MRI的基本知识.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Magnetic resonance imaging（MRI,）,In 1946,Felix Bloch and Edward Purcell independently discovered the magnetic resonance(MR)phenomena,(Nobel Prize in 1952),In 1971,Raymond,Damadian,showed that nuclear magnetic relaxation times of tissues and tumors differed,In 1973,By utilizing gradients in the,magnetic field,Paul,Lauterbur,demonstrated MR imaging on small test tube samples,(Nobel Prize in 2003),In 1975 Richard Ernst proposed MR imaging using phase and frequency encoding,and the Fourier Transform,which are the basis of current MRI techniques.,(Nobel Prize in 1991),In 1977,Peter Mansfield developed the echo-planar imaging technique,(Nobel Prize in 2003),In 1977,Raymond,Damadian,completed the first MR scanner(Indomitable).,HISTORY OF MRI,Felix Bloch,Edward Purcell,Raymond Damadian,Paul Lauterbur,Richard Ernst,Peter Mansfield,3.1 Basic Principles of MRI,MRI reveals,chemical compositions,of objects as well as their,molecular structure&arrangement,but not density.,Electromagnetic,wave,is used in MRI,not,ionizing radiation,wave(X-ray,-ray)or,mechanical wave(ultrasound),Nuclei suitable for MRI,are those which have an odd number of protons or neutrons and therefore possess a net charge and have angular magnetic momentum(tiny magnetic field generated by the spinning motion of atomic nuclei),Hydrogen nucleus,(one proton,no neutron)is the best element for general clinical utility of MRI due to its strongest magnetic moment and greatest abundance in human tissues.,x,y,z,Static,Field,B0,Applied RF,z is longitudinal direction,x-y is transverse plane,Mo,EXCITATION,:,causing the magnetic moments flip away from equilibrium position(z-axis),x,y,z,x,y,z,x,y,z,x,y,z,x,y,z,Relaxation:,Return to equilibrium,Energy released in the form of radio waves,T,2,relaxation,Decay of the transverse component of magnetization,T,2,relaxation constant:the elapsed time between the peak transverse signal and 37%of the peak level(1/,e,),Pulse sequences refer to the manner in which the radiofrequencies and gradients are applied with appropriate time intervals between them,in order to highlight a certain inherent tissue property.,Pulse sequence,Spin echo pulse sequence,is the most important sequence in clinical imaging and forms the basis of MR tissue characterization.,TR,TE,2,TE,2,TE,MRI,Spin echo(SE),sequence,90,0,90,0,180,0,180,0,90,0 pulse,TE/2180,0,pulseTE/2signal coding，spin echo pulse sequence,SE,TR:the time over which a basic pulse sequence is repeated.,TE:time between RF excitation and the peak of the echo signal.,Gradient echo(GRE)sequence,Inversion recovery(IR)sequence,Echo planar imaging(EPI)sequence,By altering the pulse sequence,the MR signal produced by various structures could be dramatically changed.,3.2 Basic Equipments of MRI,Data processing,image reconstruction,displaying and storage,analog converter,Computer with monitor,Storage devices(disk),printer,Data,processor,Power,supply,Manipulation and display,Scanning aperture,Main coil,RF coil,Gradient coil,MRI System,Magnet,tunnel-shaped,Imaging contrast is determined by densities of different tissues in X-ray and CT image,but in MRI it is determined by,signal intensities,of different tissues.,The proton density,T1,and T2 relaxation,constants are fundamental properties of tissues,and therefore these tissue properties can be exploited by MRI to aid in the diagnosis of pathologic conditions such as cancer,multiple sclerosis,or,hematoma,T,1,weighted imaging(T,1,WI),T,2,weighted imaging(T,1,WI),Proton density,weighted imaging(PDWI),T,1,WI after administration of,Gd-DTPA,(a complex of gadolinium with a chelating agent,which is paramagnetic and can shorten relaxation time),Flow void effect,The loss of MR signal in macroscopic structures with fluid flow(vessels,heart,etc.).,90,O,RF,180,O,RF,vessel,Imaging slice,vessel,3.4 MR Signals of Some Normal&Pathological Tissues,Body fluid,including cerebrospinal fluid,urine,edema fluid and fluid with low protein content in cavity and cyst,etc,T,1,WIlow signal intensity T,2,WIhigh signal intensity,Fat,including subcutaneous fat,bone marrow,well differentiated fatty tumors,etc,T,1,WIhigh signal intensity T,2,WIhigh signal intensity,T1,T2,Acute,(3d),hypo-or iso-intensity,hypointensity,Sub-acute,(330d),hyperintensity,(from rim to center),hyperintensity,(from rim to center),Chronic,(30d),hypointense rim+hypo-or hyper-intense center,hypointense rim+hyperintense center,Hemorhage,Its signal is essentially determined by hemoglobin and its degradation products as well as the integrity of red blood cell.,Hemorrhagem(4 days),Subcute hemorrhage(12 days),Subacute hematoma,Chronic hemorrhage,Tumor,majority:,hypointense on T,1,WI,hyperintense on T,2,WI,cause,:,more water content,minority:,hyperintense on T,1,WI,hypointense on T,2,WI,cause:,special component,eg,melanin in melanoma,Pigmented villonodular synovitis,Heart chambers,lumen of blood vessels,Signal void or very low signal intensity,Hyperintensity when using paramagnetic contrast agent or special sequences,Black blood,CE,SE,GRE,Bone,ligament,tendon,calcification,Cortical bone,ligament&muscle tendon are of low signal intensities in various sequences.,Calcification is,hypointense,in most cases,but appears hyperintensity sometimes.,posterior cruciate ligament(PCL),Tendon of quadriceps femoris,Patellar ligament,3.5 Common MR imaging Techniques,Fat suppression(FS),:,a technique to suppress the signal from fatty tissue protons in order to display a lesion better or differentiate a fatty lesion from other diseases.,T1WI,T2WI,T2WI with FS,T,1,WI,T,2,WI,C,MR hydrography,Using heavily T2 weighted sequence(with or without fat suppression),so that static or slowly flowing fluids in the body are imaged as hyperintense,bright structures against a dark background with a very low signal intensity.,MR cholangiopancreatography(MRCP),MR urography(MRU),MR myelography(MRM),Etc.,MRCP:,carcinoma at the distal end of common bile duct,MRCP,MRU after radiotherapy for pelvis,Magnetic resonance angiography(MRA),Methods of producing angiograms using MR techniques,1.Time of flight(TOF)MRA,2.Phase contrast(PC)MRA,3.Contrast enhanced(CE)MRA,no contrast,TOF-MRA,CE-MRA,Diffusion weighted imaging(DWI),And,Perfusion weighted imaging(PWI),MR,Spectroscopy,3.6 Advantages and Disadvantages of MRI,Sectional imaging in multiple planes,Imaging with multiple parameters,High soft tissue resolution,Without ionizing radiation,MRI contrast agents have no allergic reactions,Imaging heart chambers and blood vessels without using contrast agents,Functional imaging(PWI,DWI,MRS),Advantages of MRI,Disadvantages of MRI,Claustrophobia(small bore of the magnet),Long examination time(multiple sequences),Expensive and complicated equipment,Insensitive in showing calcification and tiny bone changes,Any ferromagnetic object having a location where motion of the object may be harmful to the patient,represents an absolute contraindication to MR imaging,Intracorporal ferromagnetic foreign objects and aneurysmal clips may cause serious,haemorrhage,.,The function of the pacemaker may be affected by the magnetic field,and furthermore,electric currents may be induced in the pacemaker electrode with possible heating of the,endocardium,.,Insulin pumps,medication delivery devices,etc will also be influenced by magnetic field.,Prohibitions,The transmitted radio frequency waves have a heating effect on the tissues.To avoid harmful heating,the maximum allowable energy transmitted to the patient is regulated by international recommendations.,In the first trimester of pregnancy,the foetus is surrounded by a relatively large amount of amniotic fluid,and has little capability to remove the extra heat.,(considered an contraindication by some),Prohibitions,