MR physics and safety for fMRIOutline:What is NMR?What is NMR? Nuclear magnetismPowerPoint PresentationCompass needlesGyroscopic motionEXCITATION : Displacing the spins from Equilibrium (North)Excitation: ResonanceSlide 10"Exciting" the Magnetization: tip angleDetecting the Magnetization: Faraday’s LawSlide 13The NMR SignalSlide 15Physical Foundations of MRIThree Steps in MR:Magnetization vector during MRThree places in process to make a measurement (image)Contrast in MRI: proton densityT2*-DephasingT2* DephasingSlide 23T2* WeightingGradient Echo (T2* contrast)Slide 26Gradient EchoSpin Echo (T2 contrast)Spin EchoT2 weighed spin echo imageOther contrast for MRIT1 weighting in MRI (w/ 90o excite)T1-WeightingT1 weighting in MRI (w/ 30o excite)Slide 35Slide 36Slide 37Wald, fMRI MR Physics Massachusetts General HospitalAthinoula A. Martinos Center MR physics and safety for fMRILawrence L. Wald, Ph.D.Wald, fMRI MR Physics Outline:Monday Oct 18 (LLW):MR signal, Gradient and spin echoBasic image contrastMonday Oct 25 (LLW):Fast imaging for fMRI, artifactsWed. Oct 20 (JJ):Encoding the imageWed. Oct 25 (LLW):fMRI BOLD contrastMon. Nov. 1 (JJ):Review, plus other contrast mechanisms for fMRI (CBV, CBF)Wald, fMRI MR Physics What is NMR?NUCLEAR MAGNETIC RESONANCEA magnet, a glass of water,and a radio wave source and detector….Wald, fMRI MR Physics What is NMR? Nuclear magnetism=MWald, fMRI MR Physics BNSWEEarth’sFieldprotonscompassEE= h(N↑ – N↓)/NTOT = 1 – exp(-E/kT) ≈ 10-4Wald, fMRI MR Physics Compass needlesNSWExyzMainField Bo42.58 MHz/T Earth’sFieldNorthFreq =  BWald, fMRI MR Physics Gyroscopic motionMainField BoLarmor precession freq. = 42.58 MHz/TxyzNorth• Proton has magnetic moment• Proton has spin (angular momentum)>>gyroscopic precession =  BoMWald, fMRI MR Physics EXCITATION : Displacing the spinsfrom Equilibrium (North)Problem: It must be moving for us to detect it.Solution: knock out of equilibrium so it oscillatesHow? 1) Tilt the magnet or compass suddenly 2) Drive the magnetization (compass needle)with a periodic magnetic fieldWald, fMRI MR Physics Excitation: ResonanceWhy does only one frequency efficiently tip protons?Resonant driving force.It’s like pushing a child on a swing in time with the natural oscillating frequency.Wald, fMRI MR Physics xyzStaticField, B0Applied RFField (B1)z is "longitudinal" directionx-y is "transverse" planeThe RF pulse rotates Mo the about applied fieldMoRF Field (B1) applies a torque to the spins…Wald, fMRI MR Physics xyzxyz45° 90°"Exciting" the Magnetization: tip angleStaticField, B0Wald, fMRI MR Physics Detecting the Magnetization: Faraday’s LawA moving bar magnet induces a Voltage in a coil of wire.(a generator…)The RF coil design is the #1 determinant of the system SNRxyzo90°V(t)= -d/dt= n Bspins AWald, fMRI MR Physics Detecting the NMR: the noisexyzo90°V(t)Noise comes from electrical losses in the resistance of the coil or electrical losses in the tissue.For a resistor:Pnoise = 4kTRB•Noise is white.>>Noise power  bandwidth•Noise is spatially uniform.•R is dominated by the tissue. >> big coil is bad.Wald, fMRI MR Physics The NMR SignalxyzRFtimexyzVoltage(Signal)timeoxyzo90°V(t)BoMoWald, fMRI MR Physics Signal to Noise Ratio in MRI•Most important piece of hardware is the RF coil.•SNR  voxel volume (# of spins)•SNR  SQRT( total time of data collection)•SNR depends on the amount of signal you throw away to better visualize the brain (gain image contrast)Wald, fMRI MR Physics Physical Foundations of MRINMR: 60 year old phenomena that generates thesignal from water that we detect.MRI: using NMR signal to generate an imageThree magnetic fields (generated by 3 coils)1) static magnetic field Bo2) RF field that excites the spins B13) gradient fields that encode spatial infoGx, Gy, GzWald, fMRI MR Physics Three Steps in MR:0) Equilibrium (magnetization points along Bo)1) RF Excitation (tip magn. away from equil.)2) Precession induces signal, dephasing (timescale = T2, T2*).3) Return to equilibrium (timescale = T1).Wald, fMRI MR Physics Magnetization vector during MRRFVoltage(Signal)timeencodeMzMxyWald, fMRI MR Physics Three places in process to make ameasurement (image)0) Equilibrium (magnetization points along Bo)1) RF Excitation (tip magn. away from equil.)2) Precession induces signal, allow to dephase for time TE. 3) Return to equilibrium (timescale =T1).protondensityweightingT2 or T2*weightingT1 WeightingWald, fMRI MR Physics Contrast in MRI: proton densityForm image immediately after excitation (creation of signal).Tissue with more protons per cc give more signal and is thus brighter on the image.No chance to dephase, thus no differences due to different tissue T2 values.Magnetization starts fully relaxed (full Mz), thus no T1 weighting.Wald, fMRI MR Physics T2*-DephasingWait time TE after excitation before measuring M. Shorter T2* spins have dephasedxyzxyzxyzvectorsuminitially at t= TEWald, fMRI MR Physics T2* DephasingJust the tips of the vectors…Wald, fMRI MR Physics T2* = 200T2* = 601008060402000.00.20.40.60.81.0Time (milliseconds)Transverse MagnetizationT2* decay graphsTissue #2Tissue #1Wald, fMRI MR Physics T2* WeightingPhantoms withfour different T2* decay rates...There is no contrast difference immediately after excitation, must wait (but not too long!).Choose TE for max. inten. difference.Wald, fMRI MR Physics Gradient Echo (T2* contrast)Dephasing is entirely from a spatial difference in the applied static fields.t = 0xyz90°t = TRed arrows processes faster due to its higher local fieldxyzBo + Gx xxWald, fMRI MR Physics Gradient Echo (T2* contrast)Dephasing is entirely from a spatial difference in the applied static fields.t = 0xyz90°t = TxyzBo + Gx xxBo + Gx xxt = Txyzt = 2TxyzWald, fMRI MR Physics Gradient EchoRF excitationGxtSttBoring!Wald, fMRI MR Physics Spin Echo (T2 contrast)Some dephasing can be refocused because its due to static fields.t = 0xyz90°t = TBlue arrows precesses faster due to localfield inhomogeneity than red arrowxyzt = T (+)t = 2TEcho!xyzxyz180°Wald, fMRI MR Physics Spin Echo180° pulse only helps cancel static inhomogeneityThe “runners” can have static speed distribution.If a runner trips, he will not make it back in phase with the others.Wald, fMRI MR Physics T2 weighed spin echo imagegraywhite Time to Echo , TE (ms)NMR SignalWald, fMRI MR Physics Other