Academic Radiology, University of Sheffield, C Floor Royal Hallamshire Hospital, Glossop Road, Sheffield S10 2JF, United Kingdom.
J Magn Reson. 2010 Feb;202(2):180-9. doi: 10.1016/j.jmr.2009.11.003. Epub 2009 Nov 13.
This work explores slice profile effects in 2D slice-selective gradient-echo MRI of hyperpolarized nuclei. Two different sequences were investigated: a Spoiled Gradient Echo sequence with variable flip angle (SPGR-VFA) and a balanced Steady-State Free Precession (SSFP) sequence. It is shown that in SPGR-VFA the distribution of flip angles across the slice present in any realistically shaped radiofrequency (RF) pulse leads to large excess signal from the slice edges in later RF views, which results in an undesired non-constant total transverse magnetization, potentially exceeding the initial value by almost 300% for the last RF pulse. A method to reduce this unwanted effect is demonstrated, based on dynamic scaling of the slice selection gradient. SSFP sequences with small to moderate flip angles (<40 degrees ) are also shown to preserve the slice profile better than the most commonly used SPGR sequence with constant flip angle (SPGR-CFA). For higher flip angles, the slice profile in SSFP evolves in a manner similar to SPGR-CFA, with depletion of polarization in the center of the slice.
这项工作探讨了二维切片选择梯度回波 MRI 中极化核的切片轮廓效应。研究了两种不同的序列：具有可变翻转角的扰相梯度回波序列（SPGR-VFA）和平衡稳态自由进动（SSFP）序列。结果表明，在 SPGR-VFA 中，实际形状的射频（RF）脉冲中切片上的翻转角分布导致在稍后的 RF 视图中从切片边缘产生大量多余的信号，从而导致不理想的非恒定总横向磁化，对于最后一个 RF 脉冲，初始值可能超过 300%。展示了一种基于切片选择梯度动态缩放的减少这种不希望的效果的方法。具有小到中等翻转角（<40 度）的 SSFP 序列也被证明比具有恒定翻转角（SPGR-CFA）的最常用的 SPGR 序列更好地保持切片轮廓。对于更高的翻转角，SSFP 中的切片轮廓以类似于 SPGR-CFA 的方式演变，切片中心的极化耗尽。
