MR(醫(yī)用磁共振成像)原理、技術(shù)與設(shè)備__MR參數(shù)

1、1Chapter 6MR 參數(shù)I. 信噪比Signal to Noise Ratio SNRSNR是MR非常重要的一個(gè)參數(shù)噪聲來(lái)源于病人、背景和系統(tǒng)電子噪聲等noisesignalSNR 與設(shè)備有關(guān)，如：磁場(chǎng)B0 Magnetic field線圈 coil調(diào)諧 tuning校準(zhǔn) Calibration線圈尺寸越大，SNR越低SNR與掃描參數(shù)有關(guān)：SNR (voxel)(Ny)(NEX)/BW)1/2Voxel =xyzNEX 平均激勵(lì)次數(shù)Ny Phase encoding stepsBW Band width121. Voxel xyz增加體素會(huì)提高SNR增加體素意味著一個(gè)體素內(nèi)的自旋質(zhì)子數(shù)增

2、加，所以SNR會(huì)增加。如何計(jì)算像素、體素？FOV/#N(P) =像素尺寸（PE）FOV/#N(F) =像素尺寸（FE）體素= 像素層厚如何增大體素？FOV ，保持 Nx、Ny:xy高分z , 保持Nx、Ny和FOV:縱向分辨力部分容積效應(yīng)偽影加重2. NEX （NAQ）SNR NEX1/2增加NEX 可提高SNR但是掃描時(shí)間增加3. 相位編碼步數(shù) NpSNR 與Np的關(guān)系：增加Np 并保持像素不變(不常用)SNR （SNR Np1/2)空間分辨力不變掃描時(shí)間增加Np并保持FOV不變(常見(jiàn))SNR (SNR 1/ Np1/2 )空間分辨力掃描時(shí)間234. 頻率編碼步數(shù) Nf增加Nf 、保持象素不

3、變(不常用)SNR 與Nf無(wú)關(guān)空間分辨力沒(méi)有變化增加Nf 、保持FOV (常用)SNR下降 (SNR1/Nf)空間分辨力增加掃描時(shí)間沒(méi)有變化5.帶寬 BW BandwidthrBWrBW=N/sampling time of echoThe sampling time of one MR echo is reducedby increasing the bandwidth.Increase the rBWWhen the rBW is increased, a shorter TE tobe used.A steeper gradient means that the rephasing it

4、 causes happensmore quickly, and so the echo forms faster. i.e., the samplingtime is reduced and the sampling rate is increased accordingly.SNR增加化學(xué)位移偽影增加TE延長(zhǎng)SNR (1/BW)1/2降低BW會(huì)導(dǎo)致：最大層面數(shù)下降運(yùn)動(dòng)偽影增加3RBWSNRchemicalshiftTEmotionartifact4BW與梯度場(chǎng)、采樣時(shí)間、噪聲、SNRFixing the rBW:What do you change on the scanner?GE: r

5、BW rangeSiemens: rBW/pixelPhilips: water fat shift(kHz)(Hz/pixel)(pixels)Chemical shift for your magnetReceive bandwidthFrequency matrixe.g. 16kHzThis is what you change on GE scannerChemical shif for your magnetReceive bandwidthFrequency matrixChemical shift for your magnetReceive bandwidthFrequenc

6、y matrix32kHz,256pixels125Hz/pixelThis is what you change on Siemens scanner.Chemical shift for your magnetReceive bandwidthFrequency matrix125Hz/pixel, 1.5T 224HzThis is what you want on Philips scanner.32kHz，125Hz/pixel，456. 3D圖像的SNR3D MR成像的SNR：SNR(3D) (voxel)(NyNzNEX/BW)1/2Ernst functioncos = exp

7、(-TR/T1) FAMR信號(hào)最大化的理想角度；用于組織成像時(shí)為什么不常使用該公式？組織間對(duì)比度的需求CNR Contrast to noise ratioCNR = (S1-S2)/SDb滿足一定SNR條件下，足夠的CNR是檢出病變的根本保證總結(jié)：增加SNR的方法增加TR降低TE降低BW增加NEX增加體素增加層厚增加FOV，保持掃描矩陣降低掃描矩陣，保持FOV保持像素，提高NyII. 空間分辨力在常規(guī)條件下，高空間分辨力意味著低SNR：SNR (FOVx/Nx)(FOVy)zNEX/(NyBW)1/2影響空間分辨力的因素：小FOV、大矩陣、薄層可得到高的空間分辨力(cm)VoxelSlice

8、thickness(mm)FOVmatrixSlicethickness#F：#P：FOV56保持FOV，增加Np保持像素，增加Np空間分辨力SNR掃描時(shí)間空間分辨力SNR掃描時(shí)間增加層厚空間分辨力下降SNR 增加部分容積效應(yīng)偽影增加SE：FSE：III. 掃描時(shí)間掃描時(shí)間=TR Ny NEX掃描時(shí)間=TR Ny NEX/ETL3D SE： 掃描時(shí)間=TRNy Nz NEXSNR、空間分辨力、掃描時(shí)間增加 TR：SNR 增加覆蓋范圍 增加T1W 降低PDW,T2W 增加掃描時(shí)間 增加降低TR：SNR 下降覆蓋范圍 下降T1W 增加PDW,T2W 下降掃描時(shí)間 降低增加TET2W 增加SNR下降

9、最大層面數(shù)下降掃描時(shí)間 不變降低 TET2W降低SNR 增加覆蓋范圍增加掃描時(shí)間不變VI. 覆蓋范圍、層厚、層間隔1.增加覆蓋范圍的方法：增加層厚增加層間隔增加TR或降低TE（增加TR/TE）2. 降低覆蓋范圍的方法：增加TE增加ETL6FOVTI/FAETL73. 增加層厚導(dǎo)致：部分容積效應(yīng)偽影 增加空間分辨力 下降覆蓋范圍 增加SNR 增加4. 增加層間隔會(huì)導(dǎo)致：覆蓋范圍 增加串?dāng)_偽影下降SNR增加探測(cè)小病灶的能力降低V. FOV field of view保持掃描矩陣，增大FOV會(huì)導(dǎo)致：空間分辨力下降；SNR增加VI. NEX增加NEX會(huì)導(dǎo)致：SNR 增加掃描時(shí)間 增加VII. 回波數(shù)增

10、加回波數(shù)導(dǎo)致：掃描時(shí)間 降低T2W 增加SNRuniformityimagesTissue ContrastdetailartifactParametersselectionpatientparametersNEXRBWCoilTR/TE MatrixImageOptionsScanTime#slicesMR controller放射科醫(yī)生的眼睛78放射科醫(yī)生的眼睛Off Center FOVFOV mmRectangular FOVFat/Water Shift PixelOff Center FOVFOV cmPartial FOV (PFOV)Receive BandwidthkHzOf

11、f Center ShiftFOV mmFOV Phase/Rectangular FOVBandwidthHz/PxShifting Slices Off CenterField Of ViewRectangular FOVBandwidthGapGapDistance Factor(% of slicethickness)Distance Between SlicesRepetition Time, EchoTime (in msec)Inversion Time (in msec)AveragesSimultaneous ExcitationRF Pulse in GradientEch

12、oScan Measurement TimePhilipsTR, TETINSA-Flip AngleAcquisition TimeGETR, TETINEXPOMP(Phase OffsetMultiPlanar)Flip AngleAcquisition TimeSiemensTR, TETIACQSimultaneousExcitationFlip AngleAcquisition Time, TAPhilipsOptimized BandwidthFrequencyOversamplingFold over SuppressionViews/SegmentTDHalf ScanPar

13、tial EchoTurbo FactorEcho SpacingFlow Comp, FlagGEVariable BandwidthAnti-AliasingNo Phase WrapViews per SegmentIntersegment Delay1/2 NEX, FractionalNEXPartial EchoEcho Train Length (ETL)Echo SpacingFlow CompSiemensOptimizedBandwidthOversamplingPhaseOversamplingLines/SegmentsTime DelayHalf FourierAsy

14、mmetricEchoTurbo FactorEcho SpacingFlowComp/GMRVariableBandwidthFrequencyOversamplingPhaseOversamplingSegmented K-spaceTime Delay/BlockK-SpaceHalf FourierImagingPartial EchoNumber of EchoesTime BetweenEchoesGradient MomentNullingPhilipsTONEMTCSPIRProsetRESTTravel RESTECG Triggered/VCGTrigger Delay,

15、TDTrigger, PEARPlan ScanBolusTrakGERamped PulseMTCFat Sat/Chem Sat-SATWalking SatCardiacGated/TriggeringTrigger Delay, TDRespiratory CompLocalizerSmart PrepFluoro-TriggeredMRASiemensTONEMTC, MTSFat SatWaterExcitationPresatTravel SatECG TriggeredTrigger Delay,TDRespiratoryGatedLocalizer, ScoutCARE Bo

16、lusRamped RF PulseMagnetization TransferContrastPrep Pulse-ChemicallyWater ExcitationPrep Pulse-SpatiallyMoving Sat PulseImage Sync with ECGDelay after R WaveRespiratory GatingPatient OrientationSequenceContrast BolusTiming/VisualizationGE 掃描設(shè)置界面磁體室內(nèi)Panel掃描控制與設(shè)置界面Status Display Panel: located onthe

17、magnet coverPressing Emergency Stop on the operatorsconsole:The scan abortsThe power disables the patient handling andscan related equipment89Control Panel ButtonAlignment light on themagnet control panel buttonMove to Scan: the tablemoves to patient isocenterControl Panel Button inthe magnet room1.

18、2.Desktop controlpanelSystem statusdisplay3. Area title4. Text box5. Pull down menu6. Autoview window7. Image slider8. System activitystatus9. Window togglebutton10.Pop-up windowbutton11.List/Select menu12.Scroll arrows13.Control panels14.Scan operationareaDesktop OverviewPatient Information/Positio

19、n PanelAbbreviated PatientInformationAccession Number: Typically prescribed by hospitalPatient ID: Any combination of numbers, letters and dashes. Limited to64 characters. System will notify if identical ID is found.Patient Name: Limited to 64 characters and should not be preceded bynumbers.Rad: Nam

20、e of radiologist can be used to specify PI.Refer: Name of referring physician also available for PI specification.Operator: Any combination of 3 characters.Patient Position: Typical setting: Supine, Head First, HRBRAINKeyboard1.2.Emergency stopOperatormicrophone3. Patient speaker4. Microphonevolume

21、adjust5. Press to talk6. Pause Scan7. Start Scan8. Stop Scan9. Move to Scan10.Stop MoveScan11.Host ComputerControl12.13.Functional KeysKeyboardSelecting a ProtocolSite: Standard Protocols placed by the Center.Including:BIRNfMRIfRMI-AllOther: Protocols belonging to eachgroup is placedProtocols select

22、ed from any of the protocollibraries (Site, GE) can be modified andsaved in the Site (other) Library.Also available is the Protocol Lockout thatrestricts site protocols by requiringpassword.Scout scanSelect the location you wantSelect pulse sequence and set scanparameters910The Rx ManagerThe Rx mana

23、ger is used to create, view, edit, and prepareseries for scanning. New: series is not prescribed or saved and cannot bescanned INRX: (in prescription) series is opened, not yet saved, andcannot be scanned. RXD: (prescribed) series is saved and can be scanned ACT: (active) series is ready for scan an

24、d can no longer beedited SCND: (scanned) series has been scannedand can no longer be edited but can bere-scanned. PSCD: (pre-scanned) series is pre-scannedand ready for scan.PrescanningAutomatic Prescan:Automatically adjuststhe center frequencyand the transmit andreceiver gain.Manual Prescan: Allowsmanual adjustment ofprescan parameters. Caution: You aredelivering RF to thepatient du