医学影像学课件：Magnetic Resonance Imaging.ppt

What is MRI ?
MRI - Magnetic Resonance Imaging
Excellent soft tissue contrast with high resolution
Display of several images and oblique cuts
No ionizing radiation
Theory complicated
Hardware ：
Main magnet 主磁体
永磁型磁体
常导型磁体
超导型磁体
Gradient system 梯度系统 ：用于扫描层面的空间定位，梯度线圈形成微弱的梯度磁场与主磁场重叠，可根据梯度差别确定层面位置。
RF system 射频系统 ：用来发射射频脉冲，使质子吸收能量产生共振，在驰豫过程中产生MR信号，并进行接收的一种装置。
Coils 发射和接受线圈
Control system ：包括梯度场、射频场的发生和控制，MR信号接收和控制等部分组成，起承上启下的作用。
Image processing software ：每部分都要求配备大容量的计算机和高分辨的模/数转换器，以完成数据采集、图像处理、图像显示等任务。
MR system
MRI( Magnetic Resonance Imaging)
Basic physical principles
Magnetic Field（磁场）
Radio Frequency Wave （射频脉冲）
Proton resonance signal （氢质子共振发出微弱信号）
receive the signal and transform into image
（接收信号转换为图像）
Radiofrequency pulse to perturb steady-state proton magnetization 向稳态的原子核发射射频脉冲
Transient, small radio signal emitted 原子核发射信号
Spatial encoding with magnetic field gradients 梯度磁场进行空间编码
Image map of MR signal strength 产生不同MR信号的图像
Spin is an intrinsic property of atomic particles.
What are the characteristics of nuclei?
Precession happens when an external magnetic field (B0) applies.
1H: g =
It is the result of Spin + B0 The frequency of precession is determined by Larmor Equation: w = g B0
Apply Radio Frequency that must be at or near the resonant (Precessional) frequency.
Remove the RF energy.
Electrical signal is induced in the coil because magnetization vectors return to equilibrium.
How to communicate with the nuclei?
Proton relaxation
Different tissues have their own value of T1 &T2
Different T1 &T2 mean different signal amplitude(振幅)
How to distinguish different tissues?
Time constant: T1 &T2
&
Why T1 constants different ?
自旋－晶格 驰豫（spin-lattice relaxation）
高能级的氢原子降至低能级时，将能量释放给周围的晶格的过程
Why T2 constants different ?
自旋－自旋 驰豫（spin-spin relaxation）
各个氢原子小磁场间相互影响致矢量散相位的过程
T1 and T2 constant of different composition in th