核磁共振成像原理 什么是核磁共振成像? 核磁共振的硬件 什么是核磁共振现象? 射频系统 梯度系统 Spin Echo sequence（自旋回波序列） This module covers the basic physics of MR imaging

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） To receive a signal from the body, a static magnetic field and radio frequency (high frequency) waves are neeeded. With these two, the nuclei of the tissue of the human body generates a MR Signal.

发射射频前 person in magnetic field 磁体 射频发射及接收器 The following illustrations describes the rough idea of MRI. The person laying inside the magnet is exposed to a magnetic field. The big arrow indicates the field. The person (nuclei) is in normal state

transmitting RF （发射射频） person in magnetic field RF Transceiver RF Pulse RF Transceiver RF is applied in a form of pulse. The person (nuclei) absorbs the energy. 射频发射及接收器

after transmitting RF （发射射频后） person in magnetic field RF Transceiver MR Signal （MR信号） After the RF Pulse is switched off, the person (nuclei) reemits the energy, and goes back to normal state. The energy is collected as MR Signal 射频发射及接收器

核磁共振的硬件 System Computer Scan Subsystem Gradient Subsysem RF Hardware Scan Room Hardware This is a diagram describing the generic MR System Hardware. So far we have only talked about the Magnet and the RF Transceiver. The other components are necessary to vary the magnetic field and RFfor various scan techniques, and to display the obtained image. Gradient Coil RF Coil Magnet

什么是核磁共振现象? Electron电子 Proton质子 1H Billions of Hydrogen Atoms in the body 1H proton spinning （氢质子自旋） N S Electron电子 Atomic nuclei are charged, so the spinning motion causes a magnetic moment. The magnetic moment acts similar to a bar magnet. The strength of the magnetic moment is a property of the type of nucleus, and it also determines the sensitivity of MR. Since 1H nuclei possess the strongest magnetic moment, and we also have billions of hydrogen atoms in our body, this is the main nucleus used in MR imaging. Proton质子 “Magnetic Moment” 磁矩 1H

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 When there isn’t any external magnetic field, the magnetic moments have no specific orientation. However, if an external magnetic field is applied, there is a tendency for the magnetic moments to align with the magnetic field. There are two orientations: parallel or anti-parallel. The two states are almost equally populated. The remaining difference results in a net magnetization, which is detectable using MR techniques. + - + net magnetization （净磁化） parallel > antiparallel

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 The actual individual spins align at an angle to B0. This causes the spin axis (magnetic moment) to precess like a top around B0, describing the surface of a cone. Any vector on the cone could be described by its components perpendicular and parallel to B0. By having enough spins randomly distributed on the surface of the cone, components perpendicular to B0 will cancel each other. This leaves only the contributions parallel to B0 , explaining the net magnetization parallel to B0.

High Magnetic Field (1.5T) Frequency of precession - unique to each nucleus 进动频率 Larmor Equation w= gB w : angular frequency角频率 g : magnetogyric ratio磁旋比 B : magnetic field strength 磁场强度 How fast the spins precess on the surface of the cone is given by the Larmor equation. The magnetogyric ratio is related to the strength of the magnetic moment for the type of nucleus considered. The Larmor frequency (usually referred to as Center Frequency in a MR System) is proportional to the magnetic field strength. High Magnetic Field (1.5T) 63.89 MHz (1H)高磁场 Low Magnetic Field (0.5T) 21.29 MHz (1H)低磁场

Resonance共振 is - absorbing and reemitting energy吸收和 再释放能量 example : tuning fork振动音叉 G C D E F G A B sonic wave 声波 The term “resonance” implies to absorbing and reemitting energy. Think of a tuning fork. If you ring the “G” fork, only a “G” fork “responds” to the sonic wave and starts to resonate. This is because the tuning forks responds to a specific frequency. G C D E F G A B

larmor frequency is 21.29 MHz the precession angle increases 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 1H in 0.5 T larmor frequency is 21.29 MHz RF pulse 21.29 MHz the precession angle increases （进动角增加） (absorbs energy) The same thing occurs in MR. Suppose a spin is precessing in Larmor frequency. When external energy of the same frequency is applied, the energy is absorbed. After the RF is turned off, the precession goes back to normal state while reemitting the energy. The important thing is that considering a specific magnetic field strength, Only the RF energy at Larmor frequency induces transition. RF energy at other frequencies have no effect. RF Off goes back to original state （回到原始状态） (reemits energy)

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） (Take time for Q & A of this section. Go back to previous slides if necessary)

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 In this section we will look into the details of RF Excitation RF Coil 射频线圈 Receiver 接收器

What does the RF pulse do? 1. It makes the nuclei precess “in phase” - in 0.5 T- same frequency different phase 相同频率不同相位 RF pulse 21.29 MHz One thing that happens with the RF Pulse is that the nuclei precess “in phase” By being exposed to an external magnetic field, the nuclei will precess at Larmor frequency. But this doesn’t necessarily mean that the nuclei are at the same phase. With the external energy (RF) applied, the phase will become equal (“in phase”). same frequency same phase (“in phase”) 相同频率和相位

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 By transferring the energy to the nuclei, the net magnetization will rotate to the energized state. This is referred to as “flipping”. The “flip” angle is proportional to the duration of an RF pulse, and the amplitude of the RF.

90° pulse, 180° pulse “90° pulse” “180° pulse” 90° 脉冲 180°脉 冲 Flip angles of 90 degrees and 180 degrees are of special importance of imaging

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 Immediately after the 90 degrees pulse, the net magnetization lies in the transverse plane and begins to precess around the B0 axis, at Larmor frequency. This rotating magnetization can induce an AC current in the receiver coil, and that current can be used to record the action of magnetization in the transverse plane.

After the RF pulse is removed 1. Loss of “in phase” (T2: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 横轴位感应电流 none The net magnetization rotating is actually the sum of the contributions from all of the spins in the excited sample. However, the spins at different points may not be experiencing the same B0 field, due to B0 inhomogeneity. This results in a slight defference of precessing frequency, and will cause the components of the net magnetization to spread apart over time. This loss of “in phase” leads to the decay of the signal. This is also called “transverse relaxation” FID signal自由感应信号 (Free Induction Decay)

Relaxation (T1: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 This is also called longitudinal relaxation T1 relaxatiion time is usually far more longer than T2 relaxation time time

纵向弛豫时间（T1)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 1 T1 2 T1 3 T1 4 T1 5 T1 Time (TR) 63% 86% 95% 98% 100%

横向弛豫时间（T2)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% 98% 100% 1 T2 2 T2 3 T2 4 T2 5 T2 Time (TE)

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 slow fast A B C D E The mechanism of losing “in phase” largely dues to B0 inhomogeneity. As the magnetization fans out, the fast and slow edges can be identified. By applying the 180 degree pulse, all of the magnetization is flipped to the opposite side of the transverse plane. Precession continues in the same direction, but now the fast edge is behind the slow edge, leading to rephasing. time echo FID A B C D E

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 (Take time for Q & A of this section. Go back to previous slides if necessary)

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 Z Y X A magnetic field gradient simply refers to the spatial variation of the strength of the B0 field Spatial variation is needed in 3spatial axes for 3D information

static magnetic field Bo (0.5T) 静磁场 static magnetic field Bo (0.5T) Z轴梯度 z axis gradient ON 线性梯度场 linear gradient field (z axis) How we make the gradient field: Two coils of wire, one on each end, are supplied with current which generates magnetic fields. The current is supplied in opposite direction, so one coil “adds to” and the other coil “subtracts from” the main magnetic field. At the point midway between the two coils, the magnetic field created by the gradient coils cancel each other, causing the net magnetic field to be equal to B0. The gradient coils are positioned so that this point is at the center of the magnet (isocenter). 0.5 T

X轴梯度 Y轴梯度 x axis gradient ON y axis gradient ON X X Z Z Y Y Note that the direction of the magnetic field is the same; only the strength is different. CAUTION Signa and Vectra calls X and Y axis opposite! H-F direction is Z axis for both Signa and Vectra L-R direction is X axis for Signa, Y axis for Vectra A-P direction is Y axis for Signa, X axis for Vectra

Slice Selective Excitation 层面选择激励 larmour equation : w= gB with no gradients z axis gradient ON 21.29 MHz 21.29 MHz 0.5 T All protons within the bore precesses at 21.29 MHz 孔内所有质子以21.29MHz频率进动 In 2D imaging, there is a need to restrict the signals coming from the other parts of the imaging volume. This is done by selectively exciting the desired slice. The illustration shows how to select the axial slice. By imposing the gradient on the Z axis, there will be a linear variation of the resonance frequency along the axis. By applying the RF pulse of a certain frequency, only the portion of the volume that is exposed to the corresponding magnetic field strength will be excited. 0.5 T 仅21.29MHz频率的层面质子进动

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 Y The next task is encoding the image information within the selected slice. There are two processes; phase encoding and frequency encoding. The next gradient field (called “phase gradient”) applied to the selected-”in phase”-slice will cause a phase shift among the spins within the slice. The phase difference will be memorized after the gradient is switched off. By applying the final gradient field (called “frequency gradient”) when the MR signal is obtained, the precessing frequency will vary along the final axis. Now we have different phase and frequency for all the points within the excited slice. phase shifted 相位转换 change the frequency 改变频率

Fourier Transformation 傅立叶转换 Amplitude / Time Ù Amplitude / Frequency 振幅/时间 振幅/频率 single frequency单个频率 single peak单峰 Time时间 Frequency 频率 two frequencies两个频率 two peaks双峰 In order to reconstruct the image from the “interference pattern”, Fourier Transformation is used. This will convert the “amplitude to time” information to “amplitude to frequency” information.

g MR Basic Physics of MR 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

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 Spin Echo uses the 90 degree and 180 degree RF pulse for slice selection. The small negative-going gradient accounts for dephasing which occurs during the slice selective RF pulse.

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 It is desirable to have the receiver on only when the echo is present, for sensitivity reasons. The matrix in the frequency direction determines the sampling rate of the receiver.

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。 The phase gradient is sometimes called “warp gradient”.

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 By changing the amplitude of the phase gradient as many times as the number of matrix, the complete data set is obtained. for 1st view for 2nd view for 3rd view

Raw Data 原始数据 Raw Data is the collection of views. 2D-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

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 TE TR Changing TR and TE will result in a different constrast.

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 PD image Long TR, short TE (short. The contrast mainly comes from the density of the spins. High density, stronger signal. T2 weighted image Long TR, long TE (around T2). Since TE is around T2, the contrast comes from the difference of T2 time of various tissues. T1 weighted image Short TR, short TE Proton Density (PD) weighted image 质子密度加权像 T2 weighted image T2加权像 T1 weighted image T1加权像

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 TR TE TE TE TE TE TE The RF frequency exciting each slice is different, so it will not affect the other slices. for slice 1 for slice 2 for slice 3

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 = 1000ms, TE = 25ms, 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 Number of view means the number of matrix in phase direction. When the matrix of phase direction is set larger than frequency direction, the “swap phase-frequency” function will automatically be selected by the system computer, in order to reduce time.