Principles of CT Operation: What determines the Voxel Resolution?
The spatial resolution in a CT image is limited by the voxel (= volume pixel) size V of the reconstructed volume data set given by the detector pixel pitch P divided by the geometric magnification M, i.e. V=P/M. M is equal to the ratio of focus-detector distance and focus-object distance: M=FDD/FOD.
If the entire object is to be contained in the CT volume, M is limited by the widest diameter d of the object and the detector size D because the full object must remain in the field of view whilst the object is rotated during data acquisition: Mmax = D/d. So the highest achievable resolution or minimum voxel size is V=P * d/D. For small objects, or partial scans very high magnifications can be achieved and the resolution is only limited by the focal spot size of the X-ray source.