报告时间：2024年10月10日上午10:00 (10:00, Oct10, 2024)

报告地点：物质楼C404会议室(Room C404, Material Science Building)

报告题目：Flip-Angle Selective Pulses and Applications in NMR Spectroscopy

报告人：Prof. Riqiang Fu National High Magnetic Field Laboratory, Florida State University

报告摘要：

        Frequency selective pulses, generally achieved by shaped pulses such as Gaussian and Sinc waveform, have been widely used to select resonances within a specific bandwidth in NMR spectra. In contrast, flip-angle selective pulses have yet to be fully explored. It is well known in nuclear magnetic resonance (NMR) spectroscopy that a radio-frequency (RF) pulse generates the observable signals that follow a sine wave with respect to its pulse length p, i.e.,Sabs=sin（q）, with the assumption that 1 is much larger than any internal spin interactions. Here, the flip-angle is defined as q=n₁t_p, where 1 is the B1 field generated by the RF pulse. In 1983, Bendall and Gordon [1] introduced multiple spin-echo sequences q-[t-2q-t]_n-acq to create the flip-angle selective pulses with an excitation profile of  Sabs=sin2n+1(q ) , the so-called DEPTH profiling, in order to improve spatial discrimination (i.e., detecting signals in a given in-depth position) in the applications with surface coils. 

       In this presentation, flip-angle selective pulses are designed using a train of pulses with the same pulse length but different phases. These profiles have a minimal scaling to the signals when the flip-angle is ~ 90° but have a large scaling when the flip-angle is small. They are analogous to Bendall and Gordon’s DEPTH profiling that can only be applicable for spin-1/2 nuclei, except that these newly designed flip-angle selective pulses can be also used on quadrupolar nuclei (such as 17O, 25Mg, and 23Na). Applications include: 1) suppressing the background signals and eliminating RF acoustic ringing effects [2]; 2) selectively observing signals based on different quadrupolar couplings [2-3]; 3) differentiating highly overlapped signals that have different relaxation properties. Recent results will be presented.

References

[1]  M. R. Bendall; R. E. Gordon, Depth and Refocusing Pulses Designed for Multiple NMR with Surface Coils. J. Magn. Reson. 53 (1983) 365-385.

[2]  F. F. Wang; S. K. Ramakrishna; P. C. Sun; R. Fu, Triple-pulse excitation: An efficient way for suppressing background signals and eliminating radio-frequency acoustic ringing in direct polarization NMR experiments. J. Magn. Reson. 332 (2021) 107067.

[3]  R. Zhang; T. A. Cross; X. Peng; R. Fu, Surprising Rigidity of Functionally Important Water Molecules Buried in the Lipid Headgroup Region. J. Am. Chem. Soc. 144 (2022) 7881-7888.