Chemical exchange saturation transfer (CEST) enables detection of molecules such as glycogen, whose concentrations are too low to impact the signal intensity of standard MR imaging. Detection of these molecules is achieved by selectively saturating a molecule of interest and by measuring the reduction in water signal due to saturation transfer. CEST effects are dependent on parameters such as CEST agent concentration, pH, temperature, relaxation rate, magnetic field strength as well as on experimental parameters such as repetition time, RF irradiation amplitude, and the imaging readout scheme. Measurement of molecules with exchangeable protons that resonate very close to water, e.g., hydroxyl groups in glycogen, is challenging especially at lower magnetic field strengths, mainly due to the effect of direct water saturation. Therefore, optimal RF irradiation parameters that maximize the CEST signal and reduce the competing factors are important for better quantification of glycogen-weighted CEST effects. In this study, analytical solution of the Bloch-McConnell equations was used to find optimal continuous wave RF irradiation parameters for detection of glycogen. In vivo tests were performed on a human calf muscle at different saturation powers to validate the optimal saturation parameters determined via simulation. The selected parameters were applied in vitro to CEST measurements in a phantom with varying glycogen concentrations and also in vivo in a human calf muscle. Our results show the possibility of detecting glycogen using CEST MRI at 3 T. It is further shown that the glycoCEST signal can be maximized by optimizing the RF pulse irradiation parameters (duration and power) and that different glycogen concentrations can be identified when applying the optimized saturation pulse.

中文翻译：

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化学交换饱和转移 MRI 用于糖原 (glycoCEST) 检测的最佳连续波射频辐射参数

化学交换饱和转移 (CEST) 能够检测糖原等分子，其浓度太低而无法影响标准 MR 成像的信号强度。这些分子的检测是通过选择性地使感兴趣的分子饱和并通过测量由于饱和转移引起的水信号的减少来实现的。CEST 效应取决于 CEST 试剂浓度、pH、温度、弛豫率、磁场强度等参数以及重复时间、RF 辐射幅度和成像读出方案等实验参数。测量具有非常接近水共振的可交换质子的分子（例如糖原中的羟基）具有挑战性，尤其是在较低磁场强度下，这主要是由于直接水饱和的影响。所以，最大化 CEST 信号并减少竞争因素的最佳 RF 辐射参数对于更好地量化糖原加权 CEST 效应非常重要。在本研究中，Bloch-McConnell 方程的解析解用于寻找检测糖原的最佳连续波射频辐射参数。以不同的饱和度对人体小腿肌肉进行了体内测试，以验证通过模拟确定的最佳饱和度参数。选定的参数在体外应用于具有不同糖原浓度的体模中的 CEST 测量以及在人小腿肌肉中的体内测量。我们的结果显示了在 3 T 下使用 CEST MRI 检测糖原的可能性。