Room: AAPM ePoster Library
Purpose: We are investigating the feasibility of in vivo hyperpolarized 31-phosphorus (31P) magnetic resonance imaging (MRI) for pH mapping. Hyperpolarization of nuclei temporarily boosts signal up to 1000-fold, allowing for imaging of non-proton nuclei in a reasonable time frame. Nardi-Schreiber et al. recently demonstrated hyperpolarization of 31P nuclei on inorganic phosphate. The pH-dependent chemical shift of inorganic phosphate allows it to be a useful probe for measuring pH.
Methods: Monte-Carlo simulations of the proposed image reconstruction were used to investigate the acceptable parameters and limitations involved with hyperpolarized 31P MRI at 3T. We are interested in imaging pH values in the biological range (6-8pH) and small pH changes that may be relevant to severe acidosis conditions. We calculated the standard error for pH measurement at different noise and pH levels using optimized echo times.
Results: We have found via simulation that we will be able to measure pH changes of 0.1 with an accuracy of 84% and detect pH changes of 0.5 with an accuracy of 97% in the biologically relevant pH range at 3T. Based on this model, we predict that it is possible to detect small pH changes under signal-to-noise ratios (SNR>90) that have previously been observed in 13C-pyruvate hyperpolarized imaging. Using optimized echo times, we predict the standard error of these measurements will be <6Hz, considering acceptable SNR values.
Conclusion: Reconstruction simulations show that physiological pH changes should be possible to detect using hyperpolarized 31P MRI. The relative simplicity of our proposed experiment, combined with the high gyromagnetic ratio of 31P nuclei, allows us to predict that we will be able to achieve a necessary SNR in vivo. This is the first step toward using hyperpolarized 31P imaging in vivo as a non-invasive, quantitative measure of extracellular pH.
MRI, Data Acquisition, Multiple-echo Sequence