医学影像物理学课件：15yj-MRI 03

1、Magnetic Resonance Imaging III,Magnetic Imaging Method,T1,T2-Weighted MR-Image,The signal received in an experiment contain information about the concentration of nuclei being examined and also about their relaxation times T1 and T2. So the gray scale signal in MRI image can represent concentration,

2、 T1 , or T2. If we choose T1, then get T1-weighted image.,Proton Density Weighted MR-Image,Concentration of hydrogen nuclei Bone Tissues and Air Chamber Other: Flow Effect Magnetic Susceptibility Chemical Shift (Basic of Spectrum) Diffusion,Generation of MRI signal,FID Spin Echo Inversion Recovery G

3、radient Echo*,FID,Depend on H density and T2*. Because the FID signal decay in time T2*, it is hard to get T1 and T2 from single signal. 90脉冲的终止会引起振荡，对后继信号的采集有影响,Spin Echo,90-FID-180-Echo Echo wave 的出现 TI, TR,Action of 180 Pulse,90 pulse make M0 to XY plate Different procession speed lead to dephase

4、,180 pulse make direction together(rephase) Dephase process take place and NMR signal be emitted again,TRT1、TET1，TET2,Different T Weighted Image,Adjust,流速函数：g(v),Inversion-Recovery Sequence,180 pulse make M0 reverse its direction. When 90 pulse is given nuclei system, FID signal be emitted. The init

5、ial amplitude of FID is proportional with M(t).,Measure T1,IR Weighted Image,Different tissue different T1 process Brain tissue Cerebrospinal fluid 90 pulse in different time will lead to different weighted T1 image.,IRSE Pulse Sequence,Amplitude of IR,反转点 90后的恢复，在T1TR，t=TR时 IRSE,IRSE Weighted Image

6、,PWI 长TI, 短TE，长TR T1WI 中等TI, 短TE，长TR 短时反转恢复成像/STIR 短TI,短TE,长TR,抑制脂肪信号 流动衰减反转恢复成像/FLAIR 由于流动水的T1驰豫不充分，所以T1很长，选择长TI时，可以抑制流动水信号。FLAIR如采用长TI,短TE,长TR-PWI;长TI,长TE,长TR-PWI（T2WI),TI决定T1，TE()决定T2，为保证M0的充分恢复，TR要足够长。,Main Parameter,TR：重复时间 TI： 复相脉冲时间 TE：回波时间 Alpha角：翻转角,总结：控制上述不同参数，获取不同的加权成像。,MRI Image Method,图

7、像灰度与信号的强度 空间定位 投影法 2D傅立叶变换法,MRI的坐标系,If we reconstructure a image with T1 or T2, where the NMR signal containing T1 T2 information come from must be known. This also means we need to locate the signal source of FID.,坐标系的建立 右手螺旋,z,y,x,梯度场Gradient Magnetic Field,梯度的概念 梯度磁场的产生 梯度磁场与主磁场的迭加,Consist of two

8、 current coil whose current directions are inverse. Adjust the distance to get a linear gradient field,梯度磁场与主磁场,空间迭加,The strength of magnetic field change with spatial position. Linear Change,空间编码/Space Encoding,Gz Slice choice Gy Phase Encoding Gx Frequency Encoding,To locate FID, we take following

9、 methods/空间定位:,Slice choice,The frequency of rf pulse must equate to the frequency of procession of nuclei.,Slice Width,激励波形的选择 带宽与层厚,窄带激发,方波 Sinc(t) 高斯脉冲,平面内信号的定位,Gy, Phase Encoding ，y Location Gx, Frequency Encoding, xLocation,相位编码原理,At the time of stopping rf, have the same phase,so there still i

10、s Mxy.In Gy, with different dispersing speed, phase become different.,频率编码原理,Next, stop Gy and store signals from in Gy and exert Gx to sample. Different Gx make the procession of own different frequency,Frequency, Phase in 2D,FID Planar Imaging,Signal contain many different frequency and different phase, we need 2D FT to decompose them.,Image Reconstruct,2D-FT,每一x坐标上代表的是来自不同y坐标（即不同位相，相同频率）的投影。,多次施加Gy梯度脉冲，获取y轴频谱。,K- Space,采集MR信号后的存储空间，2DFT-图像 K空间，频域空间 以空间（平面）频率为单位的空间坐标系