Other ways to image
Many different imaging methods are in use and development
One imaging technique that is now widely used by researchers is calcium imaging. When neurons fire, the concentration of calcium ions inside them increases. This can be detected in slices of brain tissue or in live animals, using fluorescent dyes that are sensitive to the concentration increases combined with two-photon microscopy (a type of microscopy used particularly to image living cells). This technique requires an operation to expose the brain and is not used in humans.
Near-infrared spectroscopy (NIRS) is a method that detects changes in blood flow around the brain. It works by transmitting light with a wavelength of 700–900 nanometres through the head; the light passes through skin, bones and brain tissue but is absorbed differently by oxygenated and deoxygenated blood. The person being scanned wears a cap containing lasers or light-emitting diodes; this allows people to do more things while their brain is examined because they are not enclosed in a scanner. NIRS is cheaper than fMRI and is particularly useful in small babies, whose skulls are thinner than those of children or adults and transmit the light used by NIRS better.
Other techniques include functional electrical impedance tomography by evoke response (fEITER). This uses electrodes attached to the head, which transmit electrical currents that are interrupted by the brain’s electrical activity. Researchers recently used fEITER to image how brain activity changes when people lose consciousness under anaesthesia.Lead image:
NIH Image Gallery/Flickr CC BY NC