MR成像要素課件

1、Imaging QualitySNRScan TimeContrastArtifactResolutionSNR Almost every thing have effective to SNRFrom applications point: Magnetic Field Strength Pulse Sequence Parameter choosing ( TR ; VB ; Voxel NEX ; .Contrast -CNR Intrinsic Contrast Pulse Sequences Parameter ChoosingIntrinsic Tissue Contrast Pr

2、oton DensityT2 (Decay (T2*)T1 (Relaxation)FlowDiffusionOthers ( MTC, .H1Proton Density (PD)The most straightforward contrast mechanismIntensity depends on how many protons per pixel ( picture element ) ( per voxel )Not the best contrast mechanism because proton densities of tissues are very similarR

3、epresents anatomy better than pathologyTime it takes for protons to lose coherence after being energizedGenerally, tissues with longer T2 are brighter in a T2-weighted imageT2 for white matter is 75 milliseconds, T2 for CSF is 400msec.T2ABCDT2 - Transverse DecayT263% of Transverse (Mxy) DecaysOut of

4、 phaseAffected byTissue TypeField Strength (at very high end)Spin-Spin RelaxationDoes not involve a loss of energyReasons for Loss of MxyT2 ( Extrinsic )inhomogenitieschemical shiftT2 ( Intrinsic )spin-spin interaction : Exchange of energy among spinsT2*Spin Echo vs. GRESpin Echo90180aGradient EchoT

5、2 vs. T2*T1 Time for energy to be released (cooling time or refractory period)Generally, tissues with shorter T1 are brighter in the imageT1 of white matter is 500ms, T1 of CSF is 2.5 seconds (1.5T)T1 Regrowth CurveABCDET1 Recovery63% of Longitudinal (Mz)RecoversAffected byTissue TypeTemperatureFiel

6、d StrengthSpin Lattice RelaxationInvolves a loss of energyT1 ContrastT1 & T2: A Graphical ViewT2 is the rate of decay of the MR signal(63% = 1 T2)T1 is the rate of recovery of the MR signal(63% = 1 T1)T2 is typically shorter than T1.Long TET2 Decay.Short TET1 Recovery.Short TET1 RecoveryExtrinsic Ti

7、ssue Contrast ParametersTR (Time to Repetition)TE (Time to Echo)TRTEMR Timing VariablesTE: Echo Time - Time between RF transmission and RF receptionTR: Repeat Time - Time between successive RF transmissionsT2 and TETE controls amount of T2 weighting in the MR imageLong TE = T2 Short TE = PDLong TR =

8、 T2Dual Echo SEProvides bothPD and T2 weightingFlow Blood flow artery, Vein CSF TOF; PCDiffusionDiffusion gradients sensitize MR Image to motion of extracellular water ( molecule )More motion = Darker image Early stroke Resolution Spacial ResolutionPhase EncodingFreq EncodingFOVMatrixSlice thinkness

9、 PixelVoxel SNRPhase EncodingFreq EncodingFOVMatrixSlice thinkness PixelVoxelResolution vs SNRScan Time SE, GRE, = TR x Phase Encoding x NEX FSE = TR x Phase / ETL x NEX3D = TR x Phase Encoding x NEX x #of SliceEPI = TR x # of shot x NEXArtifact Motion : ( Physiologic & active ) Chemical Shift and M

10、agnetic Susceptibily effects Aliasing ( Phase Encoding and Frequency Encoding ) Truncation System Performance ArtifactArtifact Motion : ( Physiologic & active )Cardiac Motion EKG / PGRespiratory Resp Comp / Resp trigger / Breath HoldBlood Flow FC / Spacial Sat CSF Pulsation PG / FCAperiodic Motion ?

11、 Cooperation / Suitable EducationIn Phase DirectionTruncation artifact: low intensity lines appearing near the boundaries of the brain / skull interface, are characteristic of a 128 encoding acquisition (A). This artifact is notpresent if 256 views are collected (B).MRI Pulse SequencesTETRRF PulseRF

12、 PulseRF Received.Gradient Pulse(Slice Selection)Gradient Pulse(Signal Readout)Gradient Pulse(Slice Selection)Spin EchoGradient EchoSE IRFSE(FSE-XL FRFSE )FSE-IRSSFSEGRE SPGRFGRE FSPGRTOF PCSSFP FIESTA2D 3D3 PlaneCINEFastEPISpiralScan ModeAcquisition ModeSpin Echo (SE),Inversion Recovery (IR)90180 r

13、efocus inhomogeneity TE, TR, scan timePD, T1, T218090echoTRRFTEGradient Echo (GRE), SPGR 90Gradient RefocusInhomogeneity sensitive TE, TR, scan timePD, T2* (GRE), T1 (SPGR)RFGZGYGXechoTRTE180TIFast Spin Echo (FSE)Spin Echo with multiple refocusing pulsesMuch faster than spin echo by a factor of ETL

14、(echo train length)Image quality determined by echo space (ESP) - smaller is betterT2, (T1, PD w/ tailored rf)TR90180180180180ESP4 ETL12341234Multi-Echo SequenceDataPartial K SpaceHomodyne ReconSSFSE : Partial K-Space (Fract. NEX)Last Echo1st Echo TE 200 msTR the longer, the betterFluid T2 - 2000msO

15、ther tissue - 50-100msIf TE 200msFluid Univ. of Wales S.Raffaele MilanStoneAbdominal Vascular ImagingEFGRE3D with contrast is the method of choice for abdominal vascular imaging.EFGRE3D is very sensitive to contrast.Short TR permits high resolution in a breath hold.ZIP increases coverage.SPECIAL fas

16、t fat suppression avoids the need for subtraction.SMARTPREP is used to avoid a test bolus and reduce contrast usage.EFGRE3D with SPECIAL (TI=22ms)TR=7.0 / TE=2.2 / 4028 sl. / 2mm / 56 Slice ZIP256x192 / 512 ZIP0.5 NEX / 26s. acq.Gad. double doseEFGRE3D( 3D fast TOF ): Contrast Enhanced MRATR/TE/FA =