5_MRI基本脉冲序列与MR图像对比度分辨率信噪比ppt课件

1、,I x y,xyS k kMImage Characteristics S tl层选层选l频率编码频率编码l相位编码相位编码lK空间空间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与与2D采集的区别采集的区别RF脉冲是非选择性脉冲是非

3、选择性的的选层梯度场选层梯度场 相位相位编码编码3D FT扫描时间增加扫描时间增加(因为因为相位编码步数增加相位编码步数增加)为节省时间，可降低为节省时间，可降低Z方向的空间分辨方向的空间分辨力力,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饱和恢复序列饱和恢复序列 Saturation recoveryl部分饱和恢复序列部分饱和恢复序列 Partial saturationl反转恢复序列反转恢复序列 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 - (长间隔长间隔) - 90l每个每个90RF后，产生一个后，产生一个FID信号信号l如果如果M从和态完全恢复，每个从和态完全恢复，每个FID都可都可达到最大值达到最大值l饱和条件饱和条件?： l TRT2 3T2 ,nzMn001 /,RnTTxyzMMen 10011/RRTTTTfzAMee11011The amplitude of the FID signal is: /RrTTI rCre11Long TR PDW 1500msl90-(短间隔短间隔)- 90l第第2个个90RF脉冲后，脉冲后，M没有完全恢复，没有完全恢复，后面的

8、后面的FID信号比第信号比第1个个FID信号小。信号小。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 脉冲，脉冲，然后等待一段时间再使用一个然后等待一段时间再使用一个 90RF 脉脉冲。冲。( )()()t Tt TzzzMtMeMe11001TI( Inversion Time) /RrrTI TT TI rree111 2?作业1l对于典型的对于典型的SR序列序列T1 恢复曲线，纵向恢复曲线，纵向弛豫恢复曲线：弛豫恢复曲线： l1-exp(-t/T1)l在在IR序列

9、中，序列中，M由由M0 恢复到恢复到M0，恢复曲线：恢复曲线：l1-2exp(-t/T1)l信号过信号过0的点叫做的点叫做null pointl过过0点的时间点的时间TI(null)l TI(null)l = (ln2)T1l = 0.693T1l选择选择TI使脂肪信号过使脂肪信号过0点点l其他组织：其他组织：M可进入可进入xy平面产生随平面产生随T2衰减的衰减的信号；信号；l脂肪：没有脂肪：没有M可以可以l 进入进入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选择选择TI使使water信号过信号过0点点l其他组织：其他组织：M可进入可进入xy平面产生随平面产生随T2衰减的衰减的信号；信号；lwater ：没有：没有M可以可以l 进入进入oxy平面

12、；平面；lTI：2000msDiffusion MRI: A New Strategy for Assessment of Cancer Therapeutic EfficacyechoRFsignalreadoutphasesliceTEGzGyGxl1：90选择性选择性RF脉冲与脉冲与 Gz 层面选择层面选择l2：-Gz ：产生反方向相移，部分补偿：产生反方向相移，部分补偿+Gz产生的正向相移产生的正向相移提高信号提高信号l3：Gy 相位编码相位编码l4:180 RF 脉冲脉冲 形成回波形成回波l5:-Gx 产生负向相移，部分补偿后面产生负向相移，部分补偿后面 + Gx 产生的正向相移产生

13、的正向相移采集对称信号采集对称信号l6:+Gx 频率编码采样、读数）频率编码采样、读数） /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 长长 TR：16003000ms短短 TE：1030ms 长长 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对对GRE成像的影成像的影响？响？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的伪影比的伪影比SE的多；的多；SE的图像质量比的图像质量比GRE的好；的好；SE可得到可得到T2 对比度，而对比度，而GRE得到得到T2*对对比度。比度。 图像 A: TR = 350 图像 B: TR = 500 图像 C: TR= 1000图像 A: TE = 9 图像B: TE = 14 图像 C: TE = 40图像 A: TR = 1500 ms B: TR = 2500 ms C: TR= 4000 TE = 30图像A: TR= 1500 B: TR= 2500 C: TR= 4000 TE = 90A: TE = 25B: TE = 50C: TE =

17、 75D: TE = 100lSaturation Recovery lInversion RecoverylSTIR，FLAIRlSE: 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复数,xyi k x k yxyS k kx y ed

18、xdy k空间实际上是空间实际上是频率空间频率空间K空间位置与信空间位置与信号的空间位置没号的空间位置没有直接的对应关有直接的对应关系；系；k 空间内每个数空间内每个数据点都对图像有据点都对图像有贡献贡献k空间中心空间中心l视野视野Field of View， FOVl空间分辨力与空间分辨力与 (FOV)和扫描矩阵的关系：和扫描矩阵的关系：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右边，大于右边，大于 B0FOV左边，小于左边，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定义定义Gxtx为为 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信号水平与噪声标准差的比值信号水平与噪声标准差的比值l分辨率高，每个像素或体素越小，分辨率高，每个像素或体素越小，SNR下降下降l带宽越高，带宽越高，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，但是代价是采集变慢代价是采集变慢l图像后处理去噪图像后处理去噪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