GE Healthcare, San Diego, California, USA.
Magn Reson Med. 2010 Aug;64(2):481-90. doi: 10.1002/mrm.22433.
Ultrashort echo time MRI requires specialized pulse sequences to overcome the short T(2) of the MR signal encountered in tissues such as ligaments, tendon, or cortical bone. Theoretical work is presented, supported by simulations and experimental data on optimizing the radiofrequency excitation to maximize signal-to-noise ratio and contrast-to-noise ratio. The theoretical calculations and simulations are based on the classic Bloch equations and lead to a closed form expression for the optimal radiofrequency pulse parameters to maximize the MR signal in the presence of rapid T(2) decay. In the steady state, the spoiled gradient recalled echo signal amplitude in response to the radiofrequency excitation pulses is not maximized by the classic Ernst angle but by a more general criterion we call "generalized Ernst angle." Finally, it is shown that T(2) contrast is maximized by flipping the magnetization at the Ernst angle with a radiofrequency pulse duration proportional to the targeted T(2). Experimental studies on short T(2) phantoms confirm these optimization criteria for both signal-to-noise ratio and contrast-to-noise ratio.
超短回波时间 MRI 需要专门的脉冲序列来克服韧带、肌腱或皮质骨等组织中遇到的 MR 信号的短 T2。本文提出了理论工作,通过模拟和实验数据支持了优化射频激发以最大化信噪比和对比噪声比的工作。理论计算和模拟基于经典的 Bloch 方程,并得出了在快速 T2 衰减存在的情况下最大化 MR 信号的最佳射频脉冲参数的闭式表达式。在稳态下,经典的 Ernst 角不能使响应射频激发脉冲的梯度重聚回波信号幅度最大化,而是由我们称之为“广义 Ernst 角”的更一般准则来最大化。最后,结果表明,通过用与目标 T2 成正比的射频脉冲持续时间翻转 Ernst 角处的磁化,可以最大化 T2 对比。针对短 T2 幻影的实验研究证实了这些用于信噪比和对比噪声比的优化标准。
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