Division of Advanced Electronics and Optical Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan.
J Magn Reson. 2010 Jun;204(2):327-32. doi: 10.1016/j.jmr.2010.03.014. Epub 2010 Mar 23.
The profile of rf pulses that nuclear spins experience inside a resonator deviates from that of rf voltage signals generated by a NMR spectrometer according to users' pulse programming, when change of the profile in time is comparable to or shorter than the time constant of the resonator. In our previous work [Takeda et al., J. Magn. Reson. 197 (2009) 242-244], we proposed active compensation of rf pulse transients, in which the amplitude transient of the rf pulse can be suppressed without sacrificing the Q factor of the probe. Here we extend the idea of active compensation toward total compensation of the amplitude as well as phase transients. By measuring the transient response of the probe to a given excitation using a pickup coil, the response function determining the transient behavior of the probe is numerically obtained. Then, by numerically solving the convolution equation with the help of Laplace transformation, one can obtain the amplitude and phase profiles of the pulse that should be programmed in the spectrometer in order to apply the rf pulses to the nuclear spins as intended. Accurate rf pulsing based on this idea is experimentally demonstrated, and prospect and requirements for coping with the receiver dead-time problem are discussed.
当腔内射频脉冲的时域轮廓变化与腔的时间常数可比拟或短于该时间常数时，其经历的射频脉冲的轮廓与根据用户脉冲编程由 NMR 谱仪生成的射频电压信号的轮廓会有所不同。在我们之前的工作中[Takeda 等人，J. Magn. Reson. 197 (2009) 242-244]，我们提出了射频脉冲瞬变的主动补偿，其中可以在不牺牲探头 Q 因子的情况下抑制射频脉冲的幅度瞬变。在这里，我们将主动补偿的思想扩展到幅度和相位瞬变的完全补偿。通过使用拾波线圈测量探头对给定激励的瞬态响应，数值获得确定探头瞬态行为的响应函数。然后，通过拉普拉斯变换的帮助数值求解卷积方程，可以获得为了将射频脉冲施加到原子核自旋上而应该在谱仪中编程的脉冲的幅度和相位轮廓。基于该思想的精确射频脉冲已在实验中得到验证，并讨论了应对接收器死区时间问题的前景和要求。
