Figure 4.

Kinetic and relaxation rate constants after administration of 15.0 mM hyperpolarized [1-¹³C]pyruvate to viable rat brain slices.(a) The integrated intensities of [1-¹³C]pyruvate (Pyr) (●) [1-¹³C]lactate (Lac) (▲), and [¹³C]bicarbonate (Bic) (x) and their fit to the kinetic model (dashed lines). Tissue settling was not considered and therefore all time points from the completion of the injection, i.e. from 10 s onwards were fit. Fitting this time range yielded T_1,pyr = 30 s, T_1,lac = 53 s and T_1,bic = 24 s. Assuming that 0.5 ml of the volume detected by the coil is occupied by 15 mM [1-¹³C]pyruvate solution (and about 0.9 ml are occupied by the tissue) metabolic rates of v_lac = 4.2 nmole/g/s and v_bic = 0.3 nmole/g/s can be determined (R²_pyr = 0.98, R²_lac = 0.99, R²_bic = 0.88). (b) The integrated intensities of the same signals and their fit to the kinetic model (dashed lines) taking into consideration the tissue settling process. In order to minimize the effect of potential tissue displacement on the resultant parameters, the [1-¹³C]pyruvate signal was fit from 90 s onwards, while the [1-¹³C]lactate and [¹³C]bicarbonate signal were fit from when the deviation of the pyruvate signal from the constant decay was 25% or less, in this case from 25 s onwards. Fitting this time range yielded T_1,pyr = 50 s, T_1,lac = 30 s and T_1,bic = 13 s. Using the same assumptions as above, the metabolic rates were v_lac = 7.5 nmole/g/s and v_bic = 0.6 nmole/g/s. (R²_pyr = 1.00, R²_lac = 1.00, R²_bic = 0.91). We note that the mismatch between the early experimental data points of [1-¹³C]pyruvate and the model curve likely reflects tissue displacement in the beginning of the experiment. Tissue displacement may also explain the slight mismatch in the early [1-¹³C]lactate and [¹³C]bicarbonate time points. (c) Longitudinal relaxation time constants for the 17 injections that were analyzed. For 2 injections severely reduced SNR prevented fitting the lactate signal and for 4 additional experiments SNR was not sufficient to fit the bicarbonate signal. Without considering tissue settling we found the following T₁ parameters: T_1,pyr = 43 ± 11 s (n = 17), T_1,lac = 49 ± 11 s (n = 15) and T_1,bic = 36 ± 3 s (n = 11). When tissue settling was considered in the manner described above the following T₁ parameters were determined: T_1,pyr = 57 ± 8 s (n = 17), T_1,lac = 28 ± 5 s (n = 15) and T_1,bic = 16 ± 6 s (n = 11). *p < 0.001, **p < 0.0001.