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fMRI: Biological Basis and Experiment Design Lecture 11: Distortion

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.

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fMRI: Biological Basis and Experiment Design Lecture 11: Distortion

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  1. fMRI: Biological Basis and Experiment DesignLecture 11: Distortion • Field maps • Bandwidth / pixel • Calculations Dali. The Persistence of Memory, 1931.

  2. Field-mapping sequence: double-echo FLASH magnitude images phase difference images

  3. “Field” map ....... Gradient map df/dz df/dx Hz Hz/3mm df/dy

  4. Through-slice dephasing review 0mm  Thinner slice, less signal loss through-slice dimension 0 - 3mm No gradient; no signal loss Strong gradient; big signal loss

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

  6. Distortion is determined by the relative magnitudes of imaging gradients and artifactual gradients

  7. Distortion boils down to read-out time and phase-encode polarity

  8. Caveat: distortion and drop-out travel together TE = 25ms; TRO = 22ms TE = 50ms; TRO = 64ms

  9. Cranking up bandwidth to defeat distortion kills SNR TE = 50ms; TRO = 64ms TE = 25ms; TRO = 16ms TE = 25ms; TRO = 22ms

  10. WA5

  11. EPI images have two pixel bandwidths • In the read-out direction: • In the phase-encode direction:

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