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1、弥散&灌注技术与应用弥散&灌注 水分子的两种运动形式 弥散(随机运动)100 m/s 灌注(血液流入毛细血管)1 m/s弥散现象 水分子的随机热能运动 100m/sec Brownian 运动 存在于任何组织弥散参数 D:弥散系数,D=rD=r*v/6v/6 自由运动指数 r:自由运动路线平均长度 v:分子运动平均速度 ADC:表观弥散系数 通过带有MPG脉冲的DWI测量D值 b:b因子 视MPG 脉冲的增幅和范围应用MPGD值的例子 自由水中D值很大 因为具有更多的运动自由D(x10-3 mm2/s)NotePure water2.5CSF2.94Gray Matter0.76White M
2、atter0.22Anisotropic(From Diffusion/Perfusion MRI,ISMRM 2000)DWI的梯度效应 在2方向而不是1方向G 产生自旋相位移动 当自旋从z=z1 向 z=z2移动的纯相位移动:G(z1-z2):提督持续时间 弥散产生像素内相位不连续&继发的信号衰减1:与G方向垂直的移动#2:G方向的移动G:Applied gradient fieldSpinmovementMPG运动探针梯度RFGradGG 90180echoRFGradGG 90180echo 应用一对平衡的双极的梯度 MPG:运动探针梯度 M/MM/M0 0=exp-(=exp-(G
3、G)2 2 DD MPG导致的信号衰减 假定各向同性,均质&自由弥散:梯度分裂:梯度持续时间G:梯度增幅b 因子 Depends on MPG amplitude&period b=b=2 2G G2 2 2 2(-/3)/3)for SE type EPI Exponential signal decay due to diffusion MMTETE/M/M0 0=exp(-bD)=exp(-bD)MTE:Signal attenuation at the time of TE The stronger the b,the more emphasized diffusion(Clinica
4、lly,b=1,000 is enough)The worse SNR&the more motion artifactb factor&Signal(1)is called IVIM(Intra Voxel Incoherent Motion),induced by the bulk flow of blood within a capillary network(2)Due to diffusion The log-signal attenuationcurve for increasing b:(1)a rapid initial decay(2)more gradual decay (
5、1)is not observable inwhite matter due to the smaller perfusion ADC ADC(Apparent Diffusion Coefficient)ADC(x,y)=-bADC(x,y)=-b*lnS(x,y,G)/S(x,y,0)lnS(x,y,G)/S(x,y,0)S(x,y,G):signal of each pixel when MPG is applied Pixels of low signal in DWI have high ADC value Calculated using two diffusion weighte
6、d images of different b factor Also from regression of many b values Effects of T2 can be cancelled T2 shine throughDiffusion Anisotrophy Depends on the tissue The largest in the WM,the smallest in the GM Different D in different directions due to highly organized arrangements of components Cerebral
7、 white matter(organized axons)Skeletal muscle(organized sarcomere)Observed signal attenuation due to MPG depends on its direction in the tissue Diffusion Tensor Larger set of parameters,diffusion tensor:D D,is used to describe the orientation-dependence of the diffusion process instead of DDiffusion
8、 Sequence&Signal Very sensitive to motion artifact Short scan is importantSequence Comparisons To use SS EPI Shortest scan time despite image distortion The shorter the ETS,the smaller the distortion To use MS EPI(Not recommended)Gated motion compensation is required To use SuperFASE FASE is used wi
9、th MPG pulse instead of EPI Applicable for base of skull regionWhat Is Visualized by DWI?Cell structure Depending on cerebral infarction stage Tumor component Solid tumor or cystic tumor Fibrous structure Anisotropic Diffusion in the fibrous structureCell StructureNormal cell:Free movementamong cell
10、s-High ADC Early stage ofcerebral infarction:Water penetratesinto the cell-restricted diffusion-Low ADCLater stage ofcerebral infarction:Cell membraneis destroyed-Free diffusion-Very high ADCADC ExampleStageTime afterclinical onsetT2 SEDWIADCHyperacute0-6 hoursNormalIncreasedDecreasedAcute6-48 hours
11、Normal-increasedIncreasedDecreasedSubacute3-10 daysIncreasedIncreasedDecreased-normalChronic 10 daysIncreasedDecreased-increasedIncreased(From Echo-Planar Imaging,Chapter14,p.471)Tumor Component Low ADC in solidtumor High ADC in cysticor necrotic tumorADC ExampleTissueADC(x10-3 mm2/s)Astrocytoma3.5C
12、yst2.65Tumor necrosis2.38Cystic2.5+-0.2Edema2.49(From Echo-Planar Imaging,Chapter 14,p.476)Primary TissueTumorComponentsPerpendicular tothe fibrous structure:Difficult to moveAlong with thefibrous structure:Easy to move Fibrous Structure To be used to check the generation of the fibrous structure in
13、 the corpus callosum Fibrous structure shows anisotropy of diffusion(Ex:White Matter)ADC ExampleMinimumMaximumGray MatterAll0.66+-0.062.0+-0.1All0.73+-0.191.7+-0.1Parallel1.00+-0.051.07+-0.06Perpendicular0.43+-0.030.64+-0.05CSFAll2.5+-0.23.44(From Echo-Planar Imaging.Chapter 14,p.467)ADC(x10-3 mm2/s
14、)White MatterTissueGradientsClinical Applications of DWI To detect the early stage of cerebral infarction Shows high signal due to the restricted diffusion To differentiate tumor components Shows low signal for cystic tumor,and high signal for solid tumor To detect fibrous structure by applying MPG
15、pulse along three axis Shows low signal when MPG is applied along perpendicular to the fibrous tissueAdvantages&Problems of DWI Advantages Only means to visualize Brownian motion of water molecules in vivo Useful for very early stage CI diagnosis Problems Too sensitive to the motion(patient&CSF/bloo
16、d flow)Small movement(less than 1 pixel)affected Contamination of IVCM(perfusion)Perfusion Phenomenon Relates to the blood delivery at the capillary level 1m/sec How much volume of blood is delivered to a given volume of tissue in a given periodPerfusion Parameters CBV(Cerebral Blood Volume)AC How much blood stays in the capillary 100 cc VOI:mL/100 cc CBF(Cerebral Blood Flow)rFlow How much supplied in a minute:mL/100cc/min MTT(Mean Transit Time)MT1 How long does it stay:min MTT=CBV/CBF(Central V
