Kingsley PB, Monahan WG
Department of Radiology, North Shore University Hospital, 300 Community Drive, Manhasset, New York, 11030, USA.
J Magn Reson. 2000 Apr;143(2):360-75. doi: 10.1006/jmre.2000.2018.
In the presence of an off-resonance radiofrequency field, recovery of longitudinal magnetization to a steady state is not purely monoexponential. Under reasonable conditions with zero initial magnetization, recovery is nearly exponential and an effective relaxation rate constant R(1eff) = 1/T(1eff) can be obtained. Exact and approximate formulas for R(1eff) and steady-state magnetization are derived from the Bloch equations for spins undergoing cross-relaxation and chemical exchange between two sites in the presence of an off-resonance radiofrequency field. The relaxation formulas require that the magnetization of one spin is constant, but not necessarily zero, while the other spin relaxes. Extension to three sites with one radiofrequency field is explained. The special cases of off-resonance effects alone and with cross-relaxation or chemical exchange, cross-relaxation alone, and chemical exchange alone are compared. The inaccuracy in saturation transfer measurements of exchange rate constants by published formulas is discussed for the creatine kinase reaction. Copyright 2000 Academic Press.
在存在失谐射频场的情况下，纵向磁化强度恢复到稳态并非纯粹的单指数形式。在初始磁化强度为零的合理条件下，恢复过程近似为指数形式，并且可以得到一个有效的弛豫速率常数R(1eff)=1/T(1eff)。从布洛赫方程推导出了在存在失谐射频场时，自旋在两个位点之间经历交叉弛豫和化学交换情况下的R(1eff)和稳态磁化强度的精确公式和近似公式。弛豫公式要求一个自旋的磁化强度保持恒定，但不一定为零，而另一个自旋发生弛豫。解释了扩展到具有一个射频场的三个位点的情况。比较了仅存在失谐效应、同时存在交叉弛豫或化学交换、仅存在交叉弛豫以及仅存在化学交换的特殊情况。针对肌酸激酶反应，讨论了已发表公式在交换速率常数饱和转移测量中的不准确性。版权所有2000年学术出版社。
