g MR Basic MRI Magnetic Resonance Imaging Magnetic

g MR Basic 什么是核磁共振成像 ? MRI - Magnetic Resonance Imaging Magnetic Field（磁场） Radio Frequency Wave (RF)（射频） Nuclei of the tissue generates MR Signal （组织的核产生MR信号） Physics of MR 2

g MR Basic 发射射频前 person in magnetic field 射频发射及接收器 Physics of MR 磁体 3

g MR Basic transmitting RF （发射射频） person in magnetic field RF Transceiver RF Pulse 射频发射及 接收器 Physics of MR 4

g MR Basic after transmitting RF （发射射频后） person in magnetic field RF Transceiver MR Signal （MR信号） 射频发射及 接收器 Physics of MR 5

g MR Basic 核磁共振的硬件 System Computer Gradient Subsysem Scan Subsystem RF Hardware Scan Room Hardware Gradient Coil RF Coil Magnet Physics of MR 6

g MR Basic 什么是核磁共振现象? Billions of Hydrogen Atoms in the body 1 H proton spinning （氢质子自旋） Electron电子 N S Proton质子 1 H Physics of MR “Magnetic Moment” 磁矩 7

g MR Basic The behavior of magnetic moment of Proton in a magnetic field （在磁场中质子磁矩的行为） in normal environment in magnetic field parallel magnetic moment （磁矩） anti-parallel net magnetization （净磁化） Physics of MR + - 0 + parallel > antiparallel 8

g MR Basic • The magnetic field (Bo) causes precession of the magnetic moment（磁场导致磁矩的进动） net magnetization净磁化 Bo Physics of MR 9

g MR Basic Frequency of precession - unique to each nucleus 进动频率 Larmor Equation w g. B = High Magnetic Field (1. 5 T) 63. 89 MHz (1 H)高磁场 Physics of MR w : angular frequency角频率 g : magnetogyric ratio磁旋比 B : magnetic field strength 磁场强度 Low Magnetic Field (0. 5 T) 21. 29 MHz (1 H)低磁场 10

g MR Basic Resonance共振 is - absorbing and reemitting energy吸收和 再释放能量 example : tuning fork振动音叉 sonic wave 声波 G G Physics of MR C C D D E E F F G G A A B B 11

g MR Basic In MR, Radio Frequency waves (RF) are absorbed and reemitted 在MR中，射频波是吸收和再释放 Magnetic resonance of 1 H in 0. 5 T RF pulse 21. 29 MHz RF Off Physics of MR larmor frequency is 21. 29 MHz the precession angle increases （进动角增加） (absorbs energy) goes back to original state （回到原始状态） (reemits energy) 12

g MR Basic review What is needed for resonance? 共振需要什么？ – large number of nuclei (with spin and magnetic moment) » Hydrogen atoms氢质子 – a static magnetic field净磁场 » Magnet磁体 – RF pulse射频脉冲 » absorb and re-emit energy • the “re-emitted energy is the “MR Signal” • （再释放的能量是MR信号） Physics of MR 13

g MR Basic RF Excitation The actual pulse transmitted to the RF coil is a sinc pulse RF Amplifier 射频放大器 RF Coil 射频线圈 RF pulse After the RF pulse is OFF, the RF coil acts as a receiving coil Receiver 接收器 Physics of MR RF Coil 射频线圈 14

g MR Basic What does the RF pulse do? 1. It makes the nuclei precess “in phase” - in 0. 5 Tsame frequency different phase 相同频率不同相位 RF pulse 21. 29 MHz same frequency same phase (“in phase”) 相同频率和相位 Physics of MR 15

g MR Basic 2. It transfers energy to the nuclei RF pulse 21. 29 MHz Net magnetization “flips” 净磁化翻转 The “flipping” motion is actually a spiralling motion: 翻转移动实际上是一个螺旋形移动 Physics of MR 16

g MR Basic 90° pulse, 180° pulse “ 90° pulse” 90° 脉冲 Physics of MR “ 180° pulse” 180°脉 冲 17

g MR Basic How is the signal detected? 1. The net magnetization induces current in the transverse plane 净磁化感应横轴位电流 (electromagnetic induction law)（电磁感应定律） 2. The RF coil picks up the current as the signal RF Coil 射频线圈 Physics of MR 18

g MR Basic After the RF pulse is removed 1. Loss of “in phase” (T 2: spin-spin relaxation自旋-自旋弛豫) out of phase 失相位 in phase 同相位 nuclei precession 核进动 View from top time current induced in transverse plane none 横轴位感应电流 FID signal自由感应信号 (Free Induction Decay) Physics of MR 19

g MR Basic 2. Relaxation (T 1: spin-lattice relaxation) 弛豫（自旋-晶格弛豫） net magnetization realigning with the main magnetic field 净磁化与主磁场再对齐 time Physics of MR 20

g MR Basic • 纵向弛豫时间（T 1)是指90度脉冲后，达到原纵向 磁化矢量 63%的时间。 S i g n a l Physics of MR 63% 1 T 1 86% 2 T 1 95% 98% 3 T 1 4 T 1 Time (TR) 100% 5 T 1 21

g MR Basic • 横向弛豫时间（T 2)是指90度脉冲后，原横向磁化 矢量值衰减到 37%的时间。 63% T i m e 86% 95% 1 T 2 Physics of MR 2 T 2 3 T 2 Time (TE) 98% 4 T 2 100% 5 T 2 22

g MR Basic Rephase相位重聚 The 180° pulse is applied to rotate the magnetization back to phase. The signal obtained is called “echo”. 180度脉冲将磁化旋转回同相位， 获得信号称为回波 180° pulse in phase again in phase fast slow fast A slow D C B time D A B FID Physics of MR E E C echo 23

g MR Basic review The RF pulse “flips” the net magnetization vector 射频脉冲翻转净磁化矢量 – actually it is a spiralling motion实际上它是螺旋形移动 When the RF coil does not send RF, it acts as a receiving coil 当射频线圈不发放射频脉冲时，它充当接收线圈 “MR signal” is actually the current induced by the magnetization vector MR信号实际上是由磁化矢量感应的电流 After the RF pulse is turned off, the current induced begins to weaken - the signal begins to decay (FID) 当射频脉冲停止后，感应电流开始变弱—信号开始衰减 Physics of MR 24

g MR Basic 梯度磁场 What are gradient fields for? Gradient fields are needed for spatial information（空间信息） 需要梯度场提供空间信息 X Z Y Physics of MR 25

g MR Basic 静磁场 static magnetic field Bo (0. 5 T) Z轴梯度 z axis gradient ON 线性梯度场 linear gradient field (z axis) 0. 5 T Physics of MR 26

g MR Basic X轴梯度 x axis gradient ON Y轴梯度 y axis gradient ON X X Z Y Physics of MR Z Y 27

g MR Basic Slice Selective Excitation 层面选择激励 larmour equation : with no gradients 0. 5 T = z axis gradient ON 21. 29 MHz All protons within the bore precesses at 21. 29 MHz w g. B 0. 5 T 孔内所有质子以 21. 29 MHz频率进动 仅21. 29 MHz频率 的层面质子进动 Physics of MR 28

g MR Basic Frequency and Phase 频率和相位 After the other two gradients, the spins in each voxel has different frequency and phase 加上其它两个梯度后，每一体素的自旋有不同频率和相位 unique signal from each voxel X 21. 29 MHz phase shifted 相位转换 Physics of MR Y change the frequency 改变频率 29

g MR Basic Fourier Transformation 傅立叶转换 Amplitude / Time Ù Amplitude / Frequency 振幅/时间 振幅/频率 single peak单峰 single frequency单个频率 Frequency 频率 Time时间 two frequencies两个频率 Physics of MR two peaks双峰 30

g MR Basic 扫描序列 Let’s combine the items that we have covered so far, into a basic sequence; “Spin Echo”自旋回波 single slice, single echo 单层，单回波 RF pulse 射频脉冲 Slice gradient 层面梯度 Phase gradient 相位梯度 Frequency gradient 频率梯度 Echo signal 回波信号 Physics of MR 32

g MR Basic RF pulses and Slice gradient 射频脉冲和层面梯度 The RF pulse is applied with the “slice gradient” to excite a single slice. 用射频脉冲和层面梯度来激发一个单一层面 The height of a gradient pulse represents the amplitude of the gradient, and the width is its duration 梯度脉冲的高度代表梯度振幅，宽度是持续时间 Physics of MR 33

g MR Basic Frequency gradient 频率梯度 The “frequency gradient” causes the frequencies of the spins to be proportional to the positions along the axis 频率梯度导致自旋的频率与沿轴的位置相一致 A “dephaser” is applied in order to prevent dephasing due to the frequency gradient 扰相器是为了防止因频率梯度引起的失相 Physics of MR 34

g MR Basic Phase gradient (1) 相位梯度 The phase change caused by this “phase gradient” will be preserved at echo time as “phase memory” 由相位梯度引起的梯度变化在回波时间里将被保留为相位记忆 This “phase gradient” will not affect the detected frequencies, because it is not on within the acquisition window 这相位梯度将不影响检测频率，因为它不在采集窗内。 Physics of MR 35

g MR Basic Phase gradient (2)相位梯度 Actually, the pulse sequence is played out many times, and the signals are stored separately 脉冲序列多次起作用，信号分开储存 These data sets (views) are identical in respect to frequency, but not with respect to phase 这些数据组频率是完全一致的，而相位是不同的 for 1 st view Physics of MR for 2 nd view for 3 rd view 36

g MR Basic Raw Data 原始数据 Raw Data is the collection of views. 2 D-FFT is used to reconstruct the image 原始数据是观察的聚集，2维-傅利叶转换被用来重建图像 Physics of MR 37

g MR Basic Scan Parameters: TE, TR 扫描参数：回波时间，重复时间 TR (Repetition Time)重复时间 The time interval from the beginning of a pulse sequence until the beginning of a next pulse sequence 一个射频脉冲的开始到下一个射频脉冲的开始的时间间隔 TE (Echo Time)回波时间 The time interval from the first RF pulse of a pulse sequence to the middle of an echo 脉冲序列的第一个射频脉冲到一个回波的中点的时间间隔 TR TE Physics of MR 38

g MR Basic Contrast 对比 Contrast depends on tissue-to tissue image intensity variations 对比依赖于组织与组织成像强度差异 Proton Density (PD) weighted image 质子密度加权像 Physics of MR T 2 weighted image T 2加权像 T 1 weighted image T 1加权像 39

g MR Basic Multislice Acquisition多层面采集 TE is much shorter than TR （TE明显短于TR）- almost all of the duration of TR is “dead time” Excitation/detection of many slices may be carried out during each TR interval每一TR间期能进行多层面激励 3 slice 2 echo sequence TE TE for slice 1 Physics of MR TR TE TE for slice 2 TE TE for slice 3 40

g MR Basic Acquisition Time 采集时间 Total Acquisition time: TR x number of views x NEX （总采集时间） NEX (number of excitation): Repeating the entire acquisition of the same slice for a number of times to gain SNR example: TR = 1000 ms, TE = 25 ms, matrix = 256 x 128, NEX = 2 acquisition time = 1000 x 128 x 2 = 256, 000 ms 采集时间 = 256 sec = 4 min 16 sec Physics of MR 41