Prospective frequency correction using outer volume suppression-localized navigator for MR Spectroscopic Imaging

Data acquisitions for magnetic resonance spectroscopic imaging (MRSI) require a long scan time to increase SNR and for spatial encoding. During the prolonged scan time, maintaining a constant static magnetic field (B0) is important for a robust MRSI measurement. However, frequency drifts occur over time even in advanced MR systems and become larger when high shim currents or rapidly switched gradients are applied. The frequency drift causes broad and distorted spectral lineshapes, reduced SNR, and quantification errors. These effects can be mitigated retrospectively and prospectively. However, in MRSI measurements, these effects can only be mitigated using the prospective frequency correction, because each spectrum is phase-encoded. The prospective frequency correction is typically achieved by incorporating a PRESS-based interleaved reference scan (PRESS-IRS) as a navigator, termed as PRESS-IRS navigator. A small excitation flip angle (10-20°) is used for the PRESS-IRS navigator to reduce the saturation-induced SNR loss on metabolite signals. Nonetheless, the SNR loss remains unavoidable and becomes notable when the imperfect refocusing pulses or a short repetition time (TR) are used in MRSI. In this study, a new prospective frequency correction method is introduced. The new method utilizes the outer volume suppression-localized navigator, termed OVS-localized navigator, resulting in no perturbations of metabolite signals and thus no saturation-induced SNR losses. Meanwhile, a precise measurement of the frequency drift and the effective correction is achieved. The presented method was demonstrated in two-dimensional (2-D) MRSI measurements under the large frequency drift induced by a fMRI experiment.