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-¹³C₆]glucose, using a simple ¹H PRESS sequence without ¹³C editing or decoupling. Point-resolved spectroscopy (PRESS) spectra revealed decreases in ¹²C-bonded protons, and increases in ¹³C-bonded protons of glutamate. To take full advantage of the simultaneous detection of ¹²C- and ¹³C-bonded protons, we implemented a quantitation procedure to properly 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 ¹²C and ¹³C. Signal changes were mainly attributed to ¹²C- and ¹³C-bonded protons of glutamate. As a result, we were able to measure the tricarboxylic acid (TCA) cycle flux in a 3.9 cm³ voxel centered in the monkey brain on a whole-body 3 Tesla system (V_TCA = 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 ¹³C radiofrequency (RF) channel.