Ulmer J L, Mathews V P, Hamilton C A, Elster A D, Moran P R
Department of Radiology, Bowman Gray School of Medicine, Winston-Salem, NC, USA.
AJNR Am J Neuroradiol. 1996 May;17(5):805-19.
To characterize near-resonance saturation pulse MR imaging on a 1.5-T scanner in order to gain insight into underlying mechanisms that alter tissue contrast and to optimize the technique for neuroimaging.
Off-resonance saturation pulses were applied to T1-weighted, spin-density-weighted, and T2-weighted sequences at frequency offsets ranging from 50 Hz to 20,000 Hz down field from water resonance. Suppression ratios were determined at each offset for phantom materials (MnCl2 solution, gadopentetate dimeglumine, corn oil, water, and agar), normal brain structures, and a variety of brain lesions.
Signal suppression of MnCl2 on T1-weighted images occurred at offsets of less than 2000 Hz even though no macromolecules were present in the solution. Only those phantom materials and tissues with short or intermediate T1 relaxation times and relatively large T1/T2 ratios were sensitive to changing frequency offsets. Suppression of brain increased from approximately 20% at 2000 Hz offset to approximately 45% when the offset was reduced to 300 Hz. In human subjects, the net effect of reducing the frequency offset was to increase T2 contrast on T1-weighted, spin-density-weighted, and T2-weighted images. Distilled water and contrast material did not suppress except at very low offsets ( < 300 Hz). A frequency offset of 300 Hz was optimal for maximizing conspicuity between most contrast-enhancing lesions and adjacent brain while preserving anatomic detail.
Suppression of MnCl2 indicates that magnetization transfer is not the sole mechanism of contrast in near-resonance saturation MR imaging. Spin-lock excitation can reasonably explain the behavior of the phantom solutions and the increase in T2 contrast of tissues achieved as the frequency offset is decreased from 2000 Hz to 300 Hz. Below 300 Hz, saturation is presumably caused by spin-tip effects. With our pulse design, an offset of 300 Hz is optimal for many routine clinical imaging examinations.
对1.5-T扫描仪上的近共振饱和脉冲磁共振成像进行特性分析，以深入了解改变组织对比度的潜在机制，并优化神经成像技术。
在从水共振向下场50 Hz至20,000 Hz的频率偏移下，将失谐饱和脉冲应用于T1加权、自旋密度加权和T2加权序列。测定每种偏移下模拟材料（氯化锰溶液、钆喷酸葡胺、玉米油、水和琼脂）、正常脑结构和各种脑病变的抑制率。
尽管溶液中不存在大分子，但在低于2000 Hz的偏移下，T1加权图像上的氯化锰信号被抑制。只有那些具有短或中等T1弛豫时间且T1/T2比值相对较大的模拟材料和组织对频率偏移变化敏感。脑抑制从2000 Hz偏移时的约20%增加到偏移降至300 Hz时的约45%。在人体受试者中，降低频率偏移的净效应是增加T1加权、自旋密度加权和T2加权图像上的T2对比度。蒸馏水和造影剂除了在非常低的偏移（<300 Hz）时不被抑制。300 Hz的频率偏移最适合在保留解剖细节的同时最大化大多数强化病变与相邻脑之间的对比度。
氯化锰的抑制表明，磁化传递不是近共振饱和磁共振成像中对比度的唯一机制。自旋锁定激发可以合理地解释模拟溶液的行为以及随着频率偏移从2000 Hz降低到300 Hz组织T2对比度的增加。低于300 Hz时，饱和可能是由自旋尖端效应引起的。采用我们的脉冲设计，300 Hz的偏移对于许多常规临床成像检查是最佳的。
