Echo-Train Pulse Sequences: EPI, RARE & Beyond

Nearly all of MR-imaging in use today is based on a k-space approach making use of the fact, that a linear spatial encoding field – i.e. a gradient – leads to a linear variation of the Larmor frequency of spins. Localization is then achieved by Fourier transformation of the acquired time domain signal into the frequency domain, which directly corresponds to the spatial domain. Consequently two-dimensional k-space data have to be acquired in order to form a two-dimensional image. The challenge for MR image formation thus is to find a way to cover the two-dimensional k-space with an intrinsically one-dimensional signal.