Multiecho approaches to spectroscopic imaging of the brain

Spectroscopic imaging (SI) with nuclear magnetic resonance (NMR) is one of the most powerful tools available for studying brain chemistry in vivo. Both proton (1H) and phosphorus (31P) NMR offer valuable biochemical information that can in principle be mapped throughout the entire brain, thereby enhancing our understanding of brain function. With the exception of protons from tissue water and the triglycerides of adipose tissue, however, nuclei contributing to the NMR signals of living tissue are in relatively small (millimolar) concentrations. The low concentration of metabolite nuclei reduces the overall sensitivity of conventional SI techniques, making high-quality metabolite mapping a lengthy procedure. This problem has led to the development and testing of nonconventional methods for reducing SI scan times, including techniques based on the collection of multiple spin-echoes. The extent to which multiecho methods can be used to decrease SI scan times and maintain high-quality metabolite mapping depends on several factors. These include the spectral transverse relaxation times, the spectral resolution required, and J-coupling interactions. We have discussed these various technical aspects of multiecho SI methods as applied to 1H and 31P spectroscopic imaging of the living brain.