化学位移伪影

1、化学位移伪影 节选自：医学影像物理学教程第三篇 磁共振成像 学习内容2. MR图像是怎样重建出的？1. 什么是化学位移？（重点） （难点） 学习目标2. 从K空间的角度解释MR图像重建准确描述K空间 化学 位移分子中的H核被电子云包围Results in a shift of resonant frequency which affects the imagee.g. 脂肪 (CH2) vs 水(H2O)?Since the secondary fields are as a result of the application of B0, they are proportional to B

2、0. Theres nothing you can do about them ! Spatial position is frequency encoded in one direction in the imaging process, and so signals from fat and water in the same spatial location will be assigned to different spatial locations on the image. This is called misregistration, and has nothing to do

3、with relaxation times.The external applied magnetic field is modified by the local structure of the molecule a proton resides in. Local variations in field mean different resonant frequencies (c.f. Larmor equation). This is sometimes stated as shielding by the electron cloud of the molecule. Shieldi

4、ng refers to the induction of secondary magnetic fields that oppose B0 by the movement of electrons due to B0.Lets look at how this happens.化学位移：在MRI中，脂肪和水的H质子的进动频率之间的差异。H 质子共振频率与主磁场B0有关水脂肪频率编码梯度MR信号是通过施加频率编码梯度造成不同相位质子的频率差异来空间定位的脂肪中的H 质子的共振频率与主磁场B0有关图像中脂肪的位置会发生轻度偏移0化学位移：由化学环境不同引起的共振频率的偏移现象。 化学位移伪影：由

5、于频率差异会随外部磁场强度 (B0)的变化而变化，化学位移常用ppm(parts-per-million) 来表示， 水和脂肪的化学位移是3.5 ppm，当B0.一定时，可以计算出频率 。（频率编码方向）脂肪水水化学位移与频率信号叠加信号丢失样品图像MR信号是通过施加频率编码梯度造成不同相位质子的频率差异实现空间定位;脂肪的频率低于水（中心频率），因此图像上脂肪信号向低频方向移动脂肪水植物油（塑料试管）（频率编码方向）低接收带宽高接收带宽化学位移与接收机带宽这幅图像的化学位移有多大?多少个像素?（接收）带宽频率编码矩阵大小化学位移e.g. 16 kHz= 32 kHze.g. 25632 x 103(H z)256 (pixels)125 Hz / pixel3.5 ppm224 Hz ( 1.5