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BE 581 Lecture 3- Intro to MRI

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BE 581 Lecture 3 - MRI

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Block Equation - T1 decay 90 pulse 180 pulse

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T1 relaxation (slow) (longitudinal or spin-lattice) 0.5T1.5T Fat200ms260 Liver Kidney White m Grey m Cerebrospinal fluid2,0002,400

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T2 relaxation (quick) 1.5T Fat60-80 Liver 40 Kidney 60 White m. 90 Grey m. 100 Cerebrospinal fluid 160

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How to measure T1 & T2? Sequence of RF pulses with a specific TE: Echo Time- time after 90 o 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 90 o RF pulses. Determines how much spin-lattice relaxation will occur before constructing the next row of the image

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Measuring T1 Magnetization Mz A 90 o RF pulse Mz->My Wait for a t time Send a new 90 o RF How long does it take for Mz to recover? Generate the Mz recovery curve

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Measuring T1 Energy transfer works when the frequency of precession of the protons overlaps with vibrational freq. of lattice Large molecules->low vibrational freq->longT1 Small molecules->broad vibrational freq->long T1 Medium/viscous fluid-> intermediate freq ->short T1 Large molecules small molecules

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Measuring T1 Large molecules->low vibrational freq -> small overlap with o Small molecules->broad vibrational freq-> larger overlap with o Medium/viscous fluid- >intermediate freq.->largest overlap with o Large molecules small molecules

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T1 and T2 Molecule size T1T2 SmallLong MediumSmall LargeLongestSmall

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T1 and T2 relaxation time

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Spin echo First 90 o nutate magnetization –spin in phase T2 and T2 * impact signal Second 180 re-phasing pulse –applied at time T ->re-phases spins

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Spin echo The 180 o pulse has the function of rotating the magnetization vector to the opposite direction of the first 90 o pulse. Spins experience OPPOSITE magnetic field inhomogeneities -> cancel its effect T2 * is cancelled

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Spin echo contrast h proton density TR repetition time TE echo time Using the same pulse seq. We get different S depending on T1 and T2

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Inversion recovery Emphasizes T1 relaxation time Extends longitudinal recovery time by a factor of 2

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180 pulse Mz => -Mz wait TI (time of inversion) 90 pulse -Mz => Mxy => FID Wait TE/2 180 pulse produces echo at TE Inversion recovery

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No T2 A factor of 2 (-Mz to Mz)

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How do you generate images? Spatial Encoding Generate magnetic gradient across the patient B decreases

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Spatial encoding Frequency of precession vary with B Resonance frequency will also vary A wise choice of RF frequency can give just one slice B decreases f1 f2 Bo

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Spatial 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

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Sensitive point technique (se) Apply slice select gradient No effect everywhere else The location is established by RF central frequency Slice thickness is established by RF bandwidth

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Phase encoding Protons 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 slower B ON B OFF WE GET A PHASE GRADIENT

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Frequency encoding

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Gradients

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Spatial 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 Fourier transforms are used to go from time to frequency

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Spatial encoding Apply slice select gradient while transmitting an RF pulse Apply phase encoding gradient Apply frequency encoding gradient Fourier transform received signal Repeat with different phase

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Spatial encoding Slice -> Z axis Frequency of returned RF signal -> x axis Phase of returned RF signal -> y axis The intersection of all planes gives us a location (voxel)

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MRI instrument

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MRI

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Magnets and coils

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Main 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

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Superconducting magnet Several tesla Conduct electrical current with little resistance Wire- wrapped cylinder (solenoid) Need high cooling (4.2K)

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Gradient coils Up to 60 mT/m In the z direction are called Helmholtz coils X and y are Saddle coils Fast switch on/of 500 µs

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RF coils Frequencies 1 MHz - 10GHz Transmitter coil - sends RF pulse Receive coils (can be same as transmitter) - receive RF signal

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Magnetic Shielding Layers of steel plates around the magnets RF shielding - faraday cage (copper sheet metal all around the MRI room.

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Homework Please write a short description of –T1 Weighting –T2 Weighting –Spin Proton Weighting (Matlab should be used to generate graphs that will help your description)

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Images References The essential physics of medical imaging (Bushberg) Lucas Parra CCNY

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Matlab exercise

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Pulse effect We 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.

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Transverse relaxation Exponential decay process of the x and y components of magnetization Mathematically this means Mx(t)=Mx(0)exp(-t/T2) My(t)=My(0)exp(-t/T2).

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Transverse 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?

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Answer [ ];

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Transverse relaxation What matrix do you need to do this in vector form? (remember your homework)

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Answer [ exp(-50/100) 0 0; 0 exp(-50/100) 0; 0 0 1];

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