Optimization of rapid acquisition with relaxation enhancement (RARE) pulse sequence parameters for ¹⁹F-MRI studies

To optimize signal-to-noise ratio (SNR) in fast spin echo (rapid acquisition with relaxation enhancement [RARE]) sequences and to improve sensitivity in ¹⁹F magnetic resonance imaging (MRI) on a 7T preclinical MRI system, based on a previous experimental evaluation of T₁ and T₂ actual relaxation times.

Relative SNR changes were theoretically calculated at given relaxation times (T₁, T₂) and mapped in RARE parameter space (TR, number of echoes, flip back pulse), at fixed acquisition times. T₁ and T₂ of KPF₆ phantom samples (solution, agar mixtures, ex vivo perfused brain) were measured and experimental SNR values were compared with simulations, at optimal and suboptimal RARE parameter values.

The optimized setting largely depended on T₁, T₂ times and the use of flip back pulse improved SNR up to 30% in case of low T₁/T₂ ratios. Relaxation times in different conditions showed negligible changes in T₁ (below 14%) and more evident changes in T₂ (-95% from water solution to ex vivo brain). Experimental data confirmed theoretical forecasts, within an error margin always below 4.1% at SNR losses of ~20% and below 8.8% at SNR losses of ~40%. The optimized settings permitted a detection threshold at a concentration of 0.5 mM, corresponding to 6.22 × 10¹⁶ fluorine atoms per voxel.

Optimal settings according to measured relaxation times can significantly improve the sensitivity threshold in ¹⁹F MRI studies. They were provided in a wide range of (T₁, T₂) values and experimentally validated showing good agreement.