4T1 relaxation (slow) (longitudinal or spin-lattice) Fat ms 260LiverKidneyWhite mGrey mCerebrospinal fluid 2,000 2,400
5T2 relaxation (quick) 1.5T Fat 60-80 Liver 40 Kidney 60 White m. 90 Grey mCerebrospinal fluid 160
6How to measure T1 & T2? Sequence of RF pulses with a specific TE: Echo Time- time after 90o RF pulse until readout. Determines how much spin-spin relaxation will occur before reading one row of the image.TR: Repetition Time– time between successive 90o RF pulses. Determines how much spin-lattice relaxation will occur before constructing the next row of the image
7Measuring T1 Magnetization Mz A 90o RF pulse Mz->My Wait for a t timeSend a new 90o RFHow long does it take for Mz to recover?Generate the Mz recovery curve
8Measuring T1 Large molecules small molecules Energy transfer works when the frequency of precession of the protons overlaps with vibrational freq. of latticeLarge molecules->low vibrational freq->longT1Small molecules->broad vibrational freq->long T1Medium/viscous fluid-> intermediate freq ->short T1Large moleculessmall molecules
9Measuring T1 Large molecules small molecules Large molecules->low vibrational freq -> small overlap with oSmall molecules->broad vibrational freq-> larger overlap with oMedium/viscous fluid->intermediate freq.->largest overlap with oLarge moleculessmall molecules
10T1 and T2Molecule sizeT1T2SmallLongMediumLargeLongest
12Spin echo First 90o nutate magnetization Second 180 re-phasing pulse spin in phase T2 and T2* impact signalSecond 180 re-phasing pulseapplied at time T ->re-phases spins
13Spin echoThe 180o pulse has the function of rotating the magnetization vector to the opposite direction of the first 90o pulse.Spins experience OPPOSITE magnetic field inhomogeneities -> cancel its effectT2* is cancelled
14Spin echo contrast h proton density Using the same pulse seq. TR repetition timeTE echo timeUsing the same pulse seq.We get different S depending on T1 and T2
15Inversion recovery Emphasizes T1 relaxation time Extends longitudinal recovery time by a factor of 2
16Inversion recovery 180 pulse Mz => -Mz wait TI (time of inversion) 90 pulse -Mz => Mxy => FIDWait TE/2180 pulse produces echo at TE
17Inversion recoveryNo T2A factor of 2 (-Mz to Mz)
18How do you generate images? Spatial EncodingGenerate magnetic gradient across the patientB decreases
19Spatial encoding Frequency of precession vary with B Resonance frequency will also varyA wise choice of RF frequency can give just one slicef1 f2BoB decreases
20Spatial encoding You can do this in all 3 planes The intersection of all planes gives us a location (voxel)A voxel becomes a value of intensity on the MRI image
21Sensitive point technique (se) Apply slice select gradientNo effect everywhere elseThe location is established by RF central frequencySlice thickness is established by RF bandwidth
22Phase encodingProtons at the end of a gradient (strong B) go faster than the one at the other end (weak B).Protons where B was higher are ahead of protons where B was slowerBONBOFFWE GET A PHASE GRADIENT
25Spatial encoding You can do this in all 3 planes The intersection of all planes gives us a location (voxel)A voxel becomes a value of intensity on the MRI imageFourier transforms are used to go from time to frequency
26Spatial encodingApply slice select gradient while transmitting an RF pulseApply phase encoding gradientApply frequency encoding gradientFourier transform received signalRepeat with different phase
27Spatial encoding Slice -> Z axis Frequency of returned RF signal -> x axisPhase of returned RF signal -> y axisThe intersection of all planes gives us a location (voxel)
31Main magnet 1 Tesla = 10,000 Gauss Earth 0.5 µT - 0.5G Magnet can be Resistive -can be turned on and off, consume a lot of electricity (0.35T)Permanent-cannot be turned off (0.5T)Superconducting - best performance need to be cooled
32Superconducting magnet Several teslaConduct electrical current with little resistanceWire- wrapped cylinder (solenoid)Need high cooling (4.2K)
33Gradient coils Up to 60 mT/m In the z direction are called Helmholtz coilsX and y are Saddle coilsFast switch on/of 500 µs
34RF coils Frequencies 1 MHz - 10GHz Transmitter coil - sends RF pulse Receive coils (can be same as transmitter) - receive RF signal
35Magnetic Shielding Layers of steel plates around the magnets RF shielding - faraday cage (copper sheet metal all around the MRI room.
36Homework Please write a short description of T1 WeightingT2 WeightingSpin Proton Weighting(Matlab should be used to generate graphs that will help your description)
37Images References The essential physics of medical imaging (Bushberg) Lucas Parra CCNY
39Pulse effectWe start by assuming that the equilibrium magnetization vector is[0, 0, 1]'If we had a perfect 90-degree excitation, about the y axis, then the vector becomes [1, 0, 0]'Try defining M=[1, 0, 0]' in Matlab, and notice the result.
40Transverse relaxation Exponential decay process of the x and y components of magnetizationMathematically this meansMx(t)=Mx(0)exp(-t/T2)My(t)=My(0)exp(-t/T2).
41Transverse relaxation Assume M consists of only an x component.Let's say that T2=100 ms.Ignoring other effects, what is the magnetization vector due to T2-decay after 50 ms?