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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)**

Fat ms 260 Liver Kidney White m Grey m Cerebrospinal fluid 2,000 2,400

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**T2 relaxation (quick) 1.5T Fat 60-80 Liver 40 Kidney 60 White m. 90**

Grey m Cerebrospinal fluid 160

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**How 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

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**Measuring T1 Magnetization Mz A 90o RF pulse Mz->My**

Wait for a t time Send a new 90o RF How long does it take for Mz to recover? Generate the Mz recovery curve

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**Measuring T1 Large molecules small molecules**

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 small molecules**

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 T1 T2 Small Long Medium Large Longest

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T1 and T2 relaxation time

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**Spin echo First 90o nutate magnetization Second 180 re-phasing pulse**

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 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 effect T2* is cancelled

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**Spin echo contrast h proton density Using the same pulse seq.**

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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**Inversion recovery 180 pulse Mz => -Mz wait TI (time of inversion)**

90 pulse -Mz => Mxy => FID Wait TE/2 180 pulse produces echo at TE

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Inversion recovery 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 f1 f2 Bo B decreases

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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) ; exp(-50/100) 0; ];

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V.G.Wimalasena Principal School of Radiography

V.G.Wimalasena Principal School of Radiography

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