As far as I am aware there is no effective way of doing this on 3D Slicer. Basically, you have to acquire the scan in high resolution (minimal field of view (FOV) and small slice thickness/interval) at the point of acquisition i.e. when taking the CT scan. Depending on the acquisition parameters used you may be able to reconstruct the CT scan with smaller slice thicknesses/intervals or FOV if you have access to the original CT machine and the scan is still on the machine (if it was taken recently). Have a read of these related posts: