Upload
tamma
View
44
Download
0
Tags:
Embed Size (px)
DESCRIPTION
fMRI: Biological Basis and Experiment Design Lecture 11: Distortion. Field maps Bandwidth / pixel Calculations. Dali. The Persistence of Memory, 1931. Field-mapping sequence: double-echo FLASH. magnitude images. phase difference images. - PowerPoint PPT Presentation
Citation preview
fMRI: Biological Basis and Experiment DesignLecture 11: Distortion
• Field maps• Bandwidth /
pixel• Calculations
Dali. The Persistence of Memory, 1931.
Field-mapping sequence: double-echo FLASH
magnitude images phase difference images
“Field” map ....... Gradient map
Hz/3mmHz
df/dz df/dx
df/dy
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
Through-slice dephasing and distortiondf/dz
df/dx
df/dy
Distortion is determined by the relative magnitudes of imaging gradients and artifactual gradients
Distortion boils down to read-out time and phase-encode polarity
Caveat: distortion and drop-out travel together
TE = 25ms; TRO = 22ms TE = 50ms; TRO = 64ms
Cranking up bandwidth to defeat distortion kills SNR
TE = 25ms; TRO = 22ms TE = 50ms; TRO = 64ms TE = 25ms; TRO = 16ms
WA5
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