Evaluating the use of a continuous approximation for model-based quantification of pulsed chemical exchange saturation transfer (CEST).
Evaluating the use of a continuous approximation for model-based quantification of pulsed chemical exchange saturation transfer (CEST).
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DOI：
10.1016/j.jmr.2012.07.003
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发表时间：
2012-09
影响因子：
2.2
通讯作者：
Chappell, M. A.
中科院分区：
文献类型：
--
作者：
Tee, Y. K.;Khrapitchev, A. A.;Sibson, N. R.;Payne, S. J.;Chappell, M. A.
关键词：
在脉冲CEST实验上进行定量模型分析（QMAs）。►不同方法拟合的重要模型参数(P)进行了比较。►离散化方法（缓慢）可以以较小的误差拟合实验数据。►但使用连续近似（快速）发现P的差异不显著。连续近似可用于酰胺质子转移成像中的QMA。近年来，人们对化学交换饱和转移（CEST）的许多潜在临床应用进行了研究。然而，由于各种限制，例如特定吸收率指南和扫描仪硬件限制，大多数拟议的应用尚未转化为常规诊断工具。目前，使用多个短脉冲进行饱和的脉冲CEST是唯一可行的临床转译照射方案。然而，对脉冲CEST进行基于模型的定量分析是费时的，因为必须考虑饱和脉冲的时间依赖性振幅。因此，通过计算脉冲CEST的等效平均场强或功率，通常将脉冲CEST视为连续CEST。然而，理论分析和模拟结果表明，不同的辐照方案所产生的磁化强度是不同的。在本研究中，使用基于定量模型的分析方法对脉冲CEST实验中重要的模型参数（如胺质子交换率）进行了定量分析。考虑了两种基于模型的方法——离散化方法和连续逼近方法。结果表明，离散化方法对实验数据的拟合效果明显优于连续化方法，但前者较小的拟合误差并未转化为对重要模型参数较好的拟合效果。对于内源性CEST效应的量化，例如在酰胺质子转移成像中，可以使用基于模型的方法，使用平均功率等效饱和来代替离散近似。
► Quantitative model-based analyses (QMAs) were performed on a pulsed CEST experiment. ► Important fitted model parameters (P) using different methods were compared. ► Discretization method (slow) could fit the experimental data with smaller error. ► But insignificant difference was found for P using continuous approximation (fast). ► Continuous approximation can be used for QMA in amide proton transfer imaging. Many potential clinical applications of chemical exchange saturation transfer (CEST) have been studied in recent years. However, due to various limitations such as specific absorption rate guidelines and scanner hardware constraints, most of the proposed applications have yet to be translated into routine diagnostic tools. Currently, pulsed CEST which uses multiple short pulses to perform the saturation is the only viable irradiation scheme for clinical translation. However, performing quantitative model-based analysis on pulsed CEST is time consuming because it is necessary to account for the time dependent amplitude of the saturation pulses. As a result, pulsed CEST is generally treated as continuous CEST by finding its equivalent average field or power. Nevertheless, theoretical analysis and simulations reveal that the resulting magnetization is different when the different irradiation schemes are applied. In this study, the quantification of important model parameters such as the amine proton exchange rate from a pulsed CEST experiment using quantitative model-based analyses were examined. Two model-based approaches were considered – discretized and continuous approximation to the time dependent RF irradiation pulses. The results showed that the discretized method was able to fit the experimental data substantially better than its continuous counterpart, but the smaller fitted error of the former did not translate to significantly better fit for the important model parameters. For quantification of the endogenous CEST effect, such as in amide proton transfer imaging, a model-based approach using the average power equivalent saturation can thus be used in place of the discretized approximation.
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