Improved tolerance to off‐resonance in spectral‐spatial EPI of hyperpolarized [1‐13C]pyruvate and metabolites. Issue 3 (29th January 2018) - Lau, Justin Y. C.
Abstract : Purpose: For 13 C 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 1 H imaging using a series of increasingly dilute 1, 4‐dioxane solutions and for 13 C imaging using an ethylene glycol phantom. Hyperpolarized [1‐ 13 C]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 1 H signal ratios that were in agreement with the independent measurements by 1 H spectroscopy for all four concentrations of 1, 4‐dioxane. The 13 C 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 13 C 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.