Relaxation dispersion in MRI induced by fictitious magnetic fields
Relaxation dispersion in MRI induced by fictitious magnetic fields
复制标题
DOI：
10.1016/j.jmr.2011.01.022
复制
发表时间：
2011-04-01
影响因子：
2.2
通讯作者：
Michaeli, Shalom
中科院分区：
文献类型：
--
作者：
Liimatainen, Timo;Mangia, Silvia;Michaeli, Shalom
为了研究秩为>= 2的旋转坐标系中的弛豫，最近提出了一种新的方法——沿虚场弛豫。RAFF通过在次绝热状态下应用正弦和余弦振幅和频率调制的扫频脉冲产生虚拟场(E)。在目前的工作中，通过改变E的方向，即E与第二个旋转框架的z”轴之间的角度E来创建MRI对比。当ε > 45度时，RAFF过程中产生的虚拟场E的振幅明显大于用于传输正弦/余弦脉冲的射频场振幅。利用不变轨迹方法和Bloch-McConnell形式主义研究了RAFF过程中的松弛。考虑了快速交换体系中相同（类）自旋之间的偶极-偶极相互作用和各向异性交换（例如，具有不同化学位移的自旋之间的交换）。在人体和小鼠脑内进行了实验验证。理论和实验结果表明，c的变化引起弛豫速率常数的色散。在epsilon约56度处，脉冲中横向分量的平均贡献最大，而纵向分量的贡献最小，弛豫速度最快。将RAFF弛豫色散与非共振自旋锁T-1 rho实验得到的弛豫色散进行了比较。与非共振自旋锁T-1 rho方法相比，RAFF方法观察到的旋转框架弛豫速率较慢，这在一定的实验条件下是可取的。(C) 2011爱思唯尔公司版权所有。
A new method entitled Relaxation Along a Fictitious Field (RAFF) was recently introduced for investigating relaxations in rotating frames of rank >= 2. RAFF generates a fictitious field (E) by applying frequency-swept pulses with sine and cosine amplitude and frequency modulation operating in a sub-adiabatic regime. In the present work, MRI contrast is created by varying the orientation of E, i.e. the angle e between E and the z '' axis of the second rotating frame. When epsilon > 45 degrees, the amplitude of the fictitious field E generated during RAFF is significantly larger than the RF field amplitude used for transmitting the sine/ cosine pulses. Relaxation during RAFF was investigated using an invariant-trajectory approach and the Bloch-McConnell formalism. Dipole-dipole interactions between identical (like) spins and anisochronous exchange (e.g., exchange between spins with different chemical shifts) in the fast exchange regime were considered. Experimental verifications were performed in vivo in human and mouse brain. Theoretical and experimental results demonstrated that changes in c induced a dispersion of the relaxation rate constants. The fastest relaxation was achieved at epsilon approximate to 56 degrees, where the averaged contributions from transverse components during the pulse are maximal and the contribution from longitudinal components are minimal. RAFF relaxation dispersion was compared with the relaxation dispersion achieved with off-resonance spin lock T-1 rho, experiments. As compared with the off-resonance spin lock T-1 rho method, a slower rotating frame relaxation rate was observed with RAFF, which under certain experimental conditions is desirable. (C) 2011 Elsevier Inc. All rights reserved.