Brain imaging is a fairly recent discipline within medicine and neuroscience. Brain imaging falls into two broad categories -- structural and functional imaging.
Structural imaging began with early radiographic techniques to image the human brain. Unfortunately, largely composed of soft tissue, the brain and brain abnormalities remained largely invisible. Crude images of the ventricular system within the brain were obtained by air injection -- a painful procedure.
With the advent of computerized axial tomography (CAT ), detailed anatomic images of the brain became available for diagnostic and research purposes. Soon after, the development of radioligands that either remained within the blood stream or entered the brain to bind to certain receptors within brain started the functional imaging revolution. Radioligands are either single photon or positron emitters. Thus, single photon emission computerized tomography (SPECT) and positron emission tomography (PET) became available as long as facilities were present to synthesize the ligands needed.
Early techniques such as xenon inhalation provided the first blood flow maps of the brain. Functional imaging took a large step forward with the development of oxygen-15 labelled water (H215O, or H20-15) imaging. H20-15 emits positrons and creates images based on regional blood flow within the brain. Since active neurons recruit a robust blood supply, H20-15 PET allowed investigators to make regional maps of brain activity during various cognitive tasks.
Concurrently, magnetic resonance imaging (MRI) was developed. Rather than radiation, MRI uses variation in signal produced largely by the body's protons when the head is placed in a strong magnetic field. At first, structural imaging benefited most from the introduction of MRI.
However, scientists soon learned that the large blood flow changes measured by H20-15 PET were also imaged by MRI. Functional magnetic resonance imaging (fMRI) was born. Since the 1990s, fMRI has come to dominate the brain mapping field due to its low invasiveness, lack of radiation exposure, and relatively wide availability.
Physicists have also developed other MRI based techniques such as magnetic resonance spectroscopy (for measuring some key metabolites such as n-acetylaspartate and lactate within living brain) and diffusion tensor imaging (for mapping white matter tracts within living brain). Structural MRI and CAT scanning have a large place in medicine, however fMRI and its brethren are still largely devoted to neuroscience research.