Considerations for hyperpolarized 13C MR at reduced field: Comparing 1.5T versus 3T

Purpose: In contrast to conventional MR, signal-to-noise ratio (SNR) is not linearly dependent on field strength in hyperpolarized MR, as polarization is generated outside the MR system. Moreover, field inhomogeneity-induced artifacts and other practical limitations associated with field strengths =3T are alleviated at lower fields. The potential of hyperpolarized 13C spectroscopy and imaging at 1.5T versus 3T is demonstrated in silico, in vitro, and in vivo for applications on clinical MR systems. Theory and Methods: Theoretical noise and SNR behavior at different field strengths are investigated based on simulations. A thorough field comparison between 1.5T and 3T is performed using thermal and hyperpolarized 13C spectroscopy and imaging. Cardiac in vivo data is obtained in pigs using hyperpolarized [1-13C]pyruvate spectroscopy and imaging at 1.5T and 3T. Results: Based on theoretical considerations and simulations, the SNRof hyperpolarized MR at identical acquisition bandwidths is independent of the field strength for typical coil setups, while adaptively changing the acquisition bandwidth proportional to the static magnetic field allows for net SNR gains of up to 40% at 1.5T compared to 3T. In vitro 13C data verified these considerations with less than 7% deviation. In vivo feasibility of hyperpolarized [1-13C]pyruvate dynamic metabolic spectroscopy and imaging at 1.5T is demonstrated in the pig heart with comparable SNR between 1.5T and 3T while B0 artifacts are noticeably reduced at 1.5T. Conclusion: Hyperpolarized 13C MR at lower field strengths is favorable in terms of SNR and off-resonance effects, which makes 1.5T a promising alternative to 3T, especially for clinical cardiac metabolic imaging.