Fifty-sixth annual meeting of the American association of physicists in medicine
SU-E-QI-11: Measurement of Renal Pyruvate-To-Lactate Exchange with Hyperpolarized 13C MRI
Previous work  modeling the metabolic flux between hyperpolarized [1-13C]pyruvate and [1-13C]lactate in magnetic resonance spectroscopic imaging (MRSI) experiments failed to account for vascular signal artifacts. Here, we investigate a method to minimize the vascular signal and its impact on the fidelity of metabolic modeling.
MRSI was simulated for renal metabolism in MATLAB both with and without bipolar gradients. The resulting data were fit to a two-site exchange model , and the effects of vascular partial volume artifacts on kinetic modeling were assessed. Bipolar gradients were then incorporated into a gradient echo sequence to validate the simulations experimentally. The degree of diffusion weighting (b = 32 s/mm2) was determined empirically from 1H imaging of murine renal vascular signal. The method was then tested in vivo using MRSI with bipolar gradients following injection of hyperpolarized [1-13C]pyruvate (∼80 mM at 20% polarization).
In simulations, vascular signal contaminated the renal metabolic signal at resolutions as high as 2 × 2 mm2 due to partial volume effects. The apparent exchange rate from pyruvate to lactate (kp) was underestimated in the presence of these artifacts due to contaminating pyruvate signal. Incorporation of bipolar gradients suppressed vascular signal and improved the accuracy of kp estimation. Experimentally, the in vivo results supported the ability of bipolar gradients to suppress vascular signal. The in vivo exchange rate increased, as predicted in simulations, from kp = 0.012 s-1 to kp = 0.020-1 after vascular signal suppression.
We have demonstrated the limited accuracy of the two-site exchange model in the presence of vascular partial volume artifacts. The addition of bipolar gradients suppressed vascular signal and improved model accuracy in simulations. Bipolar gradients largely affected kp estimation in vivo. Currently, slow-flowing spins in small vessels and capillaries are only partially suppressed, so further improvement is possible.
Funding support: Seed Grant from the Radiological Society of North America, GE Healthcare, University of Wisconsin Graduate School