Localized MRS employing radiofrequency field (B1) gradients

This article describes techniques that have been developed to localize MRS (magnetic resonance spectroscopy) spectra using the spatially dependent magnetic field (B1) produced by one or more RF coils. The work is focused on the two main techniques previously shown to be capable of producing high-quality localized spectra, which are (i) depth pulse sequences and (ii) rotating frame zeugmatography and its variant, the Fourier series window method. As compared with the now more commonly used localization techniques that exploit spatial gradients in the main field (B0), these B1-gradient techniques still offer advantages, including (i) avoidance of spectral distortions due to eddy currents, (ii) elimination of acoustic noise, and (iii) minimization of signal loss due to the rapid decay of the transverse magnetization. The drawbacks of B1-gradient methods, such as their limited flexibility to define the voxel shape, are discussed together with some ways to overcome their shortcomings. A description of the experimental procedures required to implement B1-gradient methods is also included.