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

Abstract

Purpose: To develop a short TR, short TE, large flip angle (LFA), in vivo 31 P MR spectroscopy (MRS) technique at 3T that selectively maximizes the signal-to-noise ratio (SNR) of long T 1 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 31 P MRS signal, by as much as 93%, of human brain metabolites that have T 1 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 T 1 31 P brain metabolites at a given TE and TR. LFAs ranging between 120°and 150°are shown to maximize the 31 P signal of human brain metabolites at 3T. OVER THE PAST DECADE, the 31 P nucleus has been used to study bipolar disorder-a neuropsychiatric disease that can be difficult to diagnose and affects about three million Americans over their lifetime (1). Since studies at 1.5T and 2T have shown that bipolar subjects have decreased membrane phospholipid precursor levels in their temporal (2) and frontal lobes (3), and a deficit of high-energy phosphates in their frontal lobes (4), as compared to normal subjects, the development of a quantitative technique that evaluates these particular metabolites is crucial for diagnosis. However, 31 P MR spectroscopy (MRS) quantitative techniques are confounded by low signal-to-noise ratio (SNR) and poor spatial resolution. These problems result from the inherently low sensitivity of 31 P nuclei and the long T 1 times of the metabolites, which necessitate long data acquisition times. Additionally, the low brain metabolite concentrations, as well as the short T 2 relaxation times of the 31 P metabolites, contribute to the low SNR of 31 P MRS. To address these issues, our approach was to develop a clinically feasible large flip angle (LFA) spin-echo technique at 3 T that maximizes the SNR of relevant metabolites.An increase in the field strength from 1.5T to 3T is expected to yield a two-fold gain in SNR with better spectral separation. T 1 relaxation values of 31 P brain metabolites range from 0.6 to 3 seconds (5,6) at 1.5T, and since T 1 s typically lengthen as field strength increases, the LFA technique should be very useful at the increased field strength of 3T. LFA 1 H spin-echo imaging has been shown to selectively increase the signal of ...

Alpha-180 spin-echo based line-scanning method for high resolution laminar-specific fMRI

“…This result was possibly caused by the long T1 effect (~2200 ms) in SELINE acquisition with a short TR (200 ms) 24 . As same as the theoretical predictions based on the Bloch equation 22,23 , e ! "#/"% was almost close to one and thus, the maximum intensity at the optimal flip angle wasn't changed much.…”

“…We tried to shorten TR by adjusting excitation flip angle. Based on Bloch equation 22,23 , we have estimated the appropriate angles with a short TR (i.e., 200 ms). Our results show the feasibility of the fast SELINE method which has a good sampling capability capturing dynamic BOLD signals from superficial to deeper layers.…”

“…We performed BOLD fMRI experiments with 200 ms time of repetition (TR) by applying optimized flip angles based on the Bloch equation 22,23 . For comparison, we also performed GELINE method in the same anesthetized rat.…”

Avaliação de pacientes com esclerose mesial temporal pela espectroscopia de fósforo por ressonância magnética em aparelho 3T

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