Improving Pyruvate Kinetic Measurements Using Variable Flip Angle Schemes in bSSFP Hyperpolarized Carbon-13 MR Imaging

Hyperpolarized MR imaging of 13C-labeled probes provide increased sensitivity to investigate metabolic kinetics. The characterization of [1-13C]pyruvate kinetics has been correlated to increased aggressiveness of Renal Cell Carcinoma (RCC) and pyruvate kinetics have the potential to be a valuable tool for assessing disease burden. Increased signal-to-noise ratio (SNR) has already been proven through the use of balanced steady-state free precession (bSSFP) sequences over accelerated gradient echo (GRE) acquisitions. A sigmoid-based variable flip angle scheme was implemented in a pyruvate-specific 3D bSSFP dynamic imaging protocol to further optimize the SNR across multiple acquisitions. The effects of varying flip angle across a dynamic imaging study was first simulated with a physics-based 2-site pyruvate-lactate kinetic model and then evaluated using Monte Carlo simulations for sensitivities to kinetic and imaging parameters. An increase in early lactate signal as well as a delayed pyruvate signal peak and extended signal window were confirmed with subsequent animal studies on a healthy adult rat. The resulting signal changes present the possibility of improved spatial resolution and improved kPL fitting. Additional studies and simulations to optimize associated sequence parameters are needed to further characterize the use-case for sigmoid-based variable flip angle schemes