Lactate saturation limits bicarbonate detection in hyperpolarized 13 C ‐pyruvate MRI of the brain

Purpose To investigate the potential effects of [1‐ 13 C]lactate RF saturation pulses on [ 13 C]bicarbonate detection in hyperpolarized [1‐ 13 C]pyruvate MRI of the brain. Methods Thirteen healthy rats underwent MRI with hyperpolarized [1‐ 13 C]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‐ 13 C]lactate was excited with either 0° or 90° flip angles. The [ 13 C]bicarbonate SNR and apparent [1‐ 13 C]pyruvate‐to‐[ 13 C]bicarbonate conversion ( k PB ) were determined. Furthermore, simulations were performed to identify the SNR optimal flip‐angle scheme for detection of [1‐ 13 C]lactate and [ 13 C]bicarbonate. Results In the brain, the [ 13 C]bicarbonate SNR was 64% higher when [1‐ 13 C]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‐ 13 C]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 [ 13 C]bicarbonate SNR with a TR of 1 s in the brain is obtained with [ 13 C]bicarbonate, [1‐ 13 C]lactate, and [1‐ 13 C]pyruvate flip angles of 60°, 15°, and 10°, respectively. Conclusions Radiofrequency saturation pulses on [1‐ 13 C]lactate limit [ 13 C]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 [ 13 C]bicarbonate detection is warranted.