In general, voxel size is the actual size of a 3D pixel in your actual volume dataset. This can be changed during image processing. Nominal resolution is the smallest voxel size of a scanner's reconstructed image corresponding to the beam path of the scan.
The final image resolution cannot be better than the voxel size. However, because of other factors such as spot size of the X-ray source, image reconstruction or partial volume effects of particles smaller than the voxel size, resolution is reduced and information is smeared out over neighbouring voxels. Resolution can be 2-5 times of the voxel size.
Detectability refers to the smallest object that still can be seen in an image, depending on high contrast width between object and background. For instance, a gold particle (high attenuation) on a graphite surface (low attenuation) sized even smaller than the resolution of the scanner will still be detectable, while a glass particle (medium attenuation) will not.