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. 2022 Feb;87(2):589-596.
doi: 10.1002/mrm.29008. Epub 2021 Sep 14.

Influence of editing pulse flip angle on J-difference MR spectroscopy

Sofie Tapper  1   2 , Steve C N Hui  1   2 , Muhammad G Saleh  3 , Helge J Zöllner  1   2 , Georg Oeltzschner  1   2 , Jamie Near  4   5 , Brian J Soher  6 , Richard A E Edden  1   2
Affiliations

Influence of editing pulse flip angle on J-difference MR spectroscopy

Sofie Tapper et al. Magn Reson Med. 2022 Feb.

Abstract

Purpose: To investigate the editing-pulse flip angle (FA) dependence of editing efficiency and ultimately to maximize the edited signal of commonly edited MR spectroscopy (MRS) signals, such as gamma-aminobutyric acid (GABA) and lactate.

Methods: Density-matrix simulations were performed for a range of spin systems to find the editing-pulse FA for maximal editing efficiency. Simulations were confirmed by phantom experiments and in vivo measurements in 10 healthy participants using a 3T Philips scanner. Four MEGA-PRESS in vivo measurements targeting GABA+ and lactate were performed, comparing the conventional editing-pulse FA (FA = 180°) to the optimal one suggested by simulations (FA = 210°).

Results: Simulations and phantom experiments show that edited GABA and lactate signals are maximal at FA = 210°. Compared to conventional editing (FA = 180°), in vivo signals from GABA+ and lactate signals increase on average by 8.5% and 9.3%, respectively.

Conclusion: Increasing the FA of editing-pulses in the MEGA-PRESS experiment from 180° to 210° increases the edited signals from GABA+ and lactate by about 9% in vivo.

Keywords: MEGA-PRESS; edited MRS; flip angle; optimization.

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Figures

Figure 1:
Figure 1:
Results from the simulations. A. Concatenated edit-ON spectra for GABA and co-edited metabolites. B. The normalized edit-ON integrals computed for GABA, and the co-edited metabolites Gln, Glu and Lys, simulated for GABA editing with the editing pulse at 1.9 ppm. C. Concatenated edit-ON spectra for some other editable metabolites GSH, Lac, and PE. D. The normalized edit-ON integrals computed for the other editable metabolites. For all investigated spin-systems, the maximal integral was achieved for flip angles between 200° and 210°.
Figure 2:
Figure 2:
Simulated difference spectra computed for GABA, lysine, and lactate, comparing an edit flip angle of 180° to 210°. The calculated integral areas were larger using a 210° flip angle.
Figure 3:
Figure 3:
Phantom measurements compared to the simulations. A. The normalized edit-ON integrals and fitted curves for the water calibration experiment and the GABA measurements adjusted based on the water calibration. B. The resulting concatenated GABA edit-ON spectra for the simulations and GABA phantom data. The actual editing flip angle was calculated using the results from the water calibration experiment. The gray lines plot the results of confirmatory simulations, for different editing pulse shapes and sequence timings, performed in FID-A (light) and Vespa-Simulation (dark), illustrating maximal edit-ON integral above flip angle 180°.
Figure 4:
Figure 4:
In vivo comparison of 180° and 210° flip angle. A. The voxel (30 × 30 × 30 mm3) was placed in the medial parietal lobe. B. GABA+-edited difference spectra. C. Ladc-edited difference spectra. D. The water-referenced GABA+ signal is higher for 210° FA than 180° FA in 8/10 participants. E. The water-referenced Lac signals is also higher in 8/10 participants.
See this image and copyright information in PMC

References

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