Tailored selective NMR excitation by projection presaturation

Selective excitation of spins in one region in a relatively large object is important for many NMR applications in biology and medicine. For example, excitation of spins in a slice (one‐dimensional excitation) is the basis of tomographic (section) imaging and is currently used in clinical MR imaging (MRI) for diagnosing human disease. Selective excitation in two and three dimensions also is important. It is used primarily in localized in vivo MR spectroscopy (MRS) for quantifying the metabolites or energetics of a diseased tissue, but it also is used in MRI for limited field of view imaging or for improving image quality by suppression of flow and motion artifacts. We describe a technique, projection presaturation, for selectively exciting a single volume of interest (VOI) or for simultaneous excitation of multiple VOIs, the shape and size of which can be tailored in one, two, or three dimensions. Projection presaturation has advantages over commonly used techniques: It is conformal; it provides maximum sensitivity; it is accurate even in the presence of T 1 variation, tip‐angle errors (deviation from the desired tip‐angle caused by mistuning or inhomogeneity of the radio frequency transmit field), and off‐resonance effects (variation in the resonance frequency caused by B o inhomogeneity and chemical shifts); and it is suitable for short T 2 species. Several applications of the method in MRI and MRS are described and discussed.