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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
Hz/3mmHz
df/dz df/dx
df/dy
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Through-slice dephasing review
0mm
3mm
0
-
through-slice dimension
No gradient; no signal loss
Strong gradient; big signal loss
Thinner slice, less signal loss
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Through-slice dephasing and distortiondf/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 = 25ms; TRO = 22ms TE = 50ms; TRO = 64ms TE = 25ms; TRO = 16ms
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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:
–
lineRORO
imageROpixel TN
BWBW
,,
1
imageROPERO
imagePEpixel TNN
BWBW
,,
1