Lactate saturation limits bicarbonate detection in hyperpolarized 13C-pyruvate MRI of the brain
Nikolaj Bøgh, James T. Grist, Camilla W. Rasmussen, Lotte B. Bertelsen, Esben S. S. Hansen, Jakob U. Blicher, Damian J. Tyler, Christoffer Laustsen
Abstract
Purpose
To investigate the potential effects of [1-13C]lactate RF saturation pulses on [13C]bicarbonate detection in hyperpolarized [1-13C]pyruvate MRI of the brain.
Methods
Thirteen healthy rats underwent MRI with hyperpolarized [1-13C]pyruvate of either the brain (n = 8) or the kidneys, heart, and liver (n = 5). Dynamic, metabolite-selective imaging was used in a cross-over experiment in which [1-13C]lactate was excited with either 0° or 90° flip angles. The [13C]bicarbonate SNR and apparent [1-13C]pyruvate-to-[13C]bicarbonate conversion (k PB) were determined. Furthermore, simulations were performed to identify the SNR optimal flip-angle scheme for detection of [1-13C]lactate and [13C]bicarbonate.
Results
In the brain, the [13C]bicarbonate SNR was 64% higher when [1-13C]lactate was not excited (5.8 ± 1.5 vs 3.6 ± 1.3; 1.2 to 3.3–point increase; p = 0.0027). The apparent k PB decreased 25% with [1-13C]lactate saturation (0.0047 ± 0.0008 s−1 vs 0.0034 ± 0.0006 s−1; 95% confidence interval, 0.0006–0.0019 s−1 increase; p = 0.0049). These effects were not present in the kidneys, heart, or liver. Simulations suggest that the optimal [13C]bicarbonate SNR with a TR of 1 s in the brain is obtained with [13C]bicarbonate, [1-13C]lactate, and [1-13C]pyruvate flip angles of 60°, 15°, and 10°, respectively.
Conclusions
Radiofrequency saturation pulses on [1-13C]lactate limit [13C]bicarbonate detection in the brain specifically, which could be due to shuttling of lactate from astrocytes to neurons. Our results have important implications for experimental design in studies in which [13C]bicarbonate detection is warranted.