Bottomley Paul A, Ouwerkerk Ronald, Lee Ray F, Weiss Robert G
Division of MR Research, Department of Radiology, Johns Hopkins University, Baltimore, Maryland 21287-0843, USA.
Magn Reson Med. 2002 May;47(5):850-63. doi: 10.1002/mrm.10130.
A new fast method of measuring kinetic reaction rates for two-site chemical exchange is described. The method employs saturation transfer magnetic resonance spectroscopy (MRS) and acquisition of only four spectra under partially saturated, high signal-to-noise ratio (SNR) conditions. In two acquisitions one of the exchanging species is saturated; the other two employ a control saturation. Each pair of acquisitions is applied with two different flip angles, and the equilibrium magnetization, relaxation times, and reaction rates are calculated therefrom. This four-angle saturation transfer (FAST) method is validated theoretically using the Bloch equations modified for two-state chemical exchange. Potential errors in the rate measurements due to the effects of exchange are evaluated for creatine kinase (CK) metabolism modeled for skeletal and heart muscle, and are found to be < 5% for forward CK flux rates of 0.05 < or = k(f) < or = 1.0 s(-1), and up to a 90% depletion of phosphocreatine (PCr). The effect of too much or too little saturating irradiation on FAST appears to be comparable to that of the conventional saturation transfer method, although the relative performance deteriorates when spillover irradiation cuts the PCr signal by 50% or more. "FASTer" and " FASTest" protocols are introduced for dynamic CK studies wherein [PCr] and/or k(f) changes. These protocols permit the omission of one or two of the four acquisitions in repeat experiments, and the missing information is recreated from initial data via a new iterative algorithm. The FAST method is validated empirically in phosphorus ((31)P) MRS studies of human calf muscle at 1.5 T. FAST measurements of 10 normal volunteers yielded the same CK reaction rates measured by the conventional method (0.29 +/- 0.06 s(-1)) in the same subjects, but an average of seven times faster. Application of the FASTer algorithm to these data correctly restored missing information within seven iterations. Finally, the FAST method was combined with 1D spatially localized (31)P MRS in a study of six volunteers, yielding the same k(f) values independent of depth, in total acquisition times of 17-39 min. These timesaving FAST methods are enabling because they permit localized measurements of metabolic flux, which were previously impractical due to intolerably long scan times.
本文描述了一种测量双位点化学交换动力学反应速率的新的快速方法。该方法采用饱和转移磁共振波谱法（MRS），并在部分饱和、高信噪比（SNR）条件下仅采集四个谱图。在两次采集中，使一种交换物质饱和；另外两次采用对照饱和。每对采集应用两个不同的翻转角，并据此计算平衡磁化强度、弛豫时间和反应速率。这种四角度饱和转移（FAST）方法通过对用于双态化学交换的布洛赫方程进行修正，从理论上进行了验证。针对以骨骼肌和心肌为模型的肌酸激酶（CK）代谢，评估了由于交换效应导致的速率测量中的潜在误差，发现对于0.05≤k(f)≤1.0 s(-1)的正向CK通量速率以及高达90%的磷酸肌酸（PCr）消耗，误差<5%。尽管当溢出照射使PCr信号降低50%或更多时，相对性能会下降，但过多或过少的饱和照射对FAST的影响似乎与传统饱和转移方法相当。针对[PCr]和/或k(f)发生变化的动态CK研究，引入了“FASTer”和“FASTest”方案。这些方案允许在重复实验中省略四次采集中的一次或两次，并且通过一种新的迭代算法从初始数据中重建缺失的信息。FAST方法在1.5 T下对人体小腿肌肉进行的磷（(31)P）MRS研究中得到了实证验证。对10名正常志愿者进行的FAST测量得出的CK反应速率与同一受试者通过传统方法测量的结果相同（0.29±0.06 s(-1)），但平均速度快了七倍。将FASTer算法应用于这些数据，在七次迭代内正确恢复了缺失的信息。最后在一项针对六名志愿者的研究中，FAST方法与一维空间定位(31)P MRS相结合，在17 - 39分钟的总采集时间内，得到了与深度无关的相同k(f)值。这些节省时间的FAST方法具有可行性，因为它们允许对代谢通量进行局部测量，而这在以前由于扫描时间长得令人无法忍受而不切实际。
