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