Ouwerkerk R, Bottomley P A
Division of MR Research, Department of Radiology, Johns Hopkins University, Baltimore, Maryland 21287, USA.
J Magn Reson. 2001 Feb;148(2):425-35. doi: 10.1006/jmre.2000.2166.
Signal acquisition in most MRS experiments requires a correction for partial saturation that is commonly based on a single exponential model for T(1) that ignores effects of chemical exchange. We evaluated the errors in (31)P MRS measurements introduced by this approximation in two-, three-, and four-site chemical exchange models under a range of flip-angles and pulse sequence repetition times (T(R)) that provide near-optimum signal-to-noise ratio (SNR). In two-site exchange, such as the creatine-kinase reaction involving phosphocreatine (PCr) and gamma-ATP in human skeletal and cardiac muscle, errors in saturation factors were determined for the progressive saturation method and the dual-angle method of measuring T(1). The analysis shows that these errors are negligible for the progressive saturation method if the observed T(1) is derived from a three-parameter fit of the data. When T(1) is measured with the dual-angle method, errors in saturation factors are less than 5% for all conceivable values of the chemical exchange rate and flip-angles that deliver useful SNR per unit time over the range T(1)/5 < or = T(R) < or = 2T(1). Errors are also less than 5% for three- and four-site exchange when T(R) > or = T(1)(*)/2, the so-called "intrinsic" T(1)'s of the metabolites. The effect of changing metabolite concentrations and chemical exchange rates on observed T(1)'s and saturation corrections was also examined with a three-site chemical exchange model involving ATP, PCr, and inorganic phosphate in skeletal muscle undergoing up to 95% PCr depletion. Although the observed T(1)'s were dependent on metabolite concentrations, errors in saturation corrections for T(R) = 2 s could be kept within 5% for all exchanging metabolites using a simple interpolation of two dual-angle T(1) measurements performed at the start and end of the experiment. Thus, the single-exponential model appears to be reasonably accurate for correcting (31)P MRS data for partial saturation in the presence of chemical exchange. Even in systems where metabolite concentrations change, accurate saturation corrections are possible without much loss in SNR.
在大多数磁共振波谱（MRS）实验中，信号采集需要对部分饱和进行校正，这种校正通常基于T(1)的单指数模型，该模型忽略了化学交换的影响。我们评估了在一系列翻转角和脉冲序列重复时间（T(R)）下，这种近似在二位点、三位点和四位点化学交换模型中对(31)P MRS测量引入的误差，这些翻转角和脉冲序列重复时间能提供接近最佳的信噪比（SNR）。在二位点交换中，例如人体骨骼肌和心肌中涉及磷酸肌酸（PCr）和γ-ATP的肌酸激酶反应，针对测量T(1)的渐进饱和法和双角法确定了饱和因子的误差。分析表明，如果观察到的T(1)是通过对数据进行三参数拟合得出的，那么对于渐进饱和法，这些误差可以忽略不计。当用双角法测量T(1)时，在T(1)/5 ≤ T(R) ≤ 2T(1)范围内，对于所有能提供有用单位时间信噪比的化学交换率和翻转角的可想象值，饱和因子的误差小于5%。当T(R) ≥ T(1)(*) / 2（即代谢物的所谓“固有”T(1)）时，三位点和四位点交换的误差也小于5%。还使用了一个三位点化学交换模型研究了代谢物浓度和化学交换率变化对观察到的T(1)和饱和校正的影响，该模型涉及骨骼肌中ATP、PCr和无机磷酸盐，PCr消耗高达95%。尽管观察到的T(1)取决于代谢物浓度，但对于在实验开始和结束时进行的两次双角T(1)测量进行简单插值，对于所有交换代谢物，当T(R) = 2 s时，饱和校正的误差可以保持在5%以内。因此，在存在化学交换的情况下，单指数模型似乎对于校正(31)P MRS数据的部分饱和相当准确。即使在代谢物浓度发生变化的系统中，也可以进行准确的饱和校正，而不会在信噪比上有太大损失。
