Introduction To MR Physics - Asnweb

01/18/2015Introduction to MR PhysicsJoseph V. Fritz, PhDDent Neurologic InstituteWhat are We Imaging?voxelH H H H H pixelMeasuring Proton Densityvia Net MagnetizationNH SBo 3T1.5T(More Align Parallel as Bo Increased)1

01/18/2015Precessional Motion and theLarmor FrequencyN64 MHz128 MHzH SB0 1.5TB0 3TVector Math How Much Is Pointing in Each of 3 Axes? Add Up Components, Then RecombineMaking the Net MagnetizationDetectableThis:NetsTo This:(0)Appears to Rotate inTransverse Plane2

01/18/2015Spatial Localization Using GradientsB1Transmit at Larmor fGzGyB0B0AFourier TransformEchoReceive Mix ofLarmor f’sGradientfImage Encoding: FrequencyGFE42f2f131f2f173f1f2FT2nd Acquisition, Phase Change42f2f14 3 7 (PE 0 )43 1 (PE 180 )-GPE31f2f111f1f22 1 3 (PE 0 )21 1 (PE 180 )-FT3

01/18/2015Typical Field Echo SequenceTRTERF TransmitRF ReceiveG - SliceG - PhaseG - FrequencyFundamental Principles Protons– Precession: Spinning Charge & Mass in Force Field– Larmor Frequency: Proportional to Magnetic Field Vectors and Net Magnetization– “Signal” Increases as Protons Align Electromagnetism and Faraday’s Law of Induction– Moving Charges/Electrical Current Magnetic Fields Fourier Transform– Any Signal can be Decomposed into SinusoidsSummary of Localization Strategy1.2.3.4.5.6.7.Gradient Varies Larmor Frequency along Slice Direction.Transmit Coil at Desired Slice’s Larmor Frequency to ExciteDesired Anatomy2nd Gradient: Temporarily Vary Precessional Frequencyalong PE Direction to Create Phase Distribution3rd Gradient: Vary Larmor Frequency along 3rd Directionwhile Receive Coil “Listens” (Echo)Use Fourier Transform to Decode “Echo” into FrequencyBins (along FE)Repeat for Multiple Phase Distributions along PEUse Algebra to Compute Discrete Pixel Values (2D FT)4

01/18/2015In MRI, Timing is EverythingTRTE(T1 Relaxation)(T2 Decay)Before 90ºAfter 90ºAt ReadoutWithin ViewBefore Next 90º After 90ºStart of Next ViewN(H), f(TR,T1)TRClassic Image Weighting PROTON DENSITY– Long TR, Short TE T1 Weighted– Short TR (around the Tissue T1), Short TE T2 Weighted– Long TR, Long TE (around Tissue T2)Long TR, Long TE T2 Weighted Signal Loss fromPhase Dispersion More Loss withWhite/Gray Matterthan CSF Plenty of Time forComplete T1Recovery Each ViewTR 2000, TE 1205

01/18/2015Short TR, Short TE T1 Weighted Signal Loss fromIncomplete LongitudinalMagnetization More Loss from CSFthan Gray/White MatterTR 500, TE 10Multiple Ways to Lose SignalThrough De-Phasing SPIN-SPIN Interaction (T2 Weighting)– Chemical Environment Weighs Image Intensity Fail to Realign to Reference Direction @ TE– Gradients Not BALANCED During Spatial Encoding– Hardware Error, OR Moving Tissue Magnet Inhomogeneities (Static Dephasing)– Larmor Frequency Variation Within a Voxel– “Sucsceptibility” affects of Tissue (eg Blood)– Equipment Imperfections, Implants, etcSPIN ECHO Additional RF: 180 Reverses Static De-phasing Affects T2 (SE) versus T2* (FE) Contrast6

01/18/2015Spin Echo SequenceTRTETE/2RF TransmitRF ReceiveG - SliceG - PhaseG - FrequencyFast Spin Echo Multiple PE Steps per TR Reduces Scan TimeProportional to ETL Effective TE when less PEapplied (center of k-space) Best for Long TE Can Blur for Short TE Short ETS with Wide BW,Reduces SNRInversion Recovery: STIRFat Nulled90ºTIApply 180ºAfter TIAfter 90ºStart Imaging Sequence Here7

01/18/2015Inversion Recovery: FLAIRCSF NulledTIApply 180ºAfter TIAfter 90ºStart Imaging Sequence HereIR and MagPrep SequenceTRTITETE/2RF TransmitRF ReceiveG - SliceG - PhaseG - FrequencyPreparationIR and MagPrep SequenceTRTITETE/2RF TransmitRF ReceiveG - SliceG - PhaseG - FrequencyPreparation Options:eg STIR, IR, FLAIR,Pre-Sat, ChemSat, DWI8

01/18/2015Magnetization Preparationin Diffusion ImagingApply 90ºDePhaseApply 180ºRephaseStart ofFast ImagingSequence Starting Magnetization Less if “Dephase” “Rephase” Stationary Protons Retain Signal (Restricted Diffusion)Diffusion with EPISee www.MR-TIP.com for diagrams of other sequencesMRI Sequence Pearls SE & FSE FE EPI STIR IRFLAIRChemSatStandardEcho Train Blurring with Short TELong ETL Hydrography (IAC, Myelo)Magnetic SusceptibilitiesFlowBasis for SWI (eg Blood products)Single/Multi ShotBasis for DWIFat Sat and Pathology BrighterDon’t use GdImproved T1 ContrastCSF Nulled T2WKill Fat signal before ImagingGd OK9

01/18/2015Many “Pulse Sequence”Choices TR/TE/TIFlip AngleMatrixField of ViewSlice ThicknessNEXBandwidth Spin EchoField Echo3DTurbo FactorEcho PlanarSENSE Fundamental Tradeoffs:Resolution – SNR – Speed Resolution Increases with– More Voxels in FOV (Bigger Matrix, Small FOV, Thin Slice) SNR Increases with– More data, Listen Longer, Less Resolution– Field Strength(More Net Protons, consider Safety, Artifacts, T1 Contrast)– Receive Coil Matched to Anatomy Time Decreases with:– Collecting Less Data (eg Less Resolution, Fewer Averages)– Shorter TR (contrast)– More Data per TR(eg FSE, EPI, GRASE, Parallel Imaging)Careful with Blurring, J-coupling, distortion, AliasingExample Images and Parameters10

01/18/2015Spin Echo – T1T2 Fast Spin EchoSingle Shot Fast Spin Echo11

01/18/20153D Field Echo (CE)SWI vs. long TE FEFLAIR12

01/18/2015STIRSTIR Fat Suppression(Never CE!)TOF MRA (very short TR FE)13

01/18/2015Artifacts Partial VolumeMotion, FlowWrap (Aliasing)Gibbs (Ringing)Susceptibility (including Intentionally creating SWI)Chemical ShiftCross TalkPartial VolumeMotion14

01/18/2015Motion Compensation(BLADE, PROPELLER, MultiVANE, JET, RADAR)T2 Weighted Fast Spin Echo Motion CompensationCSF Flow, Gradient Moment NullingMorelli, et al, RadioGraphics 2011; 31:849–86615

01/18/2015GibbsAliasing16

01/18/2015Parallel ImagingIf Reduce FOV by half Full FOVHalf Scan time but AliasingUnfolding(Post processing)Coil #1Coil #2No wrap aroundHalf scan timeCoil #1SensitivityDistributionMultiple receiver coils Coil #2Courtesy Toshiba Medical SystemsSusceptibility vs. Parallel ImagingNo PIPI x 3Kuhl et al, Proc. Intl. Soc. Mag. Reson. Med. 11 (2004)17

01/18/2015Motion, PresaturationMorelli, et al, RadioGraphics 2011; 31:849–866Chemical Shift vs. BandwidthMorelli, et al, RadioGraphics 2011; 31:849–86618

01/18/2015Chem-Sat Fat Suppression Lucille giardino, 12/14/2009 Edward bajdas, 2/12/2010– Find f/yu w fat suppressed orbits 2/24/10 Amanda obrien, 6/25/10– see f/u potential lipoma with fat satDWI/ADC & T2 Shine Throughe -bD e -TE/T2*e -bD e -TE/T2* e -TE/T2*b 0 (T2*)b 1000 (DWI)e -bDDADCeADCSpecialized Sequences19

01/18/2015Diffusion Tensor Imaging:Fractional Anisotropy Determine the Direction and Shape of Diffusion Repeat Diffusion “Probe” in at least 6 Directions(Dx, Dy, Dz)FA 0Isotropic Blob of Protons Going That Way(Dx, Dy, Dz)FA 1Linear Blob of Protons Going That WayResulting Graphics of White Matter FA Brightness Color Direction Connect Similar Dots Representation of Tractsthat Linearly RestrictDiffusionFunctional Imaging: BOLD Blood Oxygenation Level Dependent ImagingOxy vs. De-oxy Hg Changes Iron EnvironmentIron Affects Nearby Magnetic FieldVariation in Larmor Frequencies within VoxelChanges Net Magnetization20

01/18/2015Simple Paradigm to Tag Function Scan Entire Brain Every Couple Seconds– FE-EPI fast single shot, & susceptibility weighted Do Something that Affects Local O2 Uptake Continue Scanning While Interleaving AnotherTask With Different O2 Affect Subtract Image Sets (after a lot of averaging) Threshold “Meaningful” Differences Overlay on a Registered Anatomic ImageSurgical Guidance Using CombinedFiberTracts and fMRIAVMon T2Perfusion abnormalityArcuate Fasciculus (DTI)Broca & Wernicke (fMRI)Courtesy: Philips &University of Leuven, BelgiumVery Basic MR Spectroscopy eg Fat and Water 3.5 ppm Separation Larmor Frequency of Protons inWater 1.5T x 42.58 MHz/T 64MHzFat 3.5 ppm slower due to e- shielding about 220 Hz Lower.000220 MHz (3.5ppm)WaterFat64 MHz21

01/18/2015Water – Fat ShiftChemical Shift due to Shielding Different Larmor f ’s Tune “RF Slice Select” or “Sat Bands”to Excite or Destroy Fat or Water Phase Cycling: Fat & Water Precess In and Out ofPhase (Another Fat Suppression Method) Different Frequency Chem Shift Artifact MisplacesFat (Incorrect Frequency Encoding)Detecting Metabolite Concentrations Metabolites “Shielded” DifferentlyProtons “See” Different Bo FieldJust Excite One Big VoxelFrequency Variation Metabolites instead ofSpatial Location Fourier Transform Plot MR SpectrumExample Proton Spectra Long TE Clean, Basic– NAA, Cr, Cho: Hunter’s Angle– Lactate, LipidHunter’s Angle Short TE Methods– Add mI, Glx, etc Multivoxel– Time Efficient– Longer TE, Purists Object2 ppm22

01/18/2015Spectroscopic Imaging in Tumors(2 minutes)Courtesy Philips HealthcareThe High Field Trend 0.015T 0.35T 1.0T 1.5T 2T 2.9T 3T 7T ------ and still climbing More Net Protons: SNR Increases Linearly– Trade for Cleaner Images, Higher Resolution, Faster Acquisition Greater Chemical Shift– MRS Peak Separation, Susceptibility, Magnetization Transfer Contrast Greater SAR and Dielectric Effects– Reworked Sequences, Different Contrast, Safety Issues Equipment and Siting Expensive– Superconducting Material, Iron to Contain FieldWhy Higher Field Strength?Higher SNR Better ResolutionAnd/Or Scan TimeBetterContrastHigher Sensitivityfor Susceptibility contrastIncreasedChemical shift23

01/18/2015Physical Components, Options,Financial InsightsComponents of MRI Superconducting Electro-Magnet– Align protons in Longitudinal Direction RF Transmit Coil (Typically Built In)– Flip protons force coherence Rotating Transverse Magnetization RF Receive Coil (Several Supplied)– Detect Transverse Magnetization 3 Sets of Gradients– Vary Magnetic Field to Spatially Encode Sequencer and Image ReconstructionInside the MRI Gantry24

01/18/2015Example Receiver Coil PortfolioSENSE Head 8SENSE Knee 8Breast SupportSENSE XL Torso 16SENSE NV 16SENSE WristSENSE BreastSENSETorso/Cardiac 32SENSE Head Spine16 15SENSE Wrist 8SENSE Flex LSENSE Foot Ankle 8SENSE Breast 7SENSE TorsoSENSE Shoulder 8SENSE Breast 16SENSE Flex MSENSE ShoulderSENSE Flex SEndo coilSENSE CardiacSENSE Head 32Courtesy Philips Healthcarec 200725

01/18/2015Various Magnet DesignsSiting an MRI Scanner RoomEquipment RoomTech AreaOutsideEquipmentFringe FieldsPowerVibrationSecurityMRI System Cost Examples Low Field Open New 1T Open Refurb 1.5T New 1.5T Refurb 3T New 3T 7T 400,000 (avail?) 1.5M-------------------- 600,000 1.2M-------------------- 900,000 1.9M-------------------- 6M, plus 1M siting26

01/18/2015Example MRI Proforma (monthly) Equipment (MRI, PACS, etc) 21,000 eg 1.5M operating lease ServiceNew Installation Rent 12,000 7,500 eg 400,000 5yr capital lease 2,000 eg 20/sf x 1200sf Staff 10,000 eg RT@ 25, clerical@ 15 Legal, Management, Physicist 1,000 53,000 Technical Margin per scanBreakeven (Costs/margin) 200270 scans per month 14 scans per dayExample MR Profitability Operate 40hrs/week, ½ hr per patient320 scans per month 50 scans beyond breakeven– TC profit 200 x 50 10,000 monthly 120,000 annually– Annual PC Revenue 60/scan x 320 x 12 (about 80hrs/mo) 200K Generally,– New scanner justified at 10-15/day– 1 scan per day beyond b.e. 60 - 100K annual profit– New Neuro Patient 50% need MRI 20-30 new pat/d 4 MDs PAsSome Educational Resources https://www.youtube.com/watch?v Ok9ILIYzmaY Society of Magnetic Resonance Technologists Joe Hornack Web Site– http://www.cis.rit.edu/htbooks/mri/ WikipediaManufacturer Web Sites and Operator ManualsMRI Safety by Frank Shellock, PhDSimply Physics, Moriel NessaiverMedical Imaging Consultants, Incjfritz@dentinstitute.com, www.DentInstitute.com27