As with standard MRI, fMRI machines create magnetic fields that vary slightly throughout the brain, providing a different magnetic environment for hydrogen atoms in different areas. The differing magnetic field strengths make the spin of each hydrogen atom precess at different rates, so that when a pulse of radio waves is focused on the head, the atoms respond differently depending on location and on their particular environment. Those that absorb radio energy and then release the energy are detected by magnetic coils surrounding the head, and these signals, or "echoes," are used to produce an image of the brain.
With EPI, a single pulse of radio waves is used to excite the hydrogen atoms, but the magnetic fields are rapidly reversed several times to elicit about 50 to 100 echoes before the atoms settle down. The multiple echoes provide a high-resolution picture of the brain.
In 2002, Feinberg proposed using a sequence of two radio pulses to obtain twice the number of images in the same amount of time. Dubbed simultaneous image refocusing (SIR) EPI, it has proved useful in fMRI and for 3-D imaging of neuronal axonal fiber tracks, though the improvement in scanning speed is limited because with a train of more than four times as many echoes, the signal decays and the image resolution drops.
Another acceleration improvement, multiband excitation of several slices using multiple coil detection, was proposed in the U.K. at about the same time by David Larkmann for spinal imaging. The technique was recently used for fMRI by Steen Moeller and colleagues at the University of Minnesota. This technique, too, had limitations, primarily because the multiple coils are relatively widely spaced and cannot differentiate very closely spaced images.
In collaboration with Essa Yacoub, senior author on the paper, and Kamil Ugurbil,
|Contact: Robert Sanders|
University of California - Berkeley