Variable flip angle schedules in bSSFP imaging of hyperpolarized noble gases

Balanced steady-state free precession imaging sequences provide signal-to-noise ratio benefits for MRI of hyperpolarized nuclei. Hyperpolarized magnetization decays during the imaging sequence to thermal equilibrium, effectively necessitating imaging in a transient state characterized by nonconstant transverse magnetization and k-space filtering when using constant flip angles. This work presents an analytical method for calculation of variable flip angle schedules which maintain constant transverse magnetization in balanced steady-state free precession imaging of hyperpolarized nuclei. The approach is based on direct inversion of the Bloch equations and does not require any numerical optimization. Input parameters are pulse sequence timings and effective relaxation times, which take diffusion of hyperpolarized gas in imaging gradients into account. Provision of constant transverse magnetization is demonstrated in phantom experiments and human lung imaging using hyperpolarized (3) He. The benefit of a flat k-space filter is demonstrated by reduced blurring in (3) He and digital phantom data, and high quality (3) He ventilation images from human lungs are obtained.