Selective maximization of ³¹P MR spectroscopic signals of in vivo human brain metabolites at 3T

Purpose: To develop a short TR, short TE, large flip angle (LFA), in vivo ³¹P MR spectroscopy (MRS) technique at 3T that selectively maximizes the signal-to-noise ratio (SNR) of long T₁ human brain metabolites implicated in bipolar disorder. Materials and Methods: Two pulse sequences were evaluated for efficiency. Slice profiles acquired with the scaled, sinc-shaped, radiofrequency (RF) LFA pulses were compared to those acquired with Shinnar-Le Roux (SLR) RF LFA pulses. The SLR-based LFA pulse sequence was used to maximize the inorganic phosphate signal in a phantom, after which volunteer metabolite signals were selectively maximized and compared to their correlates acquired with conventional spin-echo methods. Results: The comparison of slice profiles acquired with sinc-shaped RF LFA pulses vs. SLR RF LFA pulses showed that SLR-based pulse sequences, with their improved excitation and slice profiles, yield significantly better results. In vivo LFA spin-echo MRS implemented with SLR pulses selectively increased the ³¹P MRS signal, by as much as 93%, of human brain metabolites that have T₁ times longer than the TR of the acquisition. Conclusion: The data show that the LFA technique can be employed in vivo to maximize the signal of long T₁ ³¹P brain metabolites at a given TE and TR. LFAs ranging between 120° and 150° are shown to maximize the ³¹P signal of human brain metabolites at 3T.