Mastropietro Alfonso, De Bernardi Elisabetta, Breschi Gian Luca, Zucca Ileana, Cametti Massimo, Soffientini Chiara Dolores, de Curtis Marco, Terraneo Giancarlo, Metrangolo Pierangelo, Spreafico Roberto, Resnati Giuseppe, Baselli Giuseppe
J Magn Reson Imaging. 2014 Jul;40(1):162-70. doi: 10.1002/jmri.24347.
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.
基于先前对 T₁ 和 T₂ 实际弛豫时间的实验评估，在 7T 临床前磁共振成像（MRI）系统上优化快速自旋回波（弛豫增强快速采集 [RARE]）序列中的信噪比（SNR），并提高¹⁹F 磁共振成像的灵敏度。
在给定的弛豫时间（T₁, T₂）下理论计算相对 SNR 变化，并在固定采集时间下映射到 RARE 参数空间（重复时间 [TR]、回波数、反转脉冲）中。测量了 KPF₆ 体模样本（溶液、琼脂混合物、离体灌注脑）的 T₁ 和 T₂，并将最佳和次最佳 RARE 参数值下的实验 SNR 值与模拟值进行比较。
优化设置在很大程度上取决于 T₁、T₂ 时间，并且在 T₁/T₂ 比值较低的情况下，使用反转脉冲可将 SNR 提高多达 30%。不同条件下的弛豫时间显示 T₁ 的变化可忽略不计（低于 14%），而 T₂ 的变化更明显（从水溶液到离体脑降低了 -95%）。实验数据证实了理论预测，在 SNR 损失约 20% 时误差幅度始终低于 4.1%，在 SNR 损失约 40% 时误差幅度低于 8.8%。优化设置允许在浓度为 0.5 mM 时达到检测阈值，相当于每体素 6.22×10¹⁶ 个氟原子。
根据测量的弛豫时间进行的最佳设置可显著提高¹⁹F MRI 研究中的灵敏度阈值。它们在广泛的（T₁, T₂）值范围内提供，并经过实验验证，显示出良好的一致性。
