The history of modern brain imaging began in the 1970s with computed axial tomography or CAT scans and proceeded at a rapid and accelerating rate for the remaining decades of the twentieth century. The idea of passing X-rays through the head from multiple directions and reconstructing a three-dimensional structural image, revolutionary at the time, was quickly adapted to radiological signals other than X-rays. These included radiation from exogenous tracers to enable imaging of brain function, as in positron emission tomography (PET) and single photon emission computed tomography (SPECT), and endogenously generated magnetic fields to image either structure or function, as in magnetic resonance imaging (MRI). Pioneering research on cognition and emotion was undertaken with PET and SPECT in the 1980s, and by the 1990s MRI, the noninvasive alternative to PET, became commonplace in research. (Casey, E. 1989)
In an MRI, atoms are first aligned by a strong static magnetic field, then knocked out of alignment by a radio frequency pulse, and then allowed to realign. The fluctuating field created as the atoms "relax" to the aligned state is the signal that is measured. Although early functional MRI used an injected contrast agent, current methods use the magnetic properties of the blood itself as a tracer, and are therefore entirely noninvasive. (Whitehouse, H. 1996) In blood oxygen level dependent (BOLD) MRI, the different magnetic susceptibility of oxygenated and deoxygenated hemoglobin provides a measure of regional brain activity. In arterial spin labeling (ASL) MRI, the atoms are aligned by a magnetic field at the neck, and relax as they circulate through the brain, indicating regional perfusion. The spatial and temporal resolution of functional MRI (fMRI) is limited by haemodynamics rather than by the physics of the method; blood flow changes over seconds in response to neural activity, and these changes extend into nearby tissue...................
