Robust hyperpolarized 13C metabolic imaging with selective non-excitation of pyruvate (SNEP)

In vivo metabolic imaging using hyperpolarized [1-C-13]pyruvate provides localized biochemical information and is particularly useful in detecting early disease changes, as well as monitoring disease progression and treatment response. However, a major limitation of hyperpolarized magnetization is its unrecoverable decay, due not only to T-1 relaxation but also to radio-frequency (RF) excitation. RF excitation schemes used in metabolic imaging must therefore be able to utilize available hyperpolarized magnetization efficiently and robustly for the optimal detection of substrate and metabolite activities. In this work, a novel RF excitation scheme called selective non-excitation of pyruvate (SNEP) is presented. This excitation scheme involves the use of a spectral selective RF pulse to specifically exclude the excitation of [1-C-13]pyruvate, while uniformly exciting the key metabolites of interest (namely [1-C-13]lactate and [1-C-13]alanine) and [1-C-13]pyruvate-hydrate. By eliminating the loss of hyperpolarized [1-C-13]pyruvate magnetization due to RF excitation, the signal from downstream metabolite pools is increased together with enhanced dynamic range. Simulation results, together with phantom measurements and in vivo experiments, demonstrated the improvement in signal-to-noise ratio (SNR) and the extension of the lifetime of the [1-C-13]lactate and [1-C-13]alanine pools when compared with conventional non-spectral selective (NS) excitation. SNEP has also been shown to perform comparably well with multi-band (MB) excitation, yet SNEP possesses distinct advantages, including ease of implementation, less stringent demands on gradient performance, increased robustness to frequency drifts and B-0 inhomogeneity as well as easier quantification involving the use of [1-C-13]pyruvate-hydrate as a proxy for the actual [1-C-13] pyruvate signal. SNEP is therefore a promising alternative for robust hyperpolarized [1-C-13]pyruvate metabolic imaging with high fidelity. Copyright (c) 2015 John Wiley & Sons, Ltd.