Stimulated echo imaging

A new form of NMR imaging is described using stimulated echoes. The technique, dubbed STEAM ( stimulated echo acquisition mode) imaging, turns out to become a versatile tool for multipurpose NMR imaging. Stimulated echoes can be excited by a sequence of at least three rf pulses, which in the basic experiment have flip angles of 90° or less. Thus no selective or nonselective 180° pulses are needed, which eliminates a variety of problems associated with such pulses in conventional spin-echo NMR imaging. Further advantages of STEAM imaging are concerned with the functional flexibility of an imaging sequence comprising three pulses and three intervals and the possibility of “storing” information prepared during the first interval into the form of longitudinal magnetization during the second interval. In general, the applied rf power is considerably reduced as compared to spin-echo-based imaging sequences. Here the general principles of the technique are outlined and first applications to multislice imaging of directly neighboring slices are demonstrated. Subsequent papers will be concerned with modifications of the basic STEAM sequence which, for example, allow multiple chemical-shift-selective (CHESS) imaging, complete imaging of the spin-lattice relaxation behavior, diffusion imaging, and single-shot real-time imaging.