核磁共振成像原理

1、,,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現(xiàn)象?射頻系統(tǒng)梯度系統(tǒng)Spin Echo sequence（自旋回波序列）,,什么是核磁共振成像 ?,MRI - Magnetic Resonance ImagingMagnetic Field（磁場）Radio Frequency Wave (RF)（射頻）Nuclei of the tissue generates MR Signal（組織

2、的核產(chǎn)生MR信號）,,射頻發(fā)射及接收器,發(fā)射射頻前,RFTransceiver,transmitting RF（發(fā)射射頻）,射頻發(fā)射及接收器,RFTransceiver,,MR Signal（MR信號）,after transmitting RF（發(fā)射射頻后）,射頻發(fā)射及接收器,,核磁共振的硬件,,SystemComputer,Scan SubsystemRF Hardware,GradientSubsysem,RF

3、Coil,Magnet,Gradient Coil,,,,,,,,,,Scan RoomHardware,,,,什么是核磁共振現(xiàn)象?,,,,Proton質(zhì)子,,,,Electron電子,1H,Billions of HydrogenAtoms in the body,,,,,1H proton spinning（氫質(zhì)子自旋）,“Magnetic Moment” 磁矩,in magnetic field,in

4、 normal environment,magnetic moment （磁矩）,,,,,,,,,,,,,,+,net magnetization （凈磁化）,,,,,,,,parallel,anti-parallel,parallel > antiparallel,The behavior of magnetic moment of Proton in a magnetic field

5、 （在磁場中質(zhì)子磁矩的行為）,The magnetic field (Bo) causes precession of the magnetic moment（磁場導致磁矩的進動）,Bo,net magnetization凈磁化,,Frequency of precession - unique to each nucleus進動頻率 Larmor Equation w= gB,,,,,High

6、Magnetic Field (1.5T)63.89 MHz(1H)高磁場,Low Magnetic Field (0.5T)21.29 MHz(1H)低磁場,w : angular frequency角頻率g : magnetogyric ratio磁旋比B : magnetic field strength 磁場強度,,Resonance共振 is - absorbing and reemitt

7、ing energy吸收和 再釋放能量,example : tuning fork振動音叉,,sonic wave聲波,C,D,E,F,G,A,B,,C,D,E,F,G,A,B,,,larmor frequency is 21.29 MHz,,,,,the precession angle increases（進動角增加）(absorbs energy),RF pulse21.29 MHz,RF Off,,,,goes

8、back to original state（回到原始狀態(tài)）(reemits energy),In MR, Radio Frequency waves (RF) are absorbed and reemitted在MR中，射頻波是吸收和再釋放,,,review,What is needed for resonance?共振需要什么？large number of nuclei (with spin and magnetic

9、 moment)Hydrogen atoms氫質(zhì)子 a static magnetic field凈磁場 Magnet磁體 RF pulse射頻脈沖absorb and re-emit energythe “re-emitted energy is the “MR Signal”（再釋放的能量是MR信號）,The actual pulse transmitted to the RF coil is a sinc pu

10、lse,RF Amplifier射頻放大器,,RF pulse,,,After the RF pulse is OFF, the RF coil acts as a receiving coil,,Receiver接收器,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,RF Excitation,1. It makes the nuclei precess “in phase”,RF pulse21.29 MHz,sa

11、me frequencydifferent phase相同頻率不同相位,same frequency same phase (“in phase”)相同頻率和相位,- in 0.5 T-,,,What does the RF pulse do?,2. It transfers energy to the nuclei,,RF pulse21.29 MHz,Net magnetization “flips”凈磁化翻轉(zhuǎn),The

12、 “flipping” motion is actually a spiralling motion:翻轉(zhuǎn)移動實際上是一個螺旋形移動,,,90° pulse, 180° pulse,“90° pulse” 90° 脈沖,“180° pulse” 180°脈 沖,,,,,,,,,,,,,,,,How is the signal detected?,1.The net mag

13、netization induces current in the transverse plane 凈磁化感應橫軸位電流(electromagnetic induction law)（電磁感應定律）,,,,,,,2.The RF coil picks up the current as the signal,,,,,,After the RF pulse is removed,,,,,FID signal自由感應信號(Free

14、Induction Decay),in phase同相位,out of phase失相位,nuclei precession 核進動View from top,current induced in transverse plane橫軸位感應電流,,time,none,,1. Loss of “in phase” (T2:spin-spin relaxation自旋-自旋弛豫),Relaxation (T1:spin

15、-lattice relaxation) 弛豫（自旋-晶格弛豫）,,,,,,,,net magnetization realigning with the main magnetic field凈磁化與主磁場再對齊,,time,縱向弛豫時間（T1)是指90度脈沖后，達到原縱向磁化矢量63%的時間。,橫向弛豫時間（T2)是指90度脈沖后，原橫向磁化矢量值衰減到37%的時間。,,,1 T2 2 T2

16、 3 T2 4 T2 5 T2Time (TE),Time,63%,86%,95%,98%,100%,,,,,,,Rephase相位重聚,The 180° pulse is applied to rotate the magnetization back to phase.The signal obtained is called “echo”. 180度脈沖將磁

17、化旋轉(zhuǎn)回同相位，獲得信號稱為回波,time,,,,,,,,,,,,180° pulse,,,,,,in phase,in phase again,fast,slow,slow,fast,The RF pulse “flips” the net magnetization vector射頻脈沖翻轉(zhuǎn)凈磁化矢量actually it is a spiralling motion實際上它是螺旋形移動When the RF co

18、il does not send RF, it acts as a receiving coil當射頻線圈不發(fā)放射頻脈沖時，它充當接收線圈“MR signal” is actually the current induced by the magnetization vectorMR信號實際上是由磁化矢量感應的電流After the RF pulse is turned off,the current induce

19、d begins to weaken - the signal begins to decay (FID)當射頻脈沖停止后，感應電流開始變?nèi)酢盘栭_始衰減,review,,梯度磁場,Gradient fields are needed for spatial information（空間信息）需要梯度場提供空間信息,What are gradient fields for?,線性梯度場linear gradient field (

20、z axis),Z軸梯度z axis gradient ON,,,,0.5 T,,,,,,,,,,靜磁場static magnetic field Bo (0.5T),,,,,Z,Y,X,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,Z,Y,X,X軸梯度x axis gradient ON,Y軸梯度y axis gradie

21、nt ON,with no gradients,All protons within the bore precesses at 21.29 MHz 孔內(nèi)所有質(zhì)子以21.29MHz頻率進動,0.5 T,z axis gradient ON,,,,0.5 T,,,,,,,,,,,,,,,21.29 MHz,,Slice Selective Excitation層面選擇激勵,僅21.29MHz頻率的層面質(zhì)子進動,Frequency a

22、nd Phase頻率和相位,After the other two gradients, the spins in each voxel has different frequency and phase 加上其它兩個梯度后，每一體素的自旋有不同頻率和相位,,21.29 MHz,,unique signal from each voxel,phase shifted相位轉(zhuǎn)換,,change the frequency改變

23、頻率,Fourier Transformation傅立葉轉(zhuǎn)換,Amplitude / Time Ù Amplitude / Frequency 振幅/時間 振幅/頻率,,Time時間,,Frequency頻率,,,single frequency單個頻率,single peak單峰,two frequencies兩個頻率,two peaks雙峰,revie

24、w,Gradient fields are needed to change the spatial variation of the magnetic field. 需要梯度場來改變磁場的空間差異With the gradient field:The resonance frequencies will vary according to the intensity of the magnetic field

25、共振頻率隨著磁場強度改變而變化Transverse magnetization will be “out of phase” 橫向磁化將失相位The MR signal is an interference pattern of various frequencies in the form of amplitude/time. MR信號是在振幅/時間形式內(nèi)不同頻率的干擾模式。Fourier Transfo

26、rmation is used to convert the signals to the form of amplitude/frequency. 利用傅立葉轉(zhuǎn)換將信號轉(zhuǎn)為振幅/頻率形式,,,掃描序列,Let’s combine the items that we have covered so far, into a basic sequence; “Spin Echo”自旋回波,RF pulse射頻脈沖,Slice gr

27、adient層面梯度,Phase gradient相位梯度,Frequency gradient 頻率梯度,Echo signal 回波信號,single slice, single echo 單層，單回波,RF pulses and Slice gradient射頻脈沖和層面梯度,The RF pulse is applied with the “slice gradient” to excite a sing

28、le slice.用射頻脈沖和層面梯度來激發(fā)一個單一層面 The height of a gradient pulse represents the amplitude of the gradient, and the width is its duration 梯度脈沖的高度代表梯度振幅，寬度是持續(xù)時間,,,,,,,,,,,,,,,,,,,,Frequency gradient頻率梯度,The “frequency gra

29、dient” causes the frequencies of the spins to be proportional to the positions along the axis頻率梯度導致自旋的頻率與沿軸的位置相一致A “dephaser” is applied in order to prevent dephasing due to the frequency gradient擾相器是為了防止因頻率梯度引起的失相,,,

30、,,,,,,,,,,,,,,,,,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變化在回波時間里將被保留為相位記憶This “phase gradient” will not affect the detected fr

31、equencies, because it is not on within the acquisition window這相位梯度將不影響檢測頻率，因為它不在采集窗內(nèi)。,,,,,,,,,,,,,,,,,,,,Phase gradient (2)相位梯度,Actually, the pulse sequence is played out many times, and the signals are stored separatel

32、y 脈沖序列多次起作用，信號分開儲存These data sets (views) are identical in respect to frequency, but not with respect to phase這些數(shù)據(jù)組頻率是完全一致的，而相位是不同的,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,for 1st view,for 2nd view,for 3rd vie

33、w,Raw Data原始數(shù)據(jù),Raw Data is the collection of views. 2D-FFT is used to reconstruct the image原始數(shù)據(jù)是觀察的聚集，2維-傅利葉轉(zhuǎn)換被用來重建圖像,,Scan Parameters: TE,TR掃描參數(shù)：回波時間，重復時間,TR (Repetition Time)重復時間The time interval from the beginning

34、 of a pulse sequence until the beginning of a next pulse sequence一個射頻脈沖的開始到下一個射頻脈沖的開始的時間間隔TE (Echo Time)回波時間The time interval from the first RF pulse of a pulse sequence to the middle of an echo脈沖序列的第一個射頻脈沖到一個回波的中點的時

35、間間隔,Contrast對比,Contrast depends on tissue-to tissue image intensity variations對比依賴于組織與組織成像強度差異,Proton Density (PD) weighted image質(zhì)子密度加權(quán)像,T2 weighted imageT2加權(quán)像,T1 weighted imageT1加權(quán)像,,,Multislice Acquisition多層面采集,T

36、E is much shorter than TR （TE明顯短于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間期能進行多層面激勵,,,,,,,,,,,,,,,,,,,,,,,TE,TE,TE,TE,TE,TE,

37、,,,,,,,,,,TR,for slice 1,for slice 2,for slice 3,3 slice 2 echo sequence,,,Acquisition Time采集時間,Total Acquisition time: TR x number of views x NEX （總采集時間）NEX (number of excitation): Repeating the enti

38、re acquisition of the same slice for a number of times to gain SNR,example:TR = 1000ms, TE = 25ms, matrix = 256 x 128, NEX = 2acquisition time = 1000 x 128 x 2 = 256,000 ms 采集時間