M. Lustig, EECS UC Berkeley

Principles of MRIEE225E / BIO265

Lecture 16

Instructor: Miki LustigUC Berkeley, EECS

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M. Lustig, EECS UC Berkeley

Projct

• Work as a team of two– Oral presentation (20min) – Or, Report as a wikepedia entry

• Work shared equally, presentation time as well• Level of presentation -- assume textbook level of

knowledge• Grading based on accuracy, clarity and normalized by

scope• Work:

–Implementation–Review of advanced topics/papers - could be from your

own research

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M. Lustig, EECS UC Berkeley

Things to do:

• Pick a partner (use Piazza)• Pick a topic (list on website)• Write a short proposal -

– 1-2 paragraphs– References

• Send me the proposal by Fri 29th.

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M. Lustig, EECS UC Berkeley

Echoes

• Early in NMR people noticed the following odd behavior (Hann, 1950)

Two 90° excitations, separated by T cause large signal to form at 2T, Why?

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�(~r, t) =

Z t

0!(~r, ⌧)d⌧

�(~r, t) = !E(~r)t| {z } + !cst|{z} + �

Z t

0

~G(⌧)~rd⌧

| {z }

M. Lustig, EECS UC Berkeley

Phase Evolution of Spins

off-res chem-shiftencoding gradients

Each of these make s(t) dephase

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�(~r, t) = �

Z t

0

~G(⌧)~rd⌧

~G(t)

M. Lustig, EECS UC Berkeley

Gradient Echoes

Consider a special case where: !E(~r) = 0, !cs = 0

is an inhomogeneity we can control!

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M. Lustig, EECS UC Berkeley

Gradient Echoes

Large signal

Low signal

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M. Lustig, EECS UC Berkeley

Spin Bench Simulation

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�(t)�G

x

x1

�G

x

x2

M. Lustig, EECS UC Berkeley

Gradient Echo

k-space

kx

(t)t

t=TE

We get a large signal when we cross k-space origin

Phase diagram: x1>0, x2<0, phase evolution at each position:

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!E(~r) !cs(~r)

M. Lustig, EECS UC Berkeley

Gradient Echo

• All Spins have zero-phase at t=TE• The signal adds coherently, producing a

“gradient echo”• Terminology:

– Gradient Recalled Echo– Gradient echo– Gradient spin-echo

• Does not refocus

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0� 0+ ⌧+⌧� 2⌧

M. Lustig, EECS UC Berkeley

Spin Echo Pulse Sequence

RF

90180

T2* ?

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M. Lustig, EECS UC Berkeley

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M. Lustig, EECS UC Berkeley

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M. Lustig, EECS UC Berkeley

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M. Lustig, EECS UC Berkeley

Spin Echo Pulse Sequence

RF

90180

T2T2*

T2*Any signal loss due to dephasingis recovered at the spin-echo

If wE(r,t) changes over time, refocusing is not perfect

Provides probe to measure molecular motion

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M. Lustig, EECS UC Berkeley

Multi-Spin Echo - CPMG

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M. Lustig, EECS UC Berkeley

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M. Lustig, EECS UC Berkeley

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M. Lustig, EECS UC Berkeley

Spin Echo Imaging

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