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1、MR成像方法和脉冲序列,MR成像方法和脉冲序列,Image Characteristics,Image Characteristics,上次课主要内容,层选频率编码相位编码K空间1D/2D/3D 采集,上次课主要内容层选,11/4/2022,4,Slice Selection,Gz,RF,ResonantFrequency,10/9/20224Slice SelectionGzRFR,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,1D 采集 - Line by Line, Line Scan,1D 采集 - Line by Line,
2、Line S,Phase Encoding,Phase Encoding,11/4/2022,9,2D Spatial Encoding Frequency and Phase Encoding,Gx,Phase encoding,Phase Shift,Frequency X-PositionPhase Y-Position,10/9/202292D Spatial Encoding,k-Space,Frequency-Encoded Signals,kx,0,kx,0,kx,0,180,k-SpaceFrequency-Encoded Signa,k-Space,Phase-Encoded
3、 Signals,t,Gy,k-SpacePhase-Encoded SignalstG,For 2D imaging,kykxFor 2D imaging,?,kykx?FT-1,3D 采集,3D与2D采集的区别RF脉冲是非选择性的选层梯度场 相位编码3D FT扫描时间增加(因为相位编码步数增加)为节省时间，可降低Z方向的空间分辨力,3D 采集3D与2D采集的区别,Image Characteristics,Image Characteristics,Which is better？What is the difference？,Which is better？What is the di,
4、Which is better？What is the difference？,Which is better？What is the d,Which is better？What is the difference？,Which is better？What is the di,Which is better？What is the difference？,Which is better？What is the di,Image QualityContrastResolutionNoise, Signal-to-Noise RatioArtifact,Image Quality,Image
5、QualityContrastResolutionNoise, Signal-to-Noise RatioArtifact,T1-weighted, T2-weighted, Proton Density,Image QualityT1-weighted, T2,In most situations (but not in principle) T1 is greater than T2.,At a main field of 1.5 TTissue,PDW T1W T2W,Contrast,PDW,MR 基本脉冲序列,饱和恢复序列 Saturation recovery部分饱和恢复序列 Pa
6、rtial saturation反转恢复序列 Inversion recoveryStandard SEGRE,MR 基本脉冲序列饱和恢复序列 Saturation r,饱和恢复序列Saturation Recovery,Saturation? In MR, saturation is a nonequilibrium state with no net magnetizationThe same amount of nuclear spins is aligned against and with the magnetic field,饱和恢复序列Saturation RecoverySat
7、u,饱和恢复序列Saturation Recovery,饱和恢复序列Saturation Recovery,饱和恢复序列Saturation Recovery,90 - (长间隔) - 90每个90RF后，产生一个FID信号如果M从和态完全恢复，每个FID都可达到最大值饱和条件?： TRT2 3T2,饱和恢复序列Saturation Recovery90,The amplitude of the FID signal is:,Long TR PDW 1500ms,The amplitude of the FID sign,II. 部分饱和恢复序列Partial Saturation Recov
8、ery,90-(短间隔)- 90第2个90RF脉冲后，M没有完全恢复，后面的FID信号比第1个FID信号小。短TR，最小的TE,T1W,Short TR T1W 500ms,II. 部分饱和恢复序列Partial Saturatio,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,III. 反转恢复序列 IR Inversion Recovery,180- 90 在IR序列中，首先使用180RF 脉冲，然后等待一段时间再使用一个 90RF 脉冲。,III. 反转恢复序列 IR Inversion Re,III. 反转恢复序列 IR Inversion Recovery,TI( Invers
9、ion Time),?作业1,III. 反转恢复序列 IR Inversion Re,对于典型的SR序列T1 恢复曲线，纵向弛豫恢复曲线： 1-exp(-t/T1)在IR序列中，M由M0 恢复到M0，恢复曲线：1-2exp(-t/T1),对于典型的SR序列T1 恢复曲线，纵向弛豫恢复曲线：,0点（Null point）,信号过0的点叫做null point过0点的时间TI(null) TI(null) = (ln2)T1 = 0.693T1,0点（Null point）信号过0的点叫做null poi,脂肪抑制 Fat suppression STIR （Short TI Inversion
10、Recovery）,选择TI使脂肪信号过0点其他组织：M可进入xy平面产生随T2衰减的信号；脂肪：没有M可以 进入xy平面；1.5T： TI 140ms0.5T： TI 100ms,脂肪抑制 Fat suppression STIR （S,Radiotherapy 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 s
11、pine to rule out metastasis and cause 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 r
12、adiotherapy, should not be mistaken for neoplastic marrow infiltration.,Radiotherapy induced fatty mar,（液体）流体抑制 FLAIR（ Fluid-Attenuated Inversion Recovery ）,选择TI使water信号过0点其他组织：M可进入xy平面产生随T2衰减的信号；water ：没有M可以 进入oxy平面；TI：2000ms,（液体）流体抑制 FLAIR（ Fluid-Attenua,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,Diffusion MRI: A
13、New Strategy for Assessment of Cancer Therapeutic Efficacy,Diffusion MRI: A New Strategy,IV. Basic Spin-Echo Imaging,IV. Basic Spin-Echo Imaging,SE Spin echo,echo,RF,signal,readout,phase,slice,TE,Gz,Gy,Gx,SE Spin echo echoRFsignalrea,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,1：90选择性RF脉冲与 Gz 层面选择2：-Gz ：产生反方向相移，部分补偿+
14、Gz产生的正向相移提高信号3：Gy 相位编码4:180 RF 脉冲 形成回波5:-Gx 产生负向相移，部分补偿后面 + Gx 产生的正向相移采集对称信号6:+Gx 频率编码（采样、读数）,1：90选择性RF脉冲与 Gz 层面选择,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,一般地:短 TR：1001000ms 长 TR：16003000ms短 TE：1030ms 长 TE：70100ms,For one slice Imaging Time = TR*Npe*Nex,TRTEPDWLongShortT1WShortShortT,SE
15、Spin echo,Saturation-Recovery Spin-Echo,SE Spin echoSaturation-Recov,Inversion-Recovery Spin-Echo,Inversion-Recovery Spin-Echo,Basic Gradient Echo,The T1 contrast is dependent mainly on the FA instead of the TR,FA对GRE成像的影响？,Basic Gradient EchoThe T1 cont,Normal tissue usually have a small variation
16、in spin density but have 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 detection,Normal tissue usually have a s,SE vs GRE,Echo formation: imaging mecha
17、nismImaging time: lower FA 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的伪影比SE的多；SE的图像质量比GRE的好；SE可得到T2 对比度，而GRE得到T2*对比度。,一般的：,Contrast,Contrast,Contrast,Contrast,图像 A: TR = 350 图像 B: TR = 500 图像 C: TR= 1000,T1像:改变TR,图像 A: TR = 350 图像 B: TR =,图像 A: TE
18、= 9 图像B: TE = 14 图像 C: TE = 40,T1像:改变TE,图像 A: TE = 9 图像B: TE,图像 A: TR = 1500 ms B: TR = 2500 ms C: TR= 4000 TE = 30,PD像:改变TR,图像 A: TR = 1500 ms B: TR,图像A: TR= 1500 B: TR= 2500 C: TR= 4000 TE = 90,T2像:改变TR,图像A: TR= 1500 B: T,T2像:改变TE,A: TE = 25,B: TE = 50,C: TE = 75,D: TE = 100,T2像:改变TEA: B: C: D:,Qu
19、estion,Saturation Recovery Inversion RecoverySTIR，FLAIRSE: To get images of T1w，T2w，PDW， How to choose TR and TEGRE: FA?,QuestionSaturation Recovery,Image QualityContrastResolutionNoise, Signal-to-Noise RatioArtifact,Image Quality,Resolution,Resolution,For 2D imaging,Pulse Sequence,For 2D imagingkyk
20、xPulse Sequen,复数,kykxFT-1复数,k 空间特点,k空间实际上是频率空间K空间位置与信号的空间位置没有直接的对应关系；k 空间内每个数据点都对图像有贡献,k空间中心,k 空间特点k空间实际上是频率空间k空间中心,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,MRI基本脉冲序列与MR图像对比度分辨率信噪比课件,空间编码与空间分辨力,视野Field of View， FOV空间分辨力与 (FOV)和扫描矩阵的关系： x = FOVx / Nx y = FOVy / Ny,FOVx,FOVy,FOVx FOVy =240mm，Nx = Ny 256，求x， y,空间编码与空
21、间分辨力视野Field of View， FOV,ky,kx,IDFT,k space sampling and image,Data space (analog), k space (digital) image,y,x,kykxIDFTk space sampling and i,FOV中央，B0FOV右边，大于 B0FOV左边，小于B0,x=Bx=Gxx fmax=GxFOVx/2or BW=GxFOVx,BandWidth BW=2 fmax,FOV中央，B0 x=B,FOV=BW/G=1/GTs 1/FOV=GTs，Ts :dwell timeX方向： 1/FOVx=Gxtx,BW=
22、GxFOVx,FOV=BW/G=1/GTs,定义Gxtx为 kxkx =Gxtx=1/FOVx kxx=1/Nxky =Gyty=1/FOVy,1/FOVx=Gxtx,定义Gxtx为 kx1/FOVx=Gxtx,降低 FOV 的方法:Stronger gradientLower BW,FOV=BW/G,kx =Gxtx=1/FOVx,降低 FOV 的方法:FOV=BW/Gkx =Gxt,Noise， SNR,Noise， SNR,信噪比Signal-to-Noise Ratio SNR,SNR信号水平与噪声标准差的比值分辨率高，每个像素或体素越小，SNR下降带宽越高，noise越高，SNR下降
23、采用多次采集平均可以提高SNR ，但是代价是采集变慢图像后处理去噪,信噪比Signal-to-Noise Ratio SNRSN,Two Bandwidth？,Two 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
24、to noise ratio (SNR) SNR = 1/squareroot (rBW)rBW=GxFOVx rBW=1/Ts (dwell time)Sometimes: rBW is defined by BW = Sampling Rate/Number of Samples (the bandwidth per pixel),Receiver Bandwidth,The receiver (or acquisition),A smaller bandwidth improves SNR, but can cause spatial distortions, also increase
25、s the chemical shift. A larger bandwidth reduces SNR (more noise from the outskirts of the spectrum), but allows faster imaging.,A smaller bandwidth improves S,Transmit Bandwidth,The transmit bandwidth refers to the RF excitation pulse required for slice selection in a pulse sequence. The slice thic
26、kness 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 transmit,Noise in MR imaging,NEX=1,NEX=2,NEX=4,Noise in MR imagingNEX=1NEX=2N,MR图像去噪,研究背景：,临床常用图像信噪
27、比基本能接受然而，还有很多情况成像的信噪比仍然较低高b值DWI成像高分辨率成像动态成像,b=1000 b=2000 b=3000,MR图像去噪研究背景：临床常用图像信噪比基本能接受,基于高阶奇异值分解的磁共振图像噪声去除算法,MR图像去噪,Xinyuan Zhang, Yanqiu Feng*, et al., Medical Image Analysis, 2019.,基于高阶奇异值分解的磁共振图像噪声去除算法TruthNois,基于高阶奇异值分解的磁共振图像噪声去除算法,Xinyuan Zhang, Yanqiu Feng*, et al., Medical Image Analysis,
28、 2019.,MR图像去噪,基于高阶奇异值分解的磁共振图像噪声去除算法Xinyuan Z,基于高阶奇异值分解的磁共振图像噪声去除算法,MR图像去噪,Xinyuan Zhang, Yanqiu Feng*, et al., Medical Image Analysis, 2019.,基于高阶奇异值分解的磁共振图像噪声去除算法MR图像去噪Xin,DWI图像噪声去除,MR图像去噪,Xinyuan Zhang, Yanqiu Feng*, et al., Submitted to Neuroimage, 2019.,DWI图像MR图像去噪Xinyuan Zhang, Yanq,DWI图像噪声去除FA Map,MR图像去噪,Xinyuan Zhang, Yanqiu Feng*, et al., Submitted to Neuroimage, 2019.,DWI图像RankEdgeNoisyPR-HOSVDb=30,
