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

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

3. “Field” map....... Gradient map Hz/3mmHz df/dzdf/dx df/dy

4. 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

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; T RO = 22msTE = 50ms; T RO = 64ms

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

10. WA5

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