Pyruvate metabolite maps from two [1-13C]-pyruvate injections, along with proton anatomic images, are shown in . The pyruvate images were collected 20 sec (top right) and 36 sec (bottom right) after start of injection. In both cases, pyruvate signal is readily observed in the brain, but at levels lower than intravascular space and similar to that in the surrounding muscle. At a delay of 36 sec, elevated intravascular pyruvate is also observed in the sagittal sinus vein at the top of the image. shows the 36-sec delay image as a full-grid spectroscopic display, along with pyruvate, lactate, and alanine maps. Under these conditions, the highest levels of lactate were detected in brain. Alanine was only detected in the tissue outside brain. In brain, lactate- to-pyruvate ratios are near 1:1.
FIG. 1 3-T Axial T2W proton reference images are shown in (a,c), and the corresponding color [1-13C] pyruvate images overlaid on grayscale proton anatomic images are shown on the right in (b,d). 8-M 13C-urea insert signal is projected across the chemical shift (more ...)
FIG. 2 Spectral grid display and image of rat data collected at 36 sec after the start of [1-13C] pyruvate injection (rat H88). Grid is displayed at a sampled resolution of 2.5mm × 2.5mm in plane. Color-overlaid metabolite maps have been Fourier interpolated (more ...)
provides a summary of dose, injection rate, liquid- state polarization, coil SNR, timing, and estimated concentrations. Pyruvate, lactate, and total carbon estimates were made using the 8-M 13
C urea standard (2.1 mM%), an estimate of polarization at dissolution (from solid-state calibration), an effective T1
of 65 sec for the dissolution to injection time, and a 3-T rat blood T1
of 42 sec (15
) to estimate intravascular pyruvate polarization loss. The largest error in these estimates is the actual polarization level during the image. Unaccounted losses in polarization will increase the estimates given.
Despite a rat cerebral blood volume (CBV) of only 3.6% (16
), the large vascular-to-brain pyruvate ratio predicted by the brain uptake index suggests a significant contribution from the CBV. The maximum pyruvate contribution from the CBV can be estimated, but a corrected value of brain tissue pyruvate is difficult without a concomitant measure of the pyruvate bolus shape and position relative to the imaging window. This can be an issue in any tissue during the passage of a high-concentration bolus but is especially difficult in brain due to the restricted transport through the blood-brain barrier. For the ranges of dose and injection-to-image delay times explored, the observed pyruvate levels ranged from 16 to 628 nmol g−1
. Pyruvate images collected 20 sec from the start of injection are likely to overlap with the passage of the bolus through the CBV and are necessarily difficult to quantify. Compared with pyruvate, lactate levels were much less varied over the doses and delay times studied, with a range of 86 to 217 nmol g−1
. Assuming a substantial contribution from pyruvate in the CBV, and the relatively large lactate-to-pyruvate levels observed, suggests that brain parenchymal pyruvate could be in limited supply under the conditions used.
Data from an injection of hyperpolarized [1-13C]-EP, and 2 h later hyperpolarized [1-13C]-pyruvate, in the same animal (H76) are illustrated in . In the EP metabolic images, the most intense signals are found dramatically located within the brain. As illustrated in the expanded spectra in , EP hydrate (175 parts per million (PPM)) is the most pronounced, followed by EP (163 PPM) and lactate (180 PPM). All appear to be uniformly distributed. Pyruvate (172 PPM), which is in the injected solution, as well as a product of the hydrolysis of EP, appears high in the intravascular space outside the brain, much like it is in the pyruvate-only run. Two unidentified peaks, U1 (179 PPM) and U2 (171 PPM) (trityl- catalyzed contaminants), colocate on the image and are part of the injected formulation. Despite the lower dose used in the EP injection, 0.53 μmol g−1 versus 1.06 μmol g−1 for pyruvate, and even without correction for the pyruvate in the CBV, total carbon in the brain is higher in the EP runs. The spectral display of the pyruvate run shows almost no alanine in brain but does indicate the presence of bicarbonate. In the case of EP, alanine signal is obscured by U1 and bicarbonate signal by EP. In the four EP studies, pyruvate levels of 92–119 nmol g−1 and lactate levels of 97–169 nmol g−1 were observed.
FIG. 3 a: Metabolic images of [1-13C]-EP and [1-13C]-pyruvate are compared at an image delay of 20 sec (rat H76). Metabolite maps for the EP injection are shown across the top, and maps for the pyruvate injection, across the bottom. Phased and baseline- corrected (more ...)
Since the pyruvate blood-brain barrier transport is rate limited (9
), an even longer injection-to-image delay was investigated for pyruvate. A comparison of metabolic response from [1-13
C]-pyruvate at an image delay of 45 sec, with a subsequent injection of [1-13
C]-EP with an image delay of 25 sec, was made on rats H91 and H92. The results from H91 are illustrated in . In the pyruvate run, the only significant signal within the brain is lactate, albeit with a somewhat limited distribution. As with the other EP studies, EP, EP-hydrate, and lactate are dramatically located within brain. The pyruvate map from the EP injection shows the dominant out-of-brain intensities but also shows higher relative brain content than the pyruvate-only run. This level is likely due to pyruvate hydrolyzed from brain EP or EP-hydrate, but we cannot fully discount the pharmacological effects of the coadministered EP on pyruvate transport.
FIG. 4 Metabolic images of [1-13C]-pyruvate with a 45-sec image delay and [1-13C]-EP with an image delay of 25 sec are compared (rat H91). Metabolite maps from the EP injection are shown at the top, and maps for the pyruvate injection, along the bottom. A phased (more ...)
shows a larger view of EP versus pyruvate under the best conditions afforded by the limits of this range-finding study. EP-hydrate versus pyruvate images are shown, along with their corresponding lactate metabolic maps. Pyruvate injected at a high dose, ~1 μmol g−1, with an imaging delay of 20 sec, and EP at a slow injection of 26 nmol g(rat)−1 sec−1 and an imaging delay of 25 sec, both generate good-quality metabolic images. Most of the carbon signal is outside brain in the pyruvate study, while most of the carbon signal is within the brain in the EP study. High intravascular levels, as seen in the large vessels for pyruvate () but not EP (), require a correction for cerebral blood content in the pyruvate case, but not for EP.
FIG. 5 Comparison of EP metabolic maps (rat H91) for lactate (a) and EP-hydrate (b), with pyruvate metabolic maps (rat H111) for lactate (c) and pyruvate (d) under optimum imaging conditions. High dose and image delay of 20 sec for pyruvate, slow injection rate (more ...)