Improved tolerance to off-resonance in spectral-spatial EPI of hyperpolarized [1-13C]pyruvate and metabolites

Abstract

Purpose: For 13C echo-planar imaging (EPI) with spectral-spatial excitation, main field inhomogeneity can result in reduced flip angle and spatial artifacts. A hybrid time-resolved pulse sequence, multi-echo spectral-spatial EPI, is proposed combining broader spectral-spatial passbands for greater off-resonance tolerance with a multi-echo acquisition to separate signals from potentially co-excited resonances. Methods: The performance of the imaging sequence and the reconstruction pipeline were evaluated for 1H imaging using a series of increasingly dilute 1,4-dioxane solutions and for 13C imaging using an ethylene glycol phantom. Hyperpolarized [1-13C]pyruvate was administered to two healthy rats. Multi-echo data of the rat kidneys were acquired to test realistic cases of off-resonance. Results: Analysis of separated images of water and 1,4-dioxane following multi-echo signal decomposition showed water-to-dioxane 1H signal ratios that were in agreement with the independent measurements by 1H spectroscopy for all four concentrations of 1,4-dioxane. The 13C signal ratio of two co-excited resonances of ethylene glycol was accurately recovered after correction for the spectral profile of the redesigned spectral-spatial pulse. In vivo, successful separation of lactate and pyruvate-hydrate signals was achieved for all except the early time points during which signal variations exceeded the temporal resolution of the multi-echo acquisition. Conclusion: Improved tolerance to off-resonance in the new 13C data acquisition pipeline was demonstrated in vitro and in vivo. Magn Reson Med 80:925–934, 2018. © 2018 International Society for Magnetic Resonance in Medicine.