fMRI: Biological Basis and Experiment DesignLecture 10: The Dreaded Drop-out

• Spin echo review• Field maps• Through-slice

dephasing

© Melissa Tilleryhttp://cyaneus.com/art/

Images acquired with a spin echo are T2-weightedRefocusing(e.g. 180)

read-out

time (ms)

M

T2*

T2

Excitation(e.g. 90)

Spin echo movie: http://www-mrsrl.stanford.edu/~brian/mri-movies/spinecho.mpg

Spin echo rescues through-slice dephasing

Spin echo refocuses dephasing near large veins

Slice selection

x

Frequency profile of pulse = spatial profile of slice

BW thk = BW/GSS

= * Gx * x

Gx = 12 mT/m

Field-mapping sequence: double-echo FLASH

magnitude image phase difference

GRE Field-mapping: (made-up) pulse sequence diagram

Nrep = 64

64 points

RF

GSS

GPE

GRO

DAC

Flip angle = 7 deg. TR = 20ms

TE1 ~ 5.76ms

64 points

TE2 ~ 7.13ms

GRE Field-mapping: (made-up) pulse sequence diagram

Nrep = 64

64 points

RF

GSS

GPE

GRO

DAC

Flip angle = 7 deg. TR = 20ms

TE1 ~ 5.76ms

64 points

TE2 ~ 7.13ms

Frequency (map) can be calculated from difference in phase between (images acquired at) 2 echo times

TE1

x

y

TE2

x

y x

y

= t

-

=

2 ms

Hz

“Field” map ....... Gradient map

Hz/3mmHz

df/dz df/dx

df/dy

Slice thickness and through-slice gradient combine to create signal loss

0mm

3mm

0

-

through-slice dimension

Through-slice dephasing and distortiondf/dz

df/dx

df/dy