Center for Magnetic Resonance Research and Department of Radiology, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Magn Reson Med. 2010 Oct;64(4):983-94. doi: 10.1002/mrm.22372.
A new method to measure rotating frame relaxation and to create contrast for MRI is introduced. The technique exploits relaxation along a fictitious field (RAFF) generated by amplitude- and frequency-modulated irradiation in a subadiabatic condition. Here, RAFF is demonstrated using a radiofrequency pulse based on sine and cosine amplitude and frequency modulations of equal amplitudes, which gives rise to a stationary fictitious magnetic field in a doubly rotating frame. According to dipolar relaxation theory, the RAFF relaxation time constant (T(RAFF)) was found to differ from laboratory frame relaxation times (T(1) and T(2)) and rotating frame relaxation times (T(1ρ) and T(2ρ)). This prediction was supported by experimental results obtained from human brain in vivo and three different solutions. Results from relaxation mapping in human brain demonstrated the ability to create MRI contrast based on RAFF. The value of T(RAFF) was found to be insensitive to the initial orientation of the magnetization vector. In the RAFF method, the useful bandwidth did not decrease as the train length increased. Finally, as compared with an adiabatic pulse train of equal duration, RAFF required less radiofrequency power and therefore can be more readily used for rotating frame relaxation studies in humans.
介绍了一种新的测量旋转框架弛豫和创建磁共振成像对比的方法。该技术利用亚绝热条件下幅度和频率调制辐照产生的虚拟场（RAFF）来实现。这里，通过基于正弦和余弦幅度和频率调制的射频脉冲演示了 RAFF，该脉冲在双旋转框架中产生稳定的虚拟磁场。根据偶极子弛豫理论，发现 RAFF 弛豫时间常数（T(RAFF)）与实验室框架弛豫时间（T(1)和 T(2))和旋转框架弛豫时间（T(1ρ)和 T(2ρ))不同。从体内人脑和三种不同溶液中获得的实验结果支持了这一预测。人脑弛豫映射的结果表明，基于 RAFF 可以创建磁共振成像对比。发现 T(RAFF)的值对磁化矢量的初始取向不敏感。在 RAFF 方法中，随着训练长度的增加，有用带宽不会减小。最后，与具有相同持续时间的绝热脉冲序列相比，RAFF 需要的射频功率更少，因此更便于在人体中进行旋转框架弛豫研究。
