Figure 2. Time course of enrichment of blood (¹³C acetate) and brain (¹³C HCO₃⁻) during Long Method.

[1-¹³C] acetate 3mg/kg/min was administered intravenously over 60 minutes with the subject inside a GE 1.5T clinical MR scanner. ¹³C brain spectra were acquired for 120 minutes (Long Method). Blood ¹³C acetate rose rapidly to achieve 80% fractional enrichment within the first 15 minutes. Cerebral ¹³C HCO₃⁻ concentration increased slowly with slope L, during the period of infusion to level off to a plateau P, then decreased. The mid-point of P (T_L) and its duration (T_s) (Figure 2) were established in 13 Long method examinations. This experimental data defined the short, SAGA method in which [1-¹³C] acetate infusion was completed while the subject was outside the MR scanner and inside the MR scanner only for 25.6 minutes. Glial metabolic rates in Table 1 were calculated from the slope L and the steady state, P of cerebral ¹³C bicarbonate in the Long method and the mean cerebral ¹³C bicarbonate in the short (SAGA) method.