MRI -- huge magnet polarizes the nucleii of your brain, and then is shut off -- as your nucleii depolarize, they send out radio-esque frequency waves in all directions which can be measured and mapped.
"Emission tomography is a visualization technique in nuclear medicine that yields an image of the distribution of a previously administered radionuclide in any desired transverse section of the body. Positron emission tomography (PET) utilizes the unique properties of the annihilation radiation generated when positrons are absorbed in matter. It is characterized by the fact that an image reconstructed from the radioactive counting data is an accurate and quantitative representation of the spatial distribution of a radionuclide in the chosen section. This approach is analogous to quantitative antoradiography performed in laboratory animals but has the added advantage of allowing in vivo studies and, hence, studies to be performed safely in human subjects." -- Marcus Raichle
"Magnetic resonance imaging (MRI) is based on the phenomenon of nuclear magnetic resonance (NMR), first described in landmark papers over fifty years ago (Rabi et al. 1938; Rabi, Millman, and Kusch 1939; Purcell et al. 1945; Bloch, Hansen, and Packard 1946). In the presence of an external magnetic field, atomic nuclei with magnetic moments, such as 1H, 13C, and 31P nuclei, encounter a separation in the energy levels of their quantum mechanically allowed orientations relative to the external field. Transitions between these orientations can be induced with electromagnetic radiation typically in the radiofrequency range. The discrete frequency associated with such a transition is proportional to the external magnetic field strength and to two parameters that are determined by the intrinsic properties of the nucleus and its chemical environments, the nuclear gyromagnetic ratio, and the chemical shift, respectively. Based on the ability to obtain discrete resonances sensitive to the chemical environment, NMR has evolved rapidly to become an indispensable tool in chemical and biological research focused on molecular composition, structure, and dynamics." -- Kamil Ugurbil
fMRI, etc... -- http://cognet.mit.edu/MITECS/Entry/ugurbil