Optimal Variable Flip Angle Schemes for Hyperpolarized MR Kinetic Modeling Robust to RF Field Variations.

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Metabolic Pathway Analysis

Background:

• Hyperpolarized carbon-13 magnetic resonance allows real-time observation of biochemical pathways in vivo.
• Pharmacokinetic modeling estimates metabolite conversion rates, aiding in distinguishing healthy from diseased tissues.
• Accurate parameter estimation is vital for reliable interpretation of hyperpolarized NMR data.

Purpose of the Study:

• To develop time-varying flip angle schemes that minimize uncertainty in pharmacokinetic model parameter estimates.
• To optimize these schemes by maximizing Fisher information while accounting for B1 field strength variations.
• To compare the performance of optimized variable flip angle (VFA) schemes against optimized constant flip angle (CFA) schemes.

Main Methods:

• Utilized Monte Carlo simulations to evaluate optimized VFA and CFA schemes.
• Incorporated prior distributions for B1 field strength variation into the optimization process.
• Validated the enzyme-driven conversion model using in vitro experiments.

Main Results:

• Optimized VFA schemes demonstrated significantly reduced uncertainty in model parameter estimates compared to optimized CFA schemes.
• The performance improvement of VFA schemes was robust across various underlying model parameters.
• Accurate estimation of B1 field strength was shown to be essential for avoiding inaccurate parameter estimates.

Conclusions:

• Optimized VFA schemes offer superior performance for pharmacokinetic modeling in hyperpolarized NMR.
• Including B1 field strength as a fitted parameter enhances the precision of metabolic parameter estimation.
• This approach improves the reliability of distinguishing between healthy and diseased tissues using hyperpolarized NMR.