Purpose: 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.
Materials and methods: 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.
Results: 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.
Conclusion: 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.