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BE 581 Lecture 3- Intro to MRI. BE 581 Lecture 3 - MRI.

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Presentation on theme: "BE 581 Lecture 3- Intro to MRI. BE 581 Lecture 3 - MRI."— Presentation transcript:

1 BE 581 Lecture 3- Intro to MRI

2 BE 581 Lecture 3 - MRI

3 Block Equation - T1 decay 90 pulse 180 pulse

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

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

6 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

7 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

8 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

9 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

10 T1 and T2 Molecule size T1T2 SmallLong MediumSmall LargeLongestSmall

11 T1 and T2 relaxation time

12 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

13 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

14 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

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

16 180 pulse Mz => -Mz wait TI (time of inversion) 90 pulse -Mz => Mxy => FID Wait TE/2 180 pulse produces echo at TE Inversion recovery

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

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

19 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

20 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

21 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

22 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

23 Frequency encoding

24 Gradients

25 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

26 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

27 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)

28 MRI instrument

29 MRI

30 Magnets and coils

31 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

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

33 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

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

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

36 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)

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

38 Matlab exercise

39 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.

40 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).

41 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?

42 Answer [ ];

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

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

45


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