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These questions are concerned with k-space.

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Missing K-Space Lines

Answer this...

Refer to the "remove even lines" page of the K-Space Tool. Has the field-of-view (FOV) changed compared to the original data? If so, how?

View the Remove Even Lines page

Yes, but only in the phase-encoding direction! (The distance between the lines in the phase encoding direction, ∆ky, is proportional to 1/FOVy.) We've sampled as far out into k-space as in the original data (and changed nothing else), and so the spatial resolution won't change. The reason you would do this is to save time, because you don't need to acquire every other line in k-space. The image produced by the scanner would be "zoomed in" to the area denoted by the red box in the following image:

50RFOV image

If the whole reconstructed image was shown, foldover as seen in the image here would be seen.

This is the same as selecting "50% RFOV" on the scanner (R = "rectangular"). (50%RFOV may also be known as "half-phase FOV".) If the object of interest extends outside of the RFOV, the foldover may obscure the region of interest.

The downside of selecting a RFOV is reduced SNR (which we'd expect since we're acquiring less data).

Here are two helpful rules to get your head around problems like these:

  • The line spacing in k-space is inversely proportional to the FOV in real space.
  • How "far out" we acquire data in k-space is inversely proportional to the resolution in real space.


Further reading on this topic:
Books: MRI From Picture to Proton p133, MRI The Basics p251, Q&A in MRI p257-259

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