fMRI: Biological Basis and Experiment Design Lecture 11: Distortion

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fMRI Biological Basis and Experiment
DesignLecture 11 Distortion

• Field maps
• Bandwidth / pixel
• Calculations

Dali. The Persistence of Memory, 1931.
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Field-mapping sequence double-echo FLASH
magnitude images
phase difference images
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Field map .......
Gradient map
df/dz
df/dx
Hz/3mm
Hz
df/dy
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Through-slice dephasing review
0mm
?
Thinner slice, less signal loss
0
through-slice dimension
-?
3mm
No gradient no signal loss
Strong gradient big signal loss
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Through-slice dephasing and distortion
df/dz
df/dx
df/dy
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Distortion is determined by the relative
magnitudes of imaging gradients and artifactual
gradients
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Distortion boils down to read-out time and
phase-encode polarity
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Caveat distortion and drop-out travel together
TE 25ms TRO 22ms
TE 50ms TRO 64ms
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Cranking up bandwidth to defeat distortion kills
SNR
TE 50ms TRO 64ms
TE 25ms TRO 16ms
TE 25ms TRO 22ms
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WA5
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EPI images have two pixel bandwidths

• In the read-out direction
• In the phase-encode direction