Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, USA.
NMR Biomed. 2013 Jul;26(7):829-38. doi: 10.1002/nbm.2874. Epub 2012 Oct 11.
Paramagnetic chemical exchange saturation transfer (PARACEST) complexes are exogenous contrast agents that have great potential to further extend the functional and molecular imaging capabilities of magnetic resonance. As a result of the presence of a central paramagnetic lanthanide ion (Ln(3+) ≠ La(3+) , Gd(3+) , Lu(3+) ) within the chelate, the resonance frequencies of exchangeable protons bound to the PARACEST agent are shifted far away from the bulk water frequency. This large chemical shift, combined with an extreme sensitivity to the chemical exchange rate, make PARACEST agents ideally suited for the reporting of significant biological metrics, such as temperature, pH and the presence of metabolites. In addition, the ability to turn PARACEST agents 'off' and 'on' using a frequency-selective saturation pulse gives them a distinct advantage over Gd(3+) -based contrast agents. A current challenge for PARACEST research is the translation of the promising in vitro results into in vivo systems. This short review article first describes the basic theory behind PARACEST contrast agents, their benefits over other contrast agents and their applications to MRI. It then describes some of the recent PARACEST research results: specifically, pH measurements using water molecule exchange rate modulation, T2 exchange contrast caused by water molecule exchange, the use of ultrashort TEs (TE < 10 µs) to overcome T2 exchange line broadening and the potential application of T2 exchange as a new contrast mechanism for MRI.
顺磁化学交换饱和转移（PARACEST）复合物是外源性对比剂，具有极大的潜力进一步扩展磁共振的功能和分子成像能力。由于螯合物内存在中心顺磁镧系离子（Ln(3+) ≠ La(3+)、Gd(3+)、Lu(3+)），与 PARACEST 试剂结合的可交换质子的共振频率远远偏离体相水的频率。这种大的化学位移，加上对化学交换率的极端敏感性，使 PARACEST 试剂非常适合报告重要的生物学指标，如温度、pH 值和代谢物的存在。此外，使用频率选择性饱和脉冲来开启和关闭 PARACEST 试剂的能力使它们与基于 Gd(3+)的对比剂相比具有明显的优势。PARACEST 研究的当前挑战是将有前途的体外结果转化为体内系统。本文首先描述了 PARACEST 对比剂的基本理论、它们相对于其他对比剂的优势及其在 MRI 中的应用。然后描述了一些最近的 PARACEST 研究结果：具体来说，使用水分子交换率调制进行 pH 值测量、水分子交换引起的 T2 交换对比、使用超短回波时间（TE < 10µs）克服 T2 交换线增宽以及 T2 交换作为 MRI 新对比机制的潜在应用。
