FMRI: Biological Basis and Experiment Design Lecture 10: The Dreaded Drop-out Spin echo review Field maps Through-slice dephasing © Melissa Tillery

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fMRI: Biological Basis and Experiment Design Lecture 10: The Dreaded Drop-out Spin echo review Field maps Through-slice dephasing © Melissa Tillery

Images acquired with a spin echo are T 2 -weighted Refocusing (e.g. 180  ) read-out time (ms) MM T2*T2* T2T2 Excitation (e.g. 90  ) Spin echo movie:

Spin echo rescues through-slice dephasing

Spin echo refocuses dephasing near large veins

Slice selection x Frequency profile of pulse = spatial profile of slice BWthk = BW/G SS =  * G x * x G x = 12 mT/m

Field-mapping sequence: double-echo FLASH magnitude imagephase difference

GRE Field-mapping: (made-up) pulse sequence diagram N rep = points RF G SS G PE G RO DAC Flip angle = 7 deg. TR = 20ms TE 1 ~ 5.76ms 64 points TE 2 ~ 7.13ms

GRE Field-mapping: (made-up) pulse sequence diagram N rep = points RF G SS G PE G RO DAC Flip angle = 7 deg. TR = 20ms TE 1 ~ 5.76ms 64 points TE 2 ~ 7.13ms

Frequency (map) can be calculated from difference in phase between (images acquired at) 2 echo times TE 1 x y TE 2 x y x y  =  t - = 2 ms Hz

“Field” map Gradient map Hz/3mmHz df/dzdf/dx df/dy

Slice thickness and through-slice gradient combine to create signal loss 0mm 3mm 0 --  through-slice dimension

Through-slice dephasing and distortion df/dz df/dx df/dy