Figure 2. Weighting of the Glutamate Spectrum by Different Contributions from Astrocyte and Neurons.

The variables a – g are illustrated. Glutamate derived from astrocyte may contain up to 9 resonances in the its C4 resonance because both [1,2-¹³C]acetate and [1,6-¹³C]glucose may be oxidized in astrocyte. The relative areas of the glutamate labeled in positions 3, 4 and 5 is defined as “a”, glutamate labeled in position 4 and 5 as “b”, glutamate labeled in position 3 and 4 as “c”, and labeled in 4 only as “d”. Similarly, the relative areas of glutamate derived from neurons labeled in position 3 and 4 is defined as “e” and the glutamate labeled in position 4 only as “f”. The observed glutamate spectrum is the sum of glutamate derived from astrocyte plus the glutamate derived from neurons, weighted by the contribution from neurons, termed “g”. A similar analysis applies to interpretation of the glutamine spectrum.