The Hahn Spin Echo!

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The Hahn Spin Echo!

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Echoes! Spin Echo Formation! CPMG! Coherence Diagrams! Gradient Echo ... 160°Y! dephase! rephase! dephase! 160°Y! rephase! Echo 2! B1! B1! B1! B1!

Bioengineering 278" Magnetic Resonance Imaging" " Winter 2013" Lecture 3! Echoes! Spin Echo Formation! CPMG! Coherence Diagrams!

Gradient Echo Imaging! Spoiled! Unspoiled steady state! Totally refocused!

!

E. Wong, BE278, UCSD Winter 2013!

The Hahn Spin Echo! M!

Z!

Z!

Z!

B1!

B1!

s! Y!

X!

Y! X!

90°x!

Y! X!

dephase!

Z!

f!

180°x!

Z!

s! f!

X!

Y!

rephase!

Y! X!

Echo!! E. Hahn, 1950!

E. Wong, BE278, UCSD Winter 2013!

The CPMG Spin Echo! Hahn! Z!

B1!

B1! Y!

dephase!

rephase!

160°X!

dephase!

160°X!

rephase!

CPMG!

Z! Y!

B1! dephase!

B1! rephase!

160°Y!

dephase!

160°Y!

rephase!

Echo 1!

E. Wong, BE278, UCSD Winter 2013!

Echo 2!

Coherence Pathways! RFx pulse operating on M:!

1st! 1st RF pulse!

A!

$1 0 0 '$M x ' & )& ) T0 = &0 cos(α ) −sin(α ))&M y ) &%0 sin(α ) cos(α ) )(&% M z )(

B!

A + B =1 A − B = cos(α ) 1+ cos(α ) A= = cos 2 (α 2) 2 1 − cos(α ) B= = sin 2 (α 2) 2

K-Space!

€

RF pulse operating on F/Z:! % cos€(α 2) e 2iφ sin 2 (α 2) −ie iφ sin(α ) ( % Fn ( ' −2iφ 2 *' * T1 = ' e sin (α 2) cos 2 (α 2) ie −iφ sin(α ) * 'F * −n * '&−i 2e −iφ sin(α ) i 2e iφ sin(α ) cos(α ) *) '& Z n *) 2

RF pulses! E. Wong, BE278, UCSD Winter 2013!

Scheffler 1999: PDF on class web site also Hennig 1991! http://www.fmrib.ox.ac.uk/Members/karla/lectures-and-teaching-material! €

Spoiled gradient echo sequence! RF!

GZ(t)! • Spoiled FLASH, Spoiled GRASS! • RF phase pseudo-randomized! • Signal comes from FIDs only! • No echo pathways build up!

GX(t)! GY(t)!

TR! E. Wong, BE278, UCSD Winter 2013!

Spoiled gradient echo sequence! α!

α!

TE!

α!

TE!

TR!

TR!

[1− e I(x, y) = ρ(x, y) [1− e

−TR /T1 (x,y )

−TR /T1 (x,y )

] sinα exp(−TE /T ) cosα ] ∗ 2

Signal intensity is maximized at the Ernst Angle!

α E = cos−1 (exp(−TR /T1 )) € Spoiled gradient echo equation assumes no coherence from shot to shot. In practice this is achieved with RF spoiling. !

€

E. Wong, BE278, UCSD Winter 2013!

Spoiled gradient echo sequence!

E. Wong, BE278, UCSD Winter 2013!

Slide Credit: T.T. Liu!

α E = cos−1 (exp(−TR /T1 ))

€

Spoiled gradient echo sequence! RF!

GZ(t)! • Spoiled FLASH, Spoiled GRASS! • RF phase pseudo-randomized! • Signal comes from FIDs only! • No echo pathways build up!

GX(t)! GY(t)!

TR! E. Wong, BE278, UCSD Winter 2013!

Unspoiled gradient echo sequence! RF!

GZ(t)! • FLASH, GRASS! • RF phase uniform! • Signal comes from FIDs and Echoes! • Signal depends on α, TE, TR, T1, T2, T2*!

GX(t)! GY(t)!

TR! E. Wong, BE278, UCSD Winter 2013!

Echo-only gradient echo sequence! RF!

GZ(t)! • CE-FAST, SSFP! • RF phase uniform! • Signal comes from echoes only! • Signal depends on α, TE, TR, T1, T2!

GX(t)! GY(t)!

TR! E. Wong, BE278, UCSD Winter 2013!

Fully refocussed gradient echo sequence! RF!

GZ(t)! Nayak et al MRM 2007!

GX(t)! GY(t)!

TR!

• FISP, FIESTA, balanced SSFP! • RF phase alternates! • No net gradient per TR! • No dephasing -> no echoes! • VERY frequency sensitive! • Zeros separated by 1/TR! • Large signal! • Signal depends on α, TE, TR, T1, T2!

http://www.fmrib.ox.ac.uk/Members/karla/lectures-and-teaching-material E. Wong, BE278, UCSD Winter 2013!