Song S K, Hotchkiss R S, Ackerman J J
Department of Chemistry, Washington University, St. Louis, Missouri 63130-4899.
Magn Reson Med. 1992 May;25(1):45-55. doi: 10.1002/mrm.1910250105.
In a series of three papers, we demonstrate and validate an approach for concurrent absolute quantification in situ of blood flow and energy metabolism with a modification of the NMR method for absolute concentration determination put forth by Thulborn and Ackerman [J. Magn. Reson. 55, 357 (1983)] and later expanded upon by Tofts and Wray. In this first paper of the series, we briefly review the theoretical basis for the concentration measurement and present, for the first time, a successful paired validation of metabolite quantification via 31P surface-coil NMR through corroborative in vitro enzymatic assays. The paired radiolabeled microsphere validation of blood flow measurement via 2H surface-coil NMR employing D2O as a freely diffusible tracer and the concurrent determination of blood flow and energy metabolism in a septic rat model are presented in the accompanying second and third paper to complete the series. In this article a classical RF tank circuit is employed to describe the effect of conductive sample loading on the NMR receiver by considering its apparent series resistance. It is shown in an easily visualized generalizable manner that the effect of sample loading on the observed NMR signal intensity can be accounted for quantitatively by monitoring changes in 90 degrees pulse width at constant power at a fixed reference point, i.e., Ssample = Sphantom (PW90phantom/PW90sample). In a series of paired experiments the absolute concentrations of high energy phosphates obtained from resting rat leg muscle (n = 4) in situ (NMR) and in vitro (enzymatic) were determined as follows: [PCr]NMR = 17.2 +/- 0.8 SD, [PCr]enzymatic = 17.3 +/- 2 SD, [ATP]NMR = 5.1 +/- 0.8 SD, [ATP]enzymatic = 5.0 +/- 0.2 SD mmol/kg tissue wet wt. Results of these two independent methods of concentration determination were not statistically different (P = 0.94 and P = 0.74 respectively) and serve to rigorously validate the Thulborn approach for absolute quantification of phosphorous metabolites in situ via NMR. Furthermore, these results strongly suggest that ATP and PCr in resting rat leg muscle under normal physiologic conditions are 100% NMR visible. The free cytosolic [ADP]NMR was estimated from the creatine kinase reaction equilibrium expression to be 0.022 +/- 0.003 SD mmol/kg tissue wet wt.
在三篇系列论文中，我们展示并验证了一种用于同时原位绝对定量血流和能量代谢的方法，该方法对Thulborn和Ackerman [《磁共振杂志》55, 357 (1983)] 提出的用于绝对浓度测定的核磁共振方法进行了改进，随后Tofts和Wray又对其进行了扩展。在该系列的第一篇论文中，我们简要回顾了浓度测量的理论基础，并首次通过确证性体外酶促分析，成功地对经由31P表面线圈核磁共振进行的代谢物定量进行了配对验证。通过使用D2O作为自由扩散示踪剂，经由2H表面线圈核磁共振对血流测量进行的配对放射性标记微球验证，以及在脓毒症大鼠模型中对血流和能量代谢的同时测定，将在随附的第二篇和第三篇论文中给出，以完成该系列。在本文中，通过考虑其表观串联电阻，采用经典的射频振荡回路来描述导电样品负载对核磁共振接收器的影响。结果表明，通过在固定参考点以恒定功率监测90度脉冲宽度的变化，即S样品 = S体模 (PW90体模 / PW90样品)，可以以一种易于可视化且可推广的方式，定量地解释样品负载对观察到的核磁共振信号强度的影响。在一系列配对实验中，从静息大鼠腿部肌肉（n = 4）原位（核磁共振）和体外（酶促法）获得的高能磷酸盐的绝对浓度测定如下：[磷酸肌酸]核磁共振 = 17.2 ± 0.8标准差，[磷酸肌酸]酶促法 = 17.3 ± 2标准差，[三磷酸腺苷]核磁共振 = 5.1 ± 0.8标准差，[三磷酸腺苷]酶促法 = 5.0 ± 0.2标准差 mmol/kg组织湿重。这两种独立浓度测定方法的结果在统计学上无差异（分别为P = 0.94和P = 0.74），并严格验证了Thulborn通过核磁共振原位绝对定量磷代谢物的方法。此外，这些结果有力地表明，在正常生理条件下，静息大鼠腿部肌肉中的三磷酸腺苷和磷酸肌酸在核磁共振中是100%可见的。根据肌酸激酶反应平衡表达式估算，游离胞质[二磷酸腺苷]核磁共振为0.022 ± 0.003标准差 mmol/kg组织湿重。
