Computed Tomography uses X-rays to look inside the body; so it cannot examine anything smaller than 2mm. Magnetic Resonance Imaging, on the other hand, creates images from the magnetic behaviour of hydrogen protons in brain tissue, and so can show much more detail (down to around 1mm). These imaging methods are mainly used to investigate anatomical structures and the processes associated with disease. CT scans are not used to examine the dynamic functioning of the brain. More on the history and current applications can be found here.