on the apparent size of long cylindrical objects (no variation within slice thickness) and also for spherical objects (maximum variation within slice thickness). Using this phantom, the authors were able to identify several commonly encountered situations in which both the apparent size and CT numbers may be seriously in error. It was found that for cylindrical objects larger than about one transverse resolution element and aligned with the scanner axis, CT numbers may be determined accurately; however, the apparent diameter changes by several millimeters as the window center setting is changed. CT numbers for spheres are inaccurate when the diameter is comparable to or less than the slice thickness and, as a result, diameters of spheres cannot be measured accurately with fixed window center settings. Diameters of spheres can be measured accurately (±1 mm) if the sphere is centered in the slice and the full width at half maximum of the CT number profile is used.