Department of Radiology, Harvard Medical School, Charlestown, MA, USA.
Magn Reson Med. 2012 Apr;67(4):936-42. doi: 10.1002/mrm.23068. Epub 2011 Aug 12.
Chemical exchange saturation transfer MRI is an emerging imaging technique capable of detecting dilute proteins/peptides and microenvironmental properties, with promising in vivo applications. However, chemical exchange saturation transfer MRI contrast is complex, varying not only with the labile proton concentration and exchange rate, but also with experimental conditions such as field strength and radiofrequency (RF) irradiation scheme. Furthermore, the optimal RF irradiation power depends on the exchange rate, which must be estimated in order to optimize the chemical exchange saturation transfer MRI experiments. Although methods including numerical fitting with modified Bloch-McConnell equations, quantification of exchange rate with RF saturation time and power (QUEST and QUESP), have been proposed to address this relationship, they require multiple-parameter non-linear fitting and accurate relaxation measurement. Our work extended the QUEST algorithm with ratiometric analysis (QUESTRA) that normalizes the magnetization transfer ratio at labile and reference frequencies, which effectively eliminates the confounding relaxation and RF spillover effects. Specifically, the QUESTRA contrast approaches its steady state mono-exponentially at a rate determined by the reverse exchange rate (k(ws) ), with little dependence on bulk water T(1) , T(2) , RF power and chemical shift. The proposed algorithm was confirmed numerically, and validated experimentally using a tissue-like phantom of serially titrated pH compartments.
化学交换饱和传递磁共振成像是一种新兴的成像技术，能够检测稀有的蛋白质/肽和微环境特性，具有广阔的体内应用前景。然而，化学交换饱和传递磁共振对比非常复杂，不仅取决于不稳定质子的浓度和交换率，还取决于实验条件，如场强和射频（RF）照射方案。此外，最优的 RF 照射功率取决于交换率，为了优化化学交换饱和传递磁共振实验，必须对其进行估计。尽管已经提出了包括使用改良的 Bloch-McConnell 方程进行数值拟合、通过 RF 饱和时间和功率定量交换率（QUEST 和 QUESP）的方法来解决这种关系，但这些方法需要多参数非线性拟合和精确的弛豫测量。我们的工作通过比率分析（QUESTRA）扩展了 QUEST 算法，该算法对不稳定和参考频率的磁化转移比进行归一化，有效地消除了混杂的弛豫和 RF 溢出效应。具体来说，QUESTRA 对比在由反向交换率（k(ws)）决定的速率下呈单指数稳态趋近，几乎不依赖于体相水 T(1)、T(2)、RF 功率和化学位移。该算法在数值上得到了验证，并使用 pH 分室的连续滴定组织样体模进行了实验验证。
