MRI基本脈沖序列與MR圖像對比度分辨率信噪比課件

1、MR成像方法和脈沖序列MR成像方法和脈沖序列Image CharacteristicsImage Characteristics上次課主要內(nèi)容層選頻率編碼相位編碼K空間1D/2D/3D 采集上次課主要內(nèi)容層選10/10/20224Slice SelectionGzRFResonantFrequency10/9/20224Slice SelectionGzRFRMRI基本脈沖序列與MR圖像對比度分辨率信噪比課件MRI基本脈沖序列與MR圖像對比度分辨率信噪比課件1D 采集 - Line by Line, Line Scan1D 采集 - Line by Line, Line SPhase Enco

2、dingPhase Encoding10/10/202292D Spatial Encoding Frequency and Phase EncodingGxPhase encodingPhase ShiftFrequency X-PositionPhase Y-Position10/9/202292D Spatial Encoding k-SpaceFrequency-Encoded SignalsGxtkx0kx0kx0180k-SpaceFrequency-Encoded Signak-SpacePhase-Encoded SignalstGykykxk-SpacePhase-Encod

3、ed SignalstGkykxFor 2D imagingkykxFor 2D imagingkykx?FT-1kykx?FT-13D 采集3D與2D采集的區(qū)別RF脈沖是非選擇性的選層梯度場 相位編碼3D FT掃描時間增加(因為相位編碼步數(shù)增加)為節(jié)省時間，可降低Z方向的空間分辨力3D 采集3D與2D采集的區(qū)別Image CharacteristicsImage CharacteristicsWhich is better？What is the difference？Which is better？What is the diWhich is better？What is the diff

4、erence？Which is better？What is the dWhich is better？What is the difference？Which is better？What is the diWhich is better？What is the difference？Which is better？What is the di Image QualityContrastResolutionNoise, Signal-to-Noise RatioArtifact Image Quality Image QualityContrastResolutionNoise, Signa

5、l-to-Noise RatioArtifactT1-weighted, T2-weighted, Proton Density Image QualityT1-weighted, T2At a main field of 1.5 TTissue TypeApproximate T1 value in msApproximate T2 value in msAdipose tissues240-25060-80Whole blood (deoxygenated)135050Whole blood (oxygenated)1350200Cerebrospinal fluid (similar t

6、o pure water)4200 - 45002100-2300Gray matter of cerebrum920100White matter of cerebrum78090Liver49040Kidneys65060-75Muscles860-90050In most situations (but not in principle) T1 is greater than T2. At a main field of 1.5 TTissuePDW T1W T2WContrastPDW MR 基本脈沖序列飽和恢復序列 Saturation recovery部分飽和恢復序列 Partia

7、l saturation反轉(zhuǎn)恢復序列 Inversion recoveryStandard SEGREMR 基本脈沖序列飽和恢復序列 Saturation r飽和恢復序列Saturation RecoverySaturation? In MR, saturation is a nonequilibrium state with no net magnetizationThe same amount of nuclear spins is aligned against and with the magnetic field飽和恢復序列Saturation RecoverySatu飽和恢復序列S

8、aturation Recovery飽和恢復序列Saturation Recovery飽和恢復序列Saturation Recovery90 - (長間隔) - 90每個90RF后，產(chǎn)生一個FID信號如果M從和態(tài)完全恢復，每個FID都可達到最大值飽和條件?： TRT2 3T2飽和恢復序列Saturation Recovery90The amplitude of the FID signal is:Long TR PDW 1500msThe amplitude of the FID signII. 部分飽和恢復序列Partial Saturation Recovery90-(短間隔)- 90第2

9、個90RF脈沖后，M沒有完全恢復，后面的FID信號比第1個FID信號小。短TR，最小的TET1WShort TR T1W 500msII. 部分飽和恢復序列Partial SaturatioMRI基本脈沖序列與MR圖像對比度分辨率信噪比課件III. 反轉(zhuǎn)恢復序列 IR Inversion Recovery180- 90 在IR序列中，首先使用180RF 脈沖，然后等待一段時間再使用一個 90RF 脈沖。III. 反轉(zhuǎn)恢復序列 IR Inversion ReIII. 反轉(zhuǎn)恢復序列 IR Inversion RecoveryTI( Inversion Time)?作業(yè)1III. 反轉(zhuǎn)恢復序列 IR

10、 Inversion Re對于典型的SR序列T1 恢復曲線，縱向弛豫恢復曲線： 1-exp(-t/T1)在IR序列中，M由M0 恢復到M0，恢復曲線：1-2exp(-t/T1)對于典型的SR序列T1 恢復曲線，縱向弛豫恢復曲線： 0點（Null point）信號過0的點叫做null point過0點的時間TI(null) TI(null) = (ln2)T1 = 0.693T10點（Null point）信號過0的點叫做null poi脂肪抑制 Fat suppression STIR （Short TI Inversion Recovery）選擇TI使脂肪信號過0點其他組織：M可進入xy平面

11、產(chǎn)生隨T2衰減的信號；脂肪：沒有M可以 進入xy平面；1.5T： TI 140ms0.5T： TI 100ms脂肪抑制 Fat suppression STIR （SRadiotherapy induced fatty marrowA 67 y o female , a known case of Ca cervix with radiotherapy. Now complains of severe backache, radiating to lower limbs. Referred to MRI lumbar spine to rule out metastasis and cause

12、 of pain.Diffusehomogeneouslyhigh marrow signals involving L5 and all sacral vertebral bodies on T1 and T2 w images with complete signal suppression on STIR implies to fatty marrow, it s a radiotherapy induced change, area of involvement corresponds to field of radiotherapy, should not be mistaken f

13、or neoplastic marrow infiltration. Radiotherapy induced fatty mar（液體）流體抑制 FLAIR（ Fluid-Attenuated Inversion Recovery ）選擇TI使water信號過0點其他組織：M可進入xy平面產(chǎn)生隨T2衰減的信號；water ：沒有M可以 進入oxy平面；TI：2000ms（液體）流體抑制 FLAIR（ Fluid-AttenuaMRI基本脈沖序列與MR圖像對比度分辨率信噪比課件Diffusion MRI: A New Strategy for Assessment of Cancer Ther

14、apeutic EfficacyDiffusion MRI: A New Strategy IV. Basic Spin-Echo ImagingIV. Basic Spin-Echo ImagingSE Spin echo echoRFsignalreadoutphasesliceTEGzGyGxSE Spin echo echoRFsignalreaMRI基本脈沖序列與MR圖像對比度分辨率信噪比課件1：90選擇性RF脈沖與 Gz 層面選擇2：-Gz ：產(chǎn)生反方向相移，部分補償+Gz產(chǎn)生的正向相移提高信號3：Gy 相位編碼4:180 RF 脈沖 形成回波5:-Gx 產(chǎn)生負向相移，部分補償后面

15、 + Gx 產(chǎn)生的正向相移采集對稱信號6:+Gx 頻率編碼（采樣、讀數(shù)）1：90選擇性RF脈沖與 Gz 層面選擇MRI基本脈沖序列與MR圖像對比度分辨率信噪比課件MRI基本脈沖序列與MR圖像對比度分辨率信噪比課件TRTEPDWLongShortT1WShortShortT2WLongLongShortLong一般地:短 TR：1001000ms 長 TR：16003000ms短 TE：1030ms 長 TE：70100msFor one slice Imaging Time = TR*Npe*NexTRTEPDWLongShortT1WShortShortTSE Spin echoSatura

16、tion-Recovery Spin-EchoSE Spin echoSaturation-RecovInversion-Recovery Spin-EchoInversion-Recovery Spin-EchoBasic Gradient EchoThe T1 contrast is dependent mainly on the FA instead of the TRFA對GRE成像的影響？Basic Gradient EchoThe T1 contNormal tissue usually have a small variation in spin density but have

17、 quite different T1values T1W image usually have a good anatomical definitionMany disease states are characterized by a change of the tissue T2 value,T2W imaging is a sensitive method for disease detectionNormal tissue usually have a sSE vs GREEcho formation: imaging mechanismImaging time: lower FA

18、and shorter TR Image contrast: SE vs GREEcho formation: imagi一般的：SE的SNR比GRE的高；GRE的速度比SE的快；GRE的偽影比SE的多；SE的圖像質(zhì)量比GRE的好；SE可得到T2 對比度，而GRE得到T2*對比度。 一般的：ContrastContrastContrastContrast圖像 A: TR = 350 圖像 B: TR = 500 圖像 C: TR= 1000T1像:改變TR圖像 A: TR = 350 圖像 B: TR =圖像 A: TE = 9 圖像B: TE = 14 圖像 C: TE = 40T1像:改變

19、TE圖像 A: TE = 9 圖像B: TE圖像 A: TR = 1500 ms B: TR = 2500 ms C: TR= 4000 TE = 30PD像:改變TR圖像 A: TR = 1500 ms B: TR 圖像A: TR= 1500 B: TR= 2500 C: TR= 4000 TE = 90T2像:改變TR圖像A: TR= 1500 B: TT2像:改變TEA: TE = 25B: TE = 50C: TE = 75D: TE = 100T2像:改變TEA: B: C: D: QuestionSaturation Recovery Inversion RecoverySTIR，

20、FLAIRSE: To get images of T1w，T2w，PDW， How to choose TR and TEGRE: FA?QuestionSaturation Recovery Image QualityContrastResolutionNoise, Signal-to-Noise RatioArtifact Image QualityResolutionResolutionFor 2D imagingkykxPulse SequenceFor 2D imagingkykxPulse SequenkykxFT-1復數(shù)kykxFT-1復數(shù)k 空間特點k空間實際上是頻率空間K空

21、間位置與信號的空間位置沒有直接的對應(yīng)關(guān)系；k 空間內(nèi)每個數(shù)據(jù)點都對圖像有貢獻k空間中心k 空間特點k空間實際上是頻率空間k空間中心MRI基本脈沖序列與MR圖像對比度分辨率信噪比課件MRI基本脈沖序列與MR圖像對比度分辨率信噪比課件空間編碼與空間分辨力視野Field of View， FOV空間分辨力與 (FOV)和掃描矩陣的關(guān)系： x = FOVx / Nx y = FOVy / NyFOVxFOVyFOVx FOVy =240mm，Nx = Ny 256，求x， y空間編碼與空間分辨力視野Field of View， FOVkykxIDFTk space sampling and image

22、Data space (analog), k space (digital) imageyxkykxIDFTk space sampling and iFOV中央，B0FOV右邊，大于 B0FOV左邊，小于B0 x=Bx=Gxx fmax=GxFOVx/2or BW=GxFOVxBandWidth BW=2 fmax FOV中央，B0 x=B FOV=BW/G=1/GTs 1/FOV=GTs，Ts :dwell timeX方向： 1/FOVx=GxtxBW=GxFOVx FOV=BW/G=1/GTs定義Gxtx為 kxkx =Gxtx=1/FOVx kxx=1/Nxky =Gyty=1/FOV

23、y1/FOVx=Gxtx定義Gxtx為 kx1/FOVx=Gxtx降低 FOV 的方法:Stronger gradientLower BWFOV=BW/Gkx =Gxtx=1/FOVx 降低 FOV 的方法:FOV=BW/Gkx =GxtNoise， SNRNoise， SNR信噪比Signal-to-Noise Ratio SNRSNR信號水平與噪聲標準差的比值分辨率高，每個像素或體素越小，SNR下降帶寬越高，noise越高，SNR下降采用多次采集平均可以提高SNR ，但是代價是采集變慢圖像后處理去噪信噪比Signal-to-Noise Ratio SNRSNTwo Bandwidth？Tw

24、o Bandwidth？The receiver (or acquisition) bandwidth (rBW) is the range of frequencies accepted by the receiver to sample the MR signal. The receiver bandwidth is changeable and has a direct relationship to the signal to noise ratio (SNR) SNR = 1/squareroot (rBW)rBW=GxFOVx rBW=1/Ts (dwell time)Someti

25、mes: rBW is defined by BW = Sampling Rate/Number of Samples (the bandwidth per pixel)Receiver BandwidthThe receiver (or acquisition) A smaller bandwidth improves SNR, but can cause spatial distortions, also increases the chemical shift. A larger bandwidth reduces SNR (more noise from the outskirts o

26、f the spectrum), but allows faster imaging.A smaller bandwidth improves STransmit BandwidthThe transmit bandwidth refers to the RF excitation pulse required for slice selection in a pulse sequence. The slice thickness is proportional to the bandwidth of the RF pulse (and inversely proportional to the applied gradient strength). Lowering the pulse bandwidth can reduce the slice thickness.Transmit BandwidthThe transmitNoise in MR ima