Radial spectroscopic MRI of hyperpolarized [1-13C] pyruvate at 7 tesla

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Abstract
Purpose The transient and nonrenewable signal from hyperpolarized metabolites necessitates extensive sequence optimization for encoding spatial, spectral, and dynamic information. In this work, we evaluate the utility of radial single‐timepoint and cumulative spectroscopic MRI of hyperpolarized [1‐13C] pyruvate and its metabolic products at 7 Tesla (T). Methods Simulations of radial echo planar spectroscopic imaging (EPSI) and multiband frequency encoding (MBFE) acquisitions were performed to confirm feasibility and evaluate performance for HP 13C imaging. Corresponding sequences were implemented on a 7T small‐animal MRI system, tested in phantom, and demonstrated in a murine model of anaplastic thyroid cancer. Results MBFE provides excellent spectral separation but is susceptible to blurring and T2* signal loss inherent to using low readout gradients. The higher readout gradients and more flexible spectral encoding for EPSI result in good spatial resolution and spectral separation. Radial acquisition throughout HP signal evolution offers the flexibility for reconstructing spatial maps of mean metabolite distribution and global dynamic time courses of multiple metabolites. Conclusion Radial EPSI and MBFE acquisitions are well‐suited for hyperpolarized 13C MRI over short and long durations. Advantages to this approach include robustness to nonstationary magnetization, insensitivity to precise acquisition timing, and versatility for reconstructing dynamically acquired spectroscopic data. Magn Reson Med 72:986–995, 2014.

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