Measurement of glucose concentration in the human brain
It is possible by using 13C NMR Spectroscopy.

It has been done after infusing enriched D-[1-13C]glucose in six healthy children. The results showed that at euglycemia glucose concentration is about 1 micromol/ml.



Source: Gruetter R, Novotny EJ, Boulware SD, Rothman DL, Mason GF, Shulman GI, Shulman RG, Tamborlane WV. Direct measurement of brain glucose concentrations in humans by 13C NMR spectroscopy. Proc. Natl. Acad. Sci. U.S.A. 1992 Feb 1;89(3):1109-12. PubMed PMID: 1736294.