NMR measurement of brain oxidative metabolism in monkeys using 13C-labeled glucose without a 13C radiofrequency channel

We detected glutamate C4 and C3 labeling in the monkey brain during an infusion of [U- 1 3 C 6 ]glucose, using a simple 1 H PRESS sequence without 1 3 C editing or decoupling. Point-resolved spectroscopy (PRESS) spectra revealed decreases in 1 2 C-bonded protons, and increases in 1 3 C-bonded protons of glutamate. To take full advantage of the simultaneous detection of 1 2 C- and 1 3 C-bonded protons, we implemented a quantitation procedure to property measure both glutamate C4 and C3 enrichments. This procedure relies on LCModel analysis with a basis set to account for simultaneous signal changes of protons bound to 1 2 C and 1 3 C. Signal changes were mainly attributed to 1 2 C- and 1 3 C-bonded protons of glutamate. As a result, we were able to measure the tricarboxylic acid (TCA) cycle flux in a 3.9 cm 3 voxel centered in the monkey brain on a whole-body 3 Tesla system (V T C A = 0.55 ′ 0.04 μmol.g - 1 .min - 1 , N = 4). This work demonstrates that oxidative metabolism can be quantified in deep structures of the brain on clinical MRI systems, without the need for a 1 3 C radiofrequency (RF) channel.