Mr Physics in 20 Minutes

8/6/2019 Mr Physics in 20 Minutes

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MR Physics in 230 Minutes A Talk from Hell

One reason why this is a talk from hell: there is a lot to say in just 20 30 minutes.

The second reason is on slide #6.

X


2/15

Changing electric fields make magneticfields, changing magnetic fields makeelectric fields.

Hydrogen nuclei (protons) are slightlymagnetic due to their spin. In water they are relatively naked of electronsso their nuclear magnetic resonance isless obscured. Water is abundant inthe body.

In the MRI scanner a very large coilcooled with liquid helium has a verystrong electric current flowingcontinuously. This makes the strong (3Tesla) unchanging magnetic fieldcalled B 0 . Protons in this strong fieldbecome (a) more magnetic and (b)lined up with B 0, rather than lining up inrandom fluctuating directions.

H 2O+

-

B 0


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There is also a radio - frequency (RF)antenna (head coil; it looks like a birdcage) that both transmits and receivesRF energy.

If we transmit a pulse at exactly the rightfrequency (127.68 MHz at 3T) we canspiral the hydrogen nuclei out of alignment with B 0 over to flip angle .More RF pulse energy is needed for larger flip angles (larger amplitude or longer duration = more energy).The head coil will receive this RF backas hydrogen nuclei spiral back uptoward B 0. T1 is the time constant for getting back this longitudinal (M z, spin -

lattice) magnetization. T2 is the timeconstant for losing transverse (M xy)magnetization due to spin - spininteractions and, when combined withfaster losses due to magnetic fieldvariation (e.g., from blood oxygenationchanges ), yields T2* .

Flip Angle

Time constant T1

Time constant(s)T2 and T2*


4/15

Three other coils in the scanner have flowingelectric currents from time to time. Thesemake weaker magnetic fields, calledgradients , that vary over time as well as over space ( field of view ). One gradient varies up -

down, another varies left - right, the third variesfore - aft.

One of those gradients is turned on at thesame time as the head coil transmits RFenergy at 127.68 MHz. This gradient makesthe magnetic field too strong near one end(3.0001 T) and too weak near the other end(2.9999 T) for 127.68 MHz to be exactly theright frequency. Only those hydrogen nucleiwhere magnetic field strength is exactly 3Tget bumped out of alignment with B 0 by thisRF. The is called slice selection because

only a thin sheet out of the whole volume fitsthe criteria. Slice thickness is determined byRF pulse bandwidth (thinner slices = longer pulses) and by gradient slope (thinner slices =more Gauss/cm). The slice - select gradientcould be up - down (axial slices, like here), left -right (sagittal slices), fore - aft (coronal slices)

or any combination (oblique slices).

Not selected, field too strong

Selected slice, field just right

Magneticvectors in theselected slice ,seen fromabove. Theyspin around at127.68 MHz

and point inthe samedirection.

Sliceselection

Not selected, field too weak


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P hase Encode & Frequency Encode (Readout) Gradients

Magnetic vectors in selected slicewhile phase encode gradient is on(one of 64 possible gradientmagnitudes illustrated). Speedchanges left to right.

Magnetic vectors in selected slicewhile frequency encode (readout)gradient is on. Speed changes topto bottom and pointing directionchanges left to right.

Magnetic vectors in selectedslice after the phase encodegradient turns off. Speed nolonger differs but pointingdirection changes left to right.

Slow Medium Fast

Fast

Medium

SlowBehind Ahead


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Why This Is a Talk from Hell

While working on this talk I recalled a jokeabout a man who died and foundhimself in Hell.

He was standing waist - deep in feces. Helooks over to the next condemned souland remarks, This sure stinks butsomehow I thought Hell would be

worse.Then the Devil gave the order, Ev eryone

stand on their head!


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D ephasing & Rephasing: ProtonsStand on Their H eads

This is where theprotons are made tostand on their head ,rotating 180 degrees outof the plane of thedrawing at time ( TI ).What forces them to dothis is a strong RFinversion pulse for spin

echo images..

D ephasing RephasingSome protons movefaster because they are ina place having a stronger magnetic field. Therecorded signal growsweaker with moredephasing (the vectors

are not pointing in thesame direction).

The vectors points arelined up again after apredictable length of time, the echo time (or TE ). The recordedsignal gets stronger withrephasing.

S tand on your head at inversion time (TI) E cho time ( T E ) = 2 * TI


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Spin Echo sequence

Repeat after time TR with a differentmagnitude of phase encode gradient

Stand on your head, protons

S pin echo

The inversion pulse is not verysensitive to t2* inhomogeneity


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G radient E cho sequence

DO NOTstand on your head, protons

No RF pulse

De - phasing: Racebackwards fast

Re - phasing: Raceforwards slow

G radient echo

Both races are very sensitive to t2* inhomogeneity


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Cl ick hererepeated lyif no image

E cho-Planar Imaging

k - spaceZoomed in view of

1 ............................................... 64

1234561 2 3 4 5 6 ...


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Adjusting Parameters to Change Image Contrast

CSF, gray matter and white matter (also water, fat, protein, etc.) all have different valuesfor T1, T2, and T2*. As a consequence there are multiple ways to reveal contrastsamong tissues (T1 - , T2 - , proton density contrasts shown). For MRA (angiogram), the TRis so short that protons stuck in the selected tissue have their vectors stuck at the flip

angle ... while protons in flowing blood escape the selected slice, relax toward B 0 andemit RF. The MRA thus makes blood vessels stand out from other tissues.

MRA

Very short T R


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Two more imaging typesArterial Spin Labeling (ASL): The common or internal carotid artery isselected either by a slice made well below the brain, or by placing asupplemental surface coil on the neck. Protons in the common/internalcarotid blood are inverted (FA = 180) with an RF pulse. Those protonswill alter magnetic signal recorded from slices in the brain into which thislabeled blood has perfused, as it dilutes signals from blood previously inthe brain that did not get inverted protons.

High Angular Resolution Diffusion Weighted Imaging (HARD): Diffusionweighting of MR images is achieved with two extra gradient pulses per TR, one strongly dephasing, the other strongly rephasing u nless the

protons have moved d u ring the time between the p u lses. Diffusion ismovement at the molecular level driven by heat. The more diffusion, the

less successful is the rephasing pulse, and the smaller the recoveredsignal. In brain tissue diffusion is directional, being channeled by the cellmembrane to move more readily along than across each axon. A voxelof white matter is sampled from many directions (the more directions thehigher the angular resolution) to determine the predominant diffusion

directions, from which the travel directions of axons passing through thatvoxel can be inferred.


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TermsB 0 = unchanging strong (3T) magnetic field, always onRF = radio frequency, approximately 127.68 MHz at 3T, Larmor frequency of H in H 2O

flip angle = tilt of proton magnetization vector away from axis of longitudinal magnetization,initially related to transmitted RF pulse energyT1 = time constant for recovery of longitudinal magnetization, a relatively large length of timeT2 = time constant for the loss of transverse magnetization, for two main reasons1/T2* = 1/T2 [spin - spin interactions, a medium length of time] + 1/T2 [magnetic fieldinhomogeneities, such as varying blood oxgenation levels, a short length of time]gradient = magnetic field that varies over space ( field of v iew ) and/or over timeslice selection gradient = makes a thin slice of tissue have the proper magnetic field for acoincident transmitted RF pulse, so as to tilt protons only within the slice to the flip angleslice thickness = determined by bandwidth of transmitted RF and gradient rate of changephase encoding gradient = temporarily speeds up or slows down precession on one sliceaxis, turned off while receiving RFfrequency encoding (readout) gradient = speeds up or slows down precession on theorthogonal slice axis, turned on while receiving RFdephasing = mismatched pointing directions of vectors that precessed at different ratesTI = inversion time, when vectors are rotated 180 degrees out of plane by an RF pulserephasing = rematched pointing directions of vectors at 2*TI or at the echo time (TE)T E = echo time(s). More than one dephasing - rephasing can be used per tilting pulse.T R = repetition time, how long to wait before tilting the protons again, perhaps using a newphase encoding gradient or perhaps selecting a new sliceE cho-planar imaging ( E P I): one method of spatial encoding, another popular method is spiralT1 -, T2 -, T2* -, proton density-, diffusion-weighted, MRA , BOLD (f MR I), AS L = types of

image contrast


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Cartoon of How RF signals are captured into k - space (via 2 - D FFT )

Selected slice before phaseor frequency encoding

psd (complex 2 - D vector)

Frequency Change from Readout Gradient

F r e q u

e n c y

C h a n g e f r o m

P h a s e E n c o d e

Readout after zero slopephase encoding, complex

plane stays flat


F r e q u

e n c y

C h a n g e f r o m


Readout after large slopephase encoding, complexplane wrinkled up/down


F r e q u

e n c y

C h a n g e f r o m


real imagx, y

real

imag

x, y

real

imag

psd (complex 2 - D vector) psd (complex 2 - D vector)

real real

real

imag

imag

imag

x, y


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Link to k - space tutorial

http://www.revisemri.com/tutoria ls/intro_kspace1.htm

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Mr Physics in 20 Minutes