Cho Z H, Hong I K, Ro Y M
Department of Radiological Sciences, University of California, Irvine 92717, USA.
Magn Reson Med. 1996 Aug;36(2):197-203. doi: 10.1002/mrm.1910360205.
Self-diffusion of nuclear spins has been suggested to cause edge enhancement in images especially on a microscopic scale. According to previously published work, theory suggests that edge enhancement is caused by motional narrowing due to the boundaries and spin self-diffusion during the data acquisition period. More careful examination reveals that edge enhancement due to motional narrowing develops only under a few specific conditions. This lack of generality of motional narrowing theory, as well as experimental observations, indicate that edge enhancement due to effects other than motional narrowing alone can exist. It is found that edge enhancement depends greatly on the data acquisition mode; therefore, the images obtained are different depending on the pulse sequence employed. For example, excessive attenuation of DC components due to diffusion can result in edge enhancement in the spin echo signal. However, in the case of FID-like signals, DC components are preserved while positive high frequency parts are attenuated, thereby degrading resolution. The new phenomenon observed has been termed selective spectral suppression since the observed edge enhancement results from the selective attenuation of certain frequency components in the nuclear signals due to diffusion-dependent signal attenuation for a given pulse sequence.
核自旋的自扩散被认为会导致图像中的边缘增强，尤其是在微观尺度上。根据先前发表的研究，理论表明边缘增强是由于数据采集期间边界和自旋自扩散引起的运动窄化所致。更仔细的研究表明，由于运动窄化导致的边缘增强仅在少数特定条件下才会出现。运动窄化理论的这种缺乏普遍性以及实验观察结果表明，除了运动窄化之外，其他效应也可能导致边缘增强。研究发现，边缘增强很大程度上取决于数据采集模式；因此，根据所采用的脉冲序列，获得的图像会有所不同。例如，由于扩散导致的直流分量过度衰减会导致自旋回波信号中的边缘增强。然而，在类似FID信号的情况下，直流分量得以保留，而正高频部分则被衰减，从而降低分辨率。由于观察到的边缘增强是由于给定脉冲序列中扩散相关信号衰减导致核信号中某些频率分量的选择性衰减，因此所观察到的新现象被称为选择性频谱抑制。
