Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, Texas, USA.
Department of Radiology, University of Texas Southwestern Medical Center, Dallas, Texas, USA.
Magn Reson Med. 2022 Mar;87(3):1150-1164. doi: 10.1002/mrm.29051. Epub 2021 Oct 17.
J-Difference editing (MEGA) provides an effective spectroscopic means of selectively measuring low-concentration metabolites having weakly coupled spins. The fractional inphase and antiphase coherences are determined by the radiofrequency (RF) pulses and inter-RF pulse intervals of the sequence. We examined the timings of the spectrally selective editing 180° pulses (E180) in MEGA-PRESS to maximize the edited signal amplitude in lactate at 3T.
The time evolution of the lactate spin coherences was analytically and numerically calculated for non-volume localized and single-voxel localized MEGA sequences. Single-voxel localized MEGA-PRESS simulations and phantom experiments were conducted for echo time (TE) 60-160 ms and for all possible integer-millisecond timings of the E180 pulses. Optimized E180 timings of 144, 103, and 109 ms TEs, tailored with simulation and phantom data, were tested in brain tumor patients in vivo. Lactate signals, broadened to singlet linewidths (~6 Hz), were compared between simulation, phantom, and in vivo data.
Theoretical and experimental data indicated consistently that the MEGA-edited signal amplitude and width are sensitive to the E180 timings. In volume-localized MEGA, the lactate peak amplitudes in E180-on and difference spectra were maximized at specific E180 timings for individual TEs, largely due to the chemical-shift displacement effects. The E180 timings for maximum lactate peak amplitude were different from those of maximum inphase coherence in in vivo linewidth situations.
In in vivo MEGA editing, the E180 pulse timings can be effectively used for manipulating the inphase and antiphase coherences and increasing the edited signal amplitude, following TE optimization.
J-Difference 编辑（MEGA）提供了一种有效的光谱手段，可选择性测量具有弱耦合自旋的低浓度代谢物。同相和反相相干分数由序列的射频（RF）脉冲和 RF 脉冲之间的间隔决定。我们检查了 MEGA-PRESS 中光谱选择性编辑 180°脉冲（E180）的时间，以最大化 3T 时乳酸的编辑信号幅度。
通过非容积局部化和单体素局部化 MEGA 序列对乳酸自旋相干的时间演化进行了分析和数值计算。对单体素局部化 MEGA-PRESS 进行了模拟和体模实验，echo time (TE) 为 60-160ms，E180 脉冲的所有可能整数毫秒定时。根据模拟和体模数据优化的 E180 定时为 144、103 和 109ms TE，并在脑肿瘤患者体内进行了测试。将扩宽至单峰线宽（~6Hz）的乳酸信号与模拟、体模和体内数据进行了比较。
理论和实验数据一致表明，MEGA 编辑信号幅度和宽度对 E180 定时敏感。在容积局部化 MEGA 中，在特定的 E180 定时下，个体 TE 的 E180-on 和差谱中的乳酸峰幅度最大，这主要是由于化学位移位移效应所致。在体内线宽情况下，最大乳酸峰幅度的 E180 定时与同相相干的最大定时不同。
在体内 MEGA 编辑中，E180 脉冲定时可在优化 TE 后有效地用于控制同相和反相相干，增加编辑信号幅度。
