Fransson A, Ericsson A, Jung B, Sperber G O
Department of Radiation Physics, Karolinska Institutet, Stockholm, Sweden.
Magn Reson Imaging. 1993;11(6):771-84. doi: 10.1016/0730-725x(93)90195-j.
The effects of imperfect radiofrequency (RF) pulses on the echo amplitudes from the Carr-Purcell (CP), Carr-Purcell-Meiboom-Gill (CPMG), and the PHase-Alternating Phase-Shift (PHAPS; combination of CP and CPMG) multiple spin-echo schemes were studied. Properties of the PHAPS scheme for transverse relaxation time measurements was emphasized. Numerical simulations on non-relaxing spin systems were performed to assess the properties of selective (damped sinc shaped) and nonselective refocusing pulses in terms of effective spatial selectivity and generation of secondary echo signal. Analytical solutions of the Bloch equations were applied to study the generation and propagation of stimulated echo signal caused by nonideal 180 degrees phase reversals, and the results were used to analyse the numerical simulations in terms of primary and stimulated echo components. Finally, the simulated echo train patterns from the different MSE schemes were compared with MR imaging measurements. It was found that the underestimation of T2 values by the PHAPS protocol with selective refocusing pulses is mainly an effect of an "artificial" echo amplitude decay in the CP scheme, while the CPMG scheme produces a typical even-odd echo pattern (different from corresponding echo patterns in conventional high resolution NMR). Both effects are related to the flip angle error and phase dispersion along the slice selection direction from selective RF pulses, and are not significantly influenced by stimulated echo interference for nonrelaxing spin systems. However, the presence of stimulated echoes at the time of the primary echoes implies a dependence on T1 of the PHAPS echo amplitudes. In the CPMG protocol, different gradient schemes have been implemented to defocus stimulated echoes. However, the results indicate that there exists stimulated components that will not be affected by such gradients, and that the optimization of the RF refocusing pulses then remain the main objective.
研究了不完美射频（RF）脉冲对来自 Carr-Purcell（CP）、Carr-Purcell-Meiboom-Gill（CPMG）以及相位交替相移（PHAPS；CP 和 CPMG 的组合）多重自旋回波序列回波幅度的影响。重点强调了用于横向弛豫时间测量的 PHAPS 序列的特性。对非弛豫自旋系统进行了数值模拟，以评估选择性（阻尼 sinc 形）和非选择性重聚焦脉冲在有效空间选择性和二次回波信号产生方面的特性。应用布洛赫方程的解析解来研究由非理想 180 度相位反转引起的受激回波信号的产生和传播，并将结果用于根据初级回波和受激回波分量分析数值模拟。最后，将不同多重自旋回波序列模拟的回波串模式与磁共振成像测量结果进行了比较。结果发现，使用选择性重聚焦脉冲的 PHAPS 协议对 T2 值的低估主要是 CP 序列中“人为”回波幅度衰减的影响，而 CPMG 序列产生典型的奇偶回波模式（不同于传统高分辨率核磁共振中的相应回波模式）。这两种影响都与选择性 RF 脉冲沿切片选择方向的翻转角误差和相位色散有关，并且对于非弛豫自旋系统，不受激回波干扰的显著影响。然而，在初级回波时刻存在受激回波意味着 PHAPS 回波幅度依赖于 T1。在 CPMG 协议中，已经实施了不同的梯度方案来散焦受激回波。然而，结果表明存在不受此类梯度影响的受激分量，因此 RF 重聚焦脉冲的优化仍然是主要目标。
