Storage of magnetization as singlet order by optimal control designed pulses.

ABSTRACT

PURPOSE: The use of hyperpolarization to enhance the sensitivity of MRI has so far been limited by the decay of the polarization through T1 relaxation. Recently, methods have been proposed that extend the lifetime of the hyperpolarization by storing the spin order in slowly relaxing singlet states. METHODS: With this aim, optimal control theory was applied to create pulses that for near-equivalent spins accomplish transfers in and out of the singlet state with maximum efficiency while ensuring robustness toward variations in the nuclear spin system Hamiltonian (chemical shift, J-couplings, B1 and B0 magnetic field inhomogeneity). RESULTS: The pulses are designed to accomplish efficient transfer with low B1 amplitude, essential for applications on preclinical and clinical MR scanners. CONCLUSION: It is demonstrated that significantly improved efficiency and robustness can be obtained within the limitations of typical MR scanner performance.