Numerical solution of the Bloch equations provides insights into the optimum design of PARACEST agents for MRI
Numerical solution of the Bloch equations provides insights into the optimum design of PARACEST agents for MRI
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DOI：
10.1002/mrm.20408
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发表时间：
2005-04-01
影响因子：
3.3
通讯作者：
Sherry, AD
中科院分区：
文献类型：
--
作者：
Woessner, DE;Zhang, SR;Sherry, AD
利用化学交换饱和转移(CEST)技术，顺磁性稀土络合物可以作为一类新的磁共振造影剂，表现出内球配位与主体水之间异常缓慢的水交换。为了帮助设计顺磁性CEST试剂以报告MRI测量中的重要生物学指标，我们基于改进的Bloch方程建立了一个理论框架，该框架将CEST试剂的化学性质(例如水交换率和结合水化学位移)和各种核磁共振参数(例如松弛速率和施加的B场)与所测量的CEST效应联系起来。对于复杂的交换系统，这种形式的数值解很容易得到，而不需要进行代数处理或简化。对于这种类型的顺磁性CEST试剂，CEST效应对束缚质子弛豫时间相对不敏感，但需要足够大的外加B-1场来高度饱和Ln(3+)束缚的水质子。这反过来又需要具有大的Ln(3+)结合水化学位移的顺磁性络合物，以避免交换的主体水质子的直接激发。虽然增加束缚质子的交换率增强了CEST效应，但这也会导致交换展宽并增加饱和所需的B。对于给定的B，存在一个产生最大CEST效应的最优汇率。将该方法应用于一个三池情形，并将其应用于一个非线性最小二乘优化程序，得到的结果与用含有两种不同类型结合质子(结合水和酰胺质子)的顺磁性CEST试剂水溶液得到的Z谱的实验结果非常吻合。《医学评论》53：790-799,2005。(C)2005年Wiley-Liss，Inc.
Paramagnetic lanthanide complexes that display unusually slow water exchange between an inner sphere coordination site and bulk water may serve as a new class of MRI contrast agents with the use of chemical exchange saturation transfer (CEST) techniques. To aid in the design of paramagnetic CEST agents for reporting important biological indices in MRI measurements, we formulated a theoretical framework based on the modified Bloch equations that relates the chemical properties of a CEST agent (e.g., water exchange rates and bound water chemical shifts) and various NMR parameters (e.g., relaxation rates and applied B, field) to the measured CEST effect. Numerical solutions of this formulation for complex exchanging systems were readily obtained without algebraic manipulation or simplification. For paramagnetic CEST agents of the type used here, the CEST effect is relatively insensitive to the bound proton relaxation times, but requires a sufficiently large applied B-1 field to highly saturate the Ln(3+) -bound water protons. This in turn requires paramagnetic complexes with large Ln(3+)-bound water chemical shifts to avoid direct excitation of the exchanging bulk water protons. Although increasing the exchange rate of the bound protons enhances the CEST effect, this also causes exchange broadening and increases the B, required for saturation. For a given B, there is an optimal exchange rate that results in a maximal CEST effect. This numerical approach, which was formulated for a three-pool case, was incorporated into a MATLAB nonlinear least-square optimization routine, and the results were in excellent agreement with experimental Z-spectra obtained with an aqueous solution of a paramagnetic CEST agent containing two different types of bound protons (bound water and amide protons). Magn Reson Med 53: 790-799, 2005. (c) 2005 Wiley-Liss, Inc.