The article contains a short presentation of the ideas underlying computer tomography. It points to the fact that when applying image analysis method to measure geometric parameters of soil aggregates displayed in tomographic images it is necessary to establish the relation between the minimum size of aggregate eligible for evaluation and the size of voxel. The analysis has been carried out for monoaggregate structures (the size of aggregates ranging from 1 to 7 mm) and encompassed two kinds of tomograms - those generated by computer tomographs as well as computer simulation. Quantitative analysis can be done if aggregate is at least 10 times bigger then voxel.