Simulation of coupled-spin systems in the steady-state free-precession acquisition mode for fast magnetic resonance (MR) spectroscopic imaging

The steady-state free-precession (SSFP) acquisition mode may be found useful for fast in vivo proton magnetic resonance spectroscopic imaging in high-field MR systems because of the achievable signal-to-noise ratio and the avoidance of RF pulses with large flip angles. Detection of signals from metabolites with coupled-spin systems under SSFP has not yet been accomplished, but should be possible in high field, albeit with substantial signal truncation. It must be expected that the spin system evolution and the spectra will be affected by the steady-state conditions, which prevent the spin systems from returning to the Boltzmann equilibrium. Computer simulation is needed for the experiment design and spectrum quantification. This work outlines a suitable simulation method (QuaM-EPG), which combines and extends two pre-existing approaches: the density matrix calculation, used in high-resolution NMR, and the extended phase graph method, used to describe cyclic excitation in fast MRI of water protons. The method is illustrated by its application to model molecules and myo-inositol, which is one of the clinically relevant target molecules. It is shown that antiphase and multiple-quantum coherences may represent a considerable portion of the steady-state magnetization in a quantum-mechanical sense and that the spectral patterns are affected thereby.