Pulse Sequences for Hyperpolarized MRS

Abstract

Hyperpolarization offers substantial gains in sensitivity for magnetic resonance (MR) spectroscopy (MRS) and imaging (MRI), which are particularly beneficial for providing novel functional information not accessible to conventional MRS and MRI. However, hyperpolarized MR requires tailored pulse sequences because of its unique magnetization behavior, including its decay back to thermal equilibrium, spin exchange, and metabolic conversion of the hyperpolarized magnetization. In addition, rapid delivery of the hyperpolarized media from the polarizer to the subject is key to success. This article begins by describing the unique behavior of hyperpolarized agents and the problems associated with their delivery, focusing on 13C-labeled pyruvate. Efficient RF and acquisition strategies that include variable flip angles, spectral-spatial RF pulses that address problems with chemical shift displacement artifacts, hyperpolarized slice profile distortions, and the effects of magnetic field inhomogeneity are presented. Acquisition strategies covered include MR spectroscopic imaging with phase encoding and time-dependent readout gradient trajectories, model-based methods, metabolite-specific imaging, steady-state free-precession, and ultrafast two-dimensional NMR. The integration of compressed sensing and parallel imaging into hyperpolarized MR for acceleration is also examined. Keywords: hyperpolarized pulse sequences; metabolic imaging; molecular imaging; variable flip angles; spectral-spatial RF pulses; rapid spectroscopic imaging; metabolite-specific imaging; ultrafast 2-D NMR