High-resolution echo-planar spectroscopic imaging at ultra-high field.

Abstract

MR spectroscopic imaging (MRSI) at ultra-high field (≥7T) benefits from improved sensitivity that allows the detection of low-concentration metabolites in the brain. However, optimized acquisition techniques are required to overcome inherent limitations of MRSI at ultra-high field. This work describes an optimized method for fast high-resolution 1 H-MRSI of the brain at 7T. The proposed acquisition sequence combines precise volume localization using semi-localization by adiabatic selective refocusing, fast spatial encoding using high-bandwidth symmetric echo-planar spectroscopic imaging (EPSI), and robust water suppression with variable power and optimized relaxation delays. This showed improved robustness to B0 and B1+ inhomogeneities, eddy currents, nuisance signal contamination and system instabilities. Furthermore, a method for correction of phase inconsistencies in symmetric EPSI enabled high-bandwidth measurements at 7T. The proposed correction effectively removed spectral ghosting using a single-shot water reference scan. This framework was tested in healthy volunteers at 7T and spectral quality was compared with lower-spatial-resolution scans, measured at 3T using the same methodology. A gain in the signal-to-noise ratio (SNR) per unit volume and unit time of 1.57 was achieved, keeping acquisition time short (5min) and the specific absorption rate within the permitted limits. This SNR enhancement obtained at ultra-high field enabled high-resolution (0.25-0.375mL) metabolite mapping of the brain within a clinically feasible scan time. The correlation of the reconstructed maps with anatomical structures was observed, showing the diagnostic potential of the technique.