The volume information thus derived can be used for an initial visual inspection for cavities, inclusions, or other internal structures.  In order to derive dimensions from this data, the precise location of the material borders or transitions (e.g. from metal to air or plastic) must be determined.  The amplitudes of the voxels surrounding each transition point of interest are used to calculate the spatial coordinates of a point on the surface or material border. By utilizing the amplitude information, the resolution for the determination of the edge location can be significantly better than the spatial resolution defined by the voxel size.  This process is known as subvoxeling, analogous to subpixeling in 2D image processing.  Subvoxeling can be performed using various methods.  For example, the edge location can be calculated by applying a locally determined threshold to the linear interpolation of the voxel amplitudes.

See in image above basic principle of linear interpolation to derive surface coordinates from voxel amplitude information: the precise location of the edge (O) is defined as the intersection of the local threshold value (S) (with respect to (A) max and (A) min).