A novel approach to probing in vivo metabolite relaxation: Linear quantification of spatially modulated magnetization.

PURPOSE: Conventional sequences for metabolite transverse relaxation quantification all generally measure signal changes at different echo times (TEs). However, quantification results obtained via these conventional methods can be very different and are highly dependent on the type of sequence being applied. TE-dependent effects such as diffusion, macromolecule baseline, and J-coupling modulation contribute significantly to these differences. Here, we propose a novel technique-multiple flip angle pulse-driven ratio of longitudinal steady states (MARzss)-for preparing magnetization with T2 /T1 weighting. Using premeasured T1 values, T2 values for metabolites can thereby be determined. The measurement procedure does not require varying TE and is TE independent; T2 , diffusion, and J-coupling effects induced by the readout sequence are cancelled.
METHOD: Longitudinal steady states at different flip angles were prepared with trains of radio frequency pulses interspersed with field gradients. The resulting spatially modulated longitudinal magnetization was acquired with a PRESS readout module. A new linear equation for quantification of MARzss was derived from Bloch equations.
RESULTS: By implementing this readout-independent method, T2 measurement of brain metabolites at 7T was demonstrated through Bloch simulations, phantom, and in vivo experiments.
CONCLUSIONS: The proposed MARzss technique can be used to largely avoid multi-TE associated interference, including diffusion, macromolecules, and J modulation. This MARzss technology, which is uniquely insensitive to readout sequence type and TE, is a promising technique for more accurately probing in vivo metabolite relaxation. Magn Reson Med 79:2491-2499, 2018.