Factors influencing the accuracy of volume measurements in spiral CT: a phantom study

The purpose of this study was to investigate the influence of different parameters (object size, shape, orientation, contrast, observer, and window setting) on the accuracy of volume measurements in spiral CT.

The phantom study consisted of two parts. First, well-circumscribed ellipsoid objects were scanned with conventional and spiral CT. Volumes were determined by two observers using manual contour delineation and summation-of-areas. The influence of object size, CT technique, and observer was assessed. Second, irregularly shaped gingerroots were scanned and the effect of observer, contrast, and orientation was investigated. Also assessed in both parts of the study was the effect of window setting.

Spiral CT offered higher accuracy and reproducibility than conventional CT did. The accuracy of volume measurements was mainly determined by the window setting used for image display. Using an "optimized" window center value halfway between object density and background density resulted in overestimation of true object volumes if the number of slices through the object (number of samples) was low. Other factors (contrast, observer-related errors in contour delineation) were less important.

Volume measurements of small objects, obtained with spiral CT and using manual contour delineation, may be grossly erroneous. Major causes of inaccuracy are an inappropriate window center selection and undersampling.