At low magnetic fields (<1 T), the low loss of radio frequency (RF) coils can cause RF pulses, in magnetic resonance (MR) experiments, to suffer from long rise and fall times. When severe, long rise and fall times can result in RF pulses with undesirable and/or unexpected shapes, lengths, and amplitudes leading to inadequate spin control during an MR experiment. Experimentally, the lack of spin control becomes evident in the shape of the nutation curve and the inversion efficiency, the degree to which full inversion is accomplished following a 180° pulse. Lowering the quality factor (Q) of tuned coil is shown to reduce the duration of the rise and fall times. The effects of long rise and fall times on the spin behavior during an MR experiment is explored. It is shown that the inversion efficiency can be used to provide a threshold that ensures high fidelity pulses with adequate spin control.

在低磁场 (<1 T) 下，射频 (RF) 线圈的低损耗会导致 RF 脉冲在磁共振 (MR) 实验中出现较长的上升和下降时间。严重时，较长的上升和下降时间会导致 RF 脉冲具有不希望的和/或意外的形状、长度和幅度，从而导致 MR 实验期间自旋控制不足。在实验上，自旋控制的缺乏在章动曲线的形状和反转效率中变得明显，反转效率是在 180° 脉冲后完成完全反转的程度。降低调谐线圈的品质因数 (Q) 可减少上升和下降时间的持续时间。探讨了 MR 实验中长上升和下降时间对自旋行为的影响。