5_MRI基本脈沖序列與MR圖像對比度分辨率信噪比ppt課件

1、,I x y,xyS k kMImage Characteristics S tl層選層選l頻率編碼頻率編碼l相位編碼相位編碼lK空間空間l1D/2D/3D 采集采集4/30/20224GzRFResonantFrequencyB xi G xtitS tx edx e0 xi G xtS tx edx1D 采集采集 - Line by Line, Line Scan4/30/20229GxPhase encodingPhase ShiftBFrequency X-PositionPhase Y-PositionlFrequency-Encoded Signals xi G xtS tx ed

2、x xik xxS kx edxxxkG t tk tGd0Gxtkx0kx0kx0180lPhase-Encoded Signals ypei G yTitS ty edy e0 yik yyS ky edyyypekG T tk tGd0tGypeTkykxkykx,xyi k x k yxyS k kI x y edxdy kykx,yik yyS x kI x y edy,I x y,yxik yik xxyS k kI x y edy edx,xyi k x k yxyS k kI x y edxdy ?FT-13D 采集采集3D與與2D采集的區(qū)別采集的區(qū)別RF脈沖是非選擇性脈沖是非

3、選擇性的的選層梯度場選層梯度場 相位相位編碼編碼3D FT掃描時間增加掃描時間增加(因為因為相位編碼步數(shù)增加相位編碼步數(shù)增加)為節(jié)省時間，可降低為節(jié)省時間，可降低Z方向的空間分辨方向的空間分辨力力,I x y,xyS k kMImage Characteristics S tWhich is better？What is the difference？Which is better？What is the difference？Which is better？What is the difference？Which is better？What is the difference？ Image

4、 QualityContrastResolutionNoise, Signal-to-Noise RatioArtifact Image QualityContrastResolutionNoise, Signal-to-Noise RatioArtifact*,I x yfT T T122T1-weighted, T2-weighted, Proton DensityAt a main field of 1.5 TTissue TypeApproximate T1 value in msApproximate T2 value in msAdipose tissues240-25060-80

5、Whole blood (deoxygenated)135050Whole blood (oxygenated)1350200Cerebrospinal fluid (similar to pure water)4200 - 45002100-2300Gray matter of cerebrum920100White matter of cerebrum78090Liver49040Kidneys65060-75Muscles860-90050In most situations (but not in principle) T1 is greater than T2. PDW T1W T2

6、WContrastl飽和恢復序列飽和恢復序列 Saturation recoveryl部分飽和恢復序列部分飽和恢復序列 Partial saturationl反轉恢復序列反轉恢復序列 Inversion recoverylStandard SElGRESaturation? In MR, saturation is a nonequilibrium state with no net magnetizationThe same amount of nuclear spins is aligned against and with the magnetic field ,nzMn001( )()

7、()t Tt TzzzMtMeMe11001l90 - (長間隔長間隔) - 90l每個每個90RF后，產生一個后，產生一個FID信號信號l如果如果M從和態(tài)完全恢復，每個從和態(tài)完全恢復，每個FID都可都可達到最大值達到最大值l飽和條件飽和條件?： l TRT2 3T2 ,nzMn001 /,RnTTxyzMMen 10011/RRTTTTfzAMee11011The amplitude of the FID signal is: /RrTTI rCre11Long TR PDW 1500msl90-(短間隔短間隔)- 90l第第2個個90RF脈沖后，脈沖后，M沒有完全恢復，沒有完全恢復，后面的

8、后面的FID信號比第信號比第1個個FID信號小。信號小。l短短TR，最小的，最小的TET1WShort TR T1W 500ms /RrTTI rCre11l180- 90 l在在IR序列中，首先使用序列中，首先使用180RF 脈沖，脈沖，然后等待一段時間再使用一個然后等待一段時間再使用一個 90RF 脈脈沖。沖。( )()()t Tt TzzzMtMeMe11001TI( Inversion Time) /RrrTI TT TI rree111 2?作業(yè)1l對于典型的對于典型的SR序列序列T1 恢復曲線，縱向恢復曲線，縱向弛豫恢復曲線：弛豫恢復曲線： l1-exp(-t/T1)l在在IR序列

9、中，序列中，M由由M0 恢復到恢復到M0，恢復曲線：恢復曲線：l1-2exp(-t/T1)l信號過信號過0的點叫做的點叫做null pointl過過0點的時間點的時間TI(null)l TI(null)l = (ln2)T1l = 0.693T1l選擇選擇TI使脂肪信號過使脂肪信號過0點點l其他組織：其他組織：M可進入可進入xy平面產生隨平面產生隨T2衰減的衰減的信號；信號；l脂肪：沒有脂肪：沒有M可以可以l 進入進入xy平面；平面；l1.5T： TI 140msl0.5T： TI 100msRadiotherapy induced fatty marrowA 67 y o female ,

10、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 of pain. Diffuse homogeneously high marrow signals involving L5 and all sacral vertebral bodies on T1 and T2 w images with complete s

11、ignal 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 for neoplastic marrow infiltration. l選擇選擇TI使使water信號過信號過0點點l其他組織：其他組織：M可進入可進入xy平面產生隨平面產生隨T2衰減的衰減的信號；信號；lwater ：沒有：沒有M可以可以l 進入進入oxy平面

12、；平面；lTI：2000msDiffusion MRI: A New Strategy for Assessment of Cancer Therapeutic EfficacyechoRFsignalreadoutphasesliceTEGzGyGxl1：90選擇性選擇性RF脈沖與脈沖與 Gz 層面選擇層面選擇l2：-Gz ：產生反方向相移，部分補償：產生反方向相移，部分補償+Gz產生的正向相移產生的正向相移提高信號提高信號l3：Gy 相位編碼相位編碼l4:180 RF 脈沖脈沖 形成回波形成回波l5:-Gx 產生負向相移，部分補償后面產生負向相移，部分補償后面 + Gx 產生的正向相移產生

13、的正向相移采集對稱信號采集對稱信號l6:+Gx 頻率編碼采樣、讀數(shù)）頻率編碼采樣、讀數(shù)） /RrrTI TT TI rree111 2 / /RRTTET rrT TTE TI rreee11221 2 / /RRTTET rrT TTE TI rreee11221 2 /RRrT TTE TTETI rree121TRTEPDWLongShortT1WShortShortT2WLongLongShortLong一般地一般地:短短 TR：1001000ms 長長 TR：16003000ms短短 TE：1030ms 長長 TE：70100ms /RrT TTE TI rree121For one

14、 slice Imaging Time = TR*Npe*NexSaturation-Recovery Spin-Echo /RrrTI TT TTE TI rreee1121 2*/sincosRRTTzTE TETTMeAee121011The T1 contrast is dependent mainly on the FA instead of the TRFA對對GRE成像的影成像的影響？響？Normal tissue usually have a small variation in spin density but have quite different T1values T1

15、W 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 detectionEcho formation: imaging mechanismImaging time: lower FA and shorter TR Image contrast: 一般的：一般的：SE的的SNR比比GRE的高；的高；GRE的速度比的速度

16、比SE的快；的快；GRE的偽影比的偽影比SE的多；的多；SE的圖像質量比的圖像質量比GRE的好；的好；SE可得到可得到T2 對比度，而對比度，而GRE得到得到T2*對對比度。比度。 圖像 A: TR = 350 圖像 B: TR = 500 圖像 C: TR= 1000圖像 A: TE = 9 圖像B: TE = 14 圖像 C: TE = 40圖像 A: TR = 1500 ms B: TR = 2500 ms C: TR= 4000 TE = 30圖像A: TR= 1500 B: TR= 2500 C: TR= 4000 TE = 90A: TE = 25B: TE = 50C: TE =

17、 75D: TE = 100lSaturation Recovery lInversion RecoverylSTIR，F(xiàn)LAIRlSE: To get images of T1w，T2w，PDW， How to choose TR and TElGRE: FA? Image QualityContrastResolutionNoise, Signal-to-Noise RatioArtifactResolutionkykxPulse Sequencekykx,I x y,xyi k x k yxyS k kI x y edxdy FT-1復數(shù),xyi k x k yxyS k kx y ed

18、xdy k空間實際上是空間實際上是頻率空間頻率空間K空間位置與信空間位置與信號的空間位置沒號的空間位置沒有直接的對應關有直接的對應關系；系；k 空間內每個數(shù)空間內每個數(shù)據(jù)點都對圖像有據(jù)點都對圖像有貢獻貢獻k空間中心空間中心l視野視野Field of View， FOVl空間分辨力與空間分辨力與 (FOV)和掃描矩陣的關系：和掃描矩陣的關系：l x = FOVx / Nxl y = FOVy / NyFOVxFOVyFOVx FOVy =240mm，Nx = Ny 256，求，求 x， ykykxIDFTData space (analog), k space (digital) imageyx

19、FOV中央，中央，B0FOV右邊，大于右邊，大于 B0FOV左邊，小于左邊，小于B0 x= Bx= Gxx fmax= GxFOVx/2or BW= GxFOVxBandWidth BW=2 fmax FOV=BW/G=1/GTs 1/FOV=GTs，Ts :dwell timeX方向：方向： 1/FOVx=GxtxBW=GxFOVx定義定義Gxtx為為 kxkx =Gxtx=1/FOVx kxx=1/Nxky =Gyty=1/FOVy1/FOVx=Gxtx降低降低 FOV 的方法的方法:Stronger gradientLower BWFOV=BW/G kx = Gx tx=1/FOVx l

20、SNR信號水平與噪聲標準差的比值信號水平與噪聲標準差的比值l分辨率高，每個像素或體素越小，分辨率高，每個像素或體素越小，SNR下降下降l帶寬越高，帶寬越高，noise越高，越高，SNR下降下降l采用多次采集平均可以提高采用多次采集平均可以提高SNR ，但是，但是代價是采集變慢代價是采集變慢l圖像后處理去噪圖像后處理去噪lThe receiver (or acquisition) bandwidth (rBW) is the range of frequencies accepted by the receiver to sample the MR signal. lThe receiver b

21、andwidth is changeable and has a direct relationship to the signal to noise ratio (SNR)l SNR = 1/squareroot (rBW)lrBW=GxFOVx lrBW=1/Ts (dwell time)lSometimes: rBW is defined by BW = Sampling Rate/Number of Samples (the bandwidth per pixel)lA smaller bandwidth improves SNR, but can cause spatial dist

22、ortions, also increases the chemical shift. A larger bandwidth reduces SNR (more noise from the outskirts of the spectrum), but allows faster imaging.lThe 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