1、Application Training 01,difper,弥散&灌注,技术与应用,Application Training 01,difper,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,Application Training 01,difper,弥散现象,水分子的随机热能运动100m/secBrownian 运动存在于任何组织,Application Training 01,difper,弥散参数,D: 弥散系数, D=r*v/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M
2、PG脉冲的DWI测量D值b: b因子视MPG 脉冲的增幅和范围应用MPG,Application Training 01,difper,D值的例子,自由水中D值很大因为具有更多的运动自由,Application Training 01,difper,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,Application Training 01,difper,MPG运动探针梯度,应用一对平衡的双极的梯度MPG:运动探
3、针梯度M/M0=exp-(G)2DMPG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,Application Training 01,difper,b 因子,Depends on MPG amplitude & period b=2G22(-/3) for SE type EPIExponential signal decay due to diffusionMTE/M0=exp(-bD)MTE: Signal attenuation at the time of TE The stronger the b, the more emphasized
4、 diffusion (Clinically, b=1,000 is enough)The worse SNR & the more motion artifact,Application Training 01,difper,b 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
5、 increasing b:(1) a rapid initial decay(2) more gradual decay (1) is not observable inwhite matter due to the smaller perfusion,Application Training 01,difper,ADC,ADC (Apparent Diffusion Coefficient)ADC(x,y)=-b*lnS(x,y,G)/S(x,y,0)S(x,y,G): signal of each pixel when MPG is appliedPixels of low signal
6、 in DWI have high ADC value Calculated using two diffusion weighted images of different b factorAlso from regression of many b valuesEffects of T2 can be cancelled T2 shine through,Application Training 01,difper,Diffusion Anisotrophy,Depends on the tissue The largest in the WM, the smallest in the G
7、MDifferent D in different directions due to highly organized arrangements of componentsCerebral white matter (organized axons)Skeletal muscle (organized sarcomere)Observed signal attenuation due to MPG depends on its direction in the tissue,Application Training 01,difper,Diffusion Tensor,Larger set
8、of parameters, diffusion tensor: D, is used to describe the orientation-dependence of the diffusion process instead of D,Application Training 01,difper,Diffusion Sequence & Signal,Very sensitive to motion artifactShort scan is important,Application Training 01,difper,Sequence Comparisons,To use SS E
9、PIShortest scan time despite image distortionThe shorter the ETS, the smaller the distortionTo use MS EPI (Not recommended)Gated motion compensation is required To use SuperFASEFASE is used with MPG pulse instead of EPI Applicable for base of skull region,Application Training 01,difper,What Is Visua
10、lized by DWI ?,Cell structureDepending on cerebral infarction stageTumor componentSolid tumor or cystic tumorFibrous structureAnisotropic Diffusion in the fibrous structure,Application Training 01,difper,Cell Structure,Normal cell:Free movementamong cells- High ADC,Early stage ofcerebral infarction:
11、Water penetratesinto the cell-restricted diffusion- Low ADC,Later stage ofcerebral infarction:Cell membraneis destroyed-Free diffusion- Very high ADC,Application Training 01,difper,ADC Example,Application Training 01,difper,Tumor Component,Low ADC in solidtumor,High ADC in cysticor necrotic tumor,Ap
12、plication Training 01,difper,ADC Example,Application Training 01,difper,Perpendicular tothe fibrous structure:Difficult to move,Along with thefibrous structure:Easy to move,Fibrous Structure,To be used to check the generation of the fibrous structure in the corpus callosumFibrous structure shows ani
13、sotropy of diffusion (Ex: White Matter),Application Training 01,difper,ADC Example,Application Training 01,difper,Clinical Applications of DWI,To detect the early stage of cerebral infarctionShows high signal due to the restricted diffusionTo differentiate tumor componentsShows low signal for cystic
14、 tumor, and high signal for solid tumorTo detect fibrous structure by applying MPG pulse along three axisShows low signal when MPG is applied along perpendicular to the fibrous tissue,Application Training 01,difper,Advantages & Problems of DWI,AdvantagesOnly means to visualize Brownian motion of wat
15、er molecules in vivoUseful for very early stage CI diagnosisProblemsToo sensitive to the motion (patient & CSF/ blood flow) Small movement (less than 1 pixel) affectedContamination of IVCM (perfusion),Application Training 01,difper,Perfusion Phenomenon,Relates to the blood delivery at the capillary
16、level1m/secHow much volume of blood is delivered to a given volume of tissue in a given period,Application Training 01,difper,Perfusion Parameters,CBV (Cerebral Blood Volume) ACHow much blood stays in the capillary 100 cc VOI: mL/100 ccCBF (Cerebral Blood Flow) rFlowHow much supplied in a minute: mL
17、/100cc/minMTT (Mean Transit Time) MT1How long does it stay: minMTT = CBV/CBF (Central Volume Theory)Transit time = volume/ flow speed,Application Training 01,difper,Perfusion Model,CBV=MTT*CBF assuming No additional blood supply to the VOI from bypassNo broken down of BBB (Blood Brain Barrier)CBF: “
18、Perfusion Rate” in more general,Application Training 01,difper,Examples of Perfusion Rate,The concept of “Perfusion” is not limited to the brain,Application Training 01,difper,Two Technologies,Dynamic Perfusion ImagingTo use Gd-DTPA as a tracerRepeated data acquisition using T2*W very fast sequences
19、 (SS EPI)ASL (Arterial Spin Labeling) To label proton spins in the bloodEasier with less reliabilityTo saturate or invert flowing-in blood spin by RF pulse,Application Training 01,difper,Measurements of Dyn. PWI,Signal decrease & recovery due to Gd arrival and pass away using T2* weighted sequence,A
20、pplication Training 01,difper,Signal & R2*, R2,R2*, R2=(1/TE)*ln S0/S(tn)To convert susceptibility contrast to Gd densityR2*, R2: When FE, SE type is used,Application Training 01,difper,1st Pass & 2nd Pass,Signalintensity,For normal brain, sharp signal decrease & recovery because BBB keeps Gd-DTPA i
21、nside the vessel,BBB (Blood Brain Barrier),Application Training 01,difper,Measurements of ASL,Imaging Plane,Inversion RF Pulse,Control,Tag,Imaging Plane,Inversion RF Pulse,Application Training 01,difper,Comparisons of Dyn. & ASL,Application Training 01,difper,Clinical Applications of PWI,To diagnose
22、 early stage of cerebral infarction or ischemia By checking blood flow supply or circulation in the capillary level,Application Training 01,difper,Advantages & Problems of PWI,AdvantagesNon invasive, no hazardousMore widely used than NMVery early stage detection of cerebral infarction ProblemsRelationship between tracer & signal is non linear Neither freely diffusible nor intravascular tracers have been available,Application Training 01,difper,DWI & PWI,Hypo perfusion,Ultra acute phase,Acute phase,Sub acute phase,Abnormal in DWI,Hyper perfusion,
