Scharfenecker A, Ardelean I, Kimmich R
Sektion Kernresonanzspektroskopie, Universität Ulm, 89069 Ulm, Germany.
J Magn Reson. 2001 Feb;148(2):363-6. doi: 10.1006/jmre.2000.2250.
Nutation echoes are generated by radiofrequency (RF) pulses with an inhomogeneous amplitude, B(1) = B(1)(r), in inhomogeneous magnetic fields, B(0) = B(0)(r). The two gradients of strengths G(1) and G(0), respectively, must be aligned in parallel for a maximum echo signal. After two RF pulses, two echoes appear at times tau(a) = 2 tau(1) + tau(2) + (G(1)/G(0))tau(1) and tau(b) = 2 tau(1) + tau(2) + 2(G(1)/G(0))tau(1), where tau(1) is the RF pulse duration and tau(2) the interpulse interval. It is shown that these echoes can favorably be employed for the determination of self-diffusion coefficients even in the poor experimental situation one often faces in low-resolution or low-field NMR. The signal intensity is comparable to that of ordinary Hahn echoes. Diffusion coefficients and spin-lattice relaxation times can be evaluated from the same experimental data set if both nutation echoes are recorded. Test experiments are in good agreement with literature data. Applications of the technique to "inside out" NMR, well logging NMR, surface coil NMR, toroid cavity NMR, etc., are suggested.
章动回波由射频（RF）脉冲产生，该脉冲在不均匀磁场B(0) = B(0)(r)中具有不均匀幅度B(1) = B(1)(r)。强度分别为G(1)和G(0)的两个梯度必须平行排列，以获得最大回波信号。在两个RF脉冲之后，在时间tau(a) = 2 tau(1) + tau(2) + (G(1)/G(0))tau(1)和tau(b) = 2 tau(1) + tau(2) + 2(G(1)/G(0))tau(1)处出现两个回波，其中tau(1)是RF脉冲持续时间，tau(2)是脉冲间隔。结果表明，即使在低分辨率或低场核磁共振中经常遇到的恶劣实验条件下，这些回波也可有利地用于确定自扩散系数。信号强度与普通哈恩回波相当。如果记录了两个章动回波，则可以从同一实验数据集评估扩散系数和自旋 - 晶格弛豫时间。测试实验与文献数据吻合良好。建议将该技术应用于“由内向外”核磁共振、测井核磁共振、表面线圈核磁共振、环形腔核磁共振等。
