What is MRI ?

MRI - Magnetic Resonance ImagingMRI - Magnetic Resonance Imaging

•Excellent soft tissue contrast with high resolution•Display of several images and oblique cuts•No ionizing radiation•Theory complicated

•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 systemMR 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: 42.58MHz/T

It is the result of Spin + B0 The frequency of precession is determined by Larmor Equation: 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 the human body

T1 and T2 constant of different composition in the human body

How to get the proton density, T1 & T2-weighted

image?

TR TE

3333333333333333333333333333

Short3333333333333333333333333333

Long

Short T1-Weighted PD- Weighted

Long Not Used T2-Weighted