Xu Y, Tang P, Zhang W, Firestone L, Winter P M
Department of Anesthesiology, University of Pittsburgh, Pennsylvania 15261, USA.
Anesthesiology. 1995 Oct;83(4):766-74. doi: 10.1097/00000542-199510000-00016.
Determination of macroscopic and microscopic distribution of general anesthetics can facilitate identification of anatomic, cellular, and molecular loci of anesthetic action. Previous attempts to measure brain anesthetic distributions with fluorine-19 (19F) nuclear magnetic resonance (NMR) imaging were conducted at magnetic field strengths lower than 2 Tesla. All have produced only silhouettes of brain tissue. Difficulties intrinsic to NMR imaging of anesthetics include higher anesthetic solubility in extracranial tissues and the lower limits to spin-echo delay times that can be used in conventional NMR imaging methods. So far, such methods have been unable to capture rapidly decaying brain 19F NMR signals.
19F NMR imaging and spectroscopy were conducted at 4.7 Tesla using a specially developed NMR probe and new imaging methods. With the new techniques, it was possible to observe directly the uptake, distribution, and elimination in brain of sevoflurane, a fluorinated general anesthetic with special advantages for NMR investigations.
19F NMR images, acquired at different times after sevoflurane administration, clearly showed the distribution of a fluorinated general anesthetic within the brain. Based on continuous transverse relaxation time measurements, sevoflurane signals could be separated into two components, attributable respectively to sevoflurane in a mobile or immobile microenvironment. During washin, there was a delayed accumulation of anesthetic in the mobile microenvironment. During washout, there was a rapid elimination from the immobile microenvironment.
At anesthetizing concentrations, sevoflurane distributes heterogeneously in the brain. Sevoflurane in the brain tissue contributes mostly to the immobile component of the 19F signal, whereas that in the surrounding adipose and muscle tissues contributes mostly to the mobile component. Imaging and spectroscopic results suggest that the immobile component of sevoflurane is associated with the general anesthetic effects of the agent.
确定全身麻醉药的宏观和微观分布有助于识别麻醉作用的解剖学、细胞和分子位点。以往使用氟-19（19F）核磁共振（NMR）成像测量脑内麻醉药分布的尝试是在低于2特斯拉的磁场强度下进行的。所有这些尝试都只产生了脑组织的轮廓图像。麻醉药NMR成像固有的困难包括麻醉药在颅外组织中的溶解度较高，以及传统NMR成像方法中可使用的自旋回波延迟时间下限较低。到目前为止，这些方法还无法捕捉快速衰减的脑19F NMR信号。
使用专门开发的NMR探头和新的成像方法在4.7特斯拉下进行19F NMR成像和光谱分析。利用这些新技术，可以直接观察七氟烷（一种对NMR研究具有特殊优势的氟化全身麻醉药）在脑内的摄取、分布和消除情况。
在给予七氟烷后的不同时间采集的19F NMR图像清楚地显示了一种氟化全身麻醉药在脑内的分布情况。基于连续横向弛豫时间测量，七氟烷信号可分为两个成分，分别归因于处于流动或固定微环境中的七氟烷。在吸入过程中，麻醉药在流动微环境中的积累存在延迟。在呼出过程中，固定微环境中的麻醉药迅速消除。
在麻醉浓度下，七氟烷在脑内分布不均。脑组织中的七氟烷主要对19F信号的固定成分有贡献，而周围脂肪和肌肉组织中的七氟烷主要对流动成分有贡献。成像和光谱分析结果表明，七氟烷的固定成分与该药物产生的全身麻醉作用相关。
