Background
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.
Methods
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.
Results
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.
Conclusions
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.
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