《彌散與灌注》PPT課件

1、Application Training 01,difper,彌散&灌注,技術(shù)與應(yīng)用,Application Training 01,difper,彌散 & 灌注,水分子的兩種運動形式 彌散 (隨機運動) 100 m/s 灌注 (血液流入毛細(xì)血管) 1 m/s,Application Training 01,difper,彌散現(xiàn)象,水分子的隨機熱能運動 100m/sec Brownian 運動 存在于任何組織,Application Training 01,difper,彌散參數(shù),D: 彌散系數(shù), D=r*v/6 自由運動指數(shù) r: 自由運動路線平均長度 v: 分子運動平均速度 ADC: 表觀

2、彌散系數(shù) 通過帶有MPG脈沖的DWI測量D值 b: b因子 視MPG 脈沖的增幅和范圍應(yīng)用MPG,Application Training 01,difper,D值的例子,自由水中D值很大 因為具有更多的運動自由,Application Training 01,difper,DWI的梯度效應(yīng),在2方向而不是1方向G 產(chǎn)生自旋相位移動 當(dāng)自旋從z=z1 向 z=z2移動的純相位移動: G(z1-z2) : 提督持續(xù)時間 彌散產(chǎn)生像素內(nèi)相位不連續(xù) & 繼發(fā)的信號衰減,1: 與G方向垂直的移動 #2: G方向的移動,Application Training 01,difper,MPG運動探針梯度,應(yīng)

3、用一對平衡的雙極的梯度 MPG:運動探針梯度 M/M0=exp-(G)2D MPG導(dǎo)致的信號衰減 假定各向同性, 均質(zhì) &自由彌散,梯度分裂 : 梯度持續(xù)時間 G: 梯度增幅,Application Training 01,difper,b 因子,Depends on MPG amplitude & period b=2G22(-/3) for SE type EPI Exponential signal decay due to diffusion MTE/M0=exp(-bD) MTE: Signal attenuation at the time of TE The stronger t

4、he b, the more emphasized 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-s

5、ignal attenuation curve for increasing b: (1) a rapid initial decay (2) more gradual decay (1) is not observable in white 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 MP

6、G is applied Pixels of low signal in DWI have high ADC value Calculated using two diffusion weighted images of different b factor Also from regression of many b values Effects of T2 can be cancelled T2 shine through,Application Training 01,difper,Diffusion Anisotrophy,Depends on the tissue The large

7、st in the WM, the smallest in the GM Different D in different directions due to highly organized arrangements of components Cerebral white matter (organized axons) Skeletal muscle (organized sarcomere) Observed signal attenuation due to MPG depends on its direction in the tissue,Application Training

8、 01,difper,Diffusion Tensor,Larger set 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 artifact Short scan is important,Application Training 01

9、,difper,Sequence 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 with MPG pulse instead of EPI Applicable for base of skull region

10、,Application Training 01,difper,What 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 structure,Application Training 01,difper,Cell Structure,Normal cell: Free movement among

11、 cells - High ADC,Early stage of cerebral infarction: Water penetrates into the cell -restricted diffusion - Low ADC,Later stage of cerebral infarction: Cell membrane is destroyed -Free diffusion - Very high ADC,Application Training 01,difper,ADC Example,Application Training 01,difper,Tumor Componen

12、t,Low ADC in solid tumor,High ADC in cystic or necrotic tumor,Application Training 01,difper,ADC Example,Application Training 01,difper,Perpendicular to the fibrous structure: Difficult to move,Along with the fibrous structure: Easy to move,Fibrous Structure,To be used to check the generation of the

13、 fibrous structure in the corpus callosum Fibrous structure shows anisotropy 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 infarction Shows high signal due to the restricted

14、diffusion To differentiate tumor components Shows low signal for cystic tumor, and high signal for solid tumor To detect fibrous structure by applying MPG pulse along three axis Shows low signal when MPG is applied along perpendicular to the fibrous tissue,Application Training 01,difper,Advantages &

15、 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/ blood flow) Small movement (less than 1 pixel) affected Contamination of IVCM (perfusion,Application Training 0

16、1,difper,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 period,Application Training 01,difper,Perfusion Parameters,CBV (Cerebral Blood Volume) AC How much blood stays in the capillary 100 c

17、c 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 Volume Theory) Transit time = volume/ flow speed,Application Training 01,difper,Perfusion Model,CBV=MTT*CBF assuming No additiona

18、l blood supply to the VOI from bypass No broken down of BBB (Blood Brain Barrier) CBF: “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 I

19、maging To use Gd-DTPA as a tracer Repeated data acquisition using T2*W very fast sequences (SS EPI) ASL (Arterial Spin Labeling) To label proton spins in the blood Easier with less reliability To saturate or invert flowing-in blood spin by RF pulse,Application Training 01,difper,Measurements of Dyn.

20、 PWI,Signal decrease & recovery due to Gd arrival and pass away using T2* weighted sequence,Application Training 01,difper,Signal & R2*, R2,R2*, R2=(1/TE)*ln S0/S(tn) To convert susceptibility contrast to Gd density R2*, R2: When FE, SE type is used,Application Training 01,difper,1st Pass & 2nd Pass,Signal intensity,For normal brain, sharp signal decrease & recovery because BBB keeps Gd-DTPA inside the vessel,BBB (Blood Brain Barrier,Application Training 01,difper,Measurements of ASL,Imaging Plane,Inversion RF Pulse,Control,Tag,Imaging Plane,Inversi