Parts of k-spaceWhy does k-space have a big spot in the middle?
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The exact middle of k-space is the point where kx = ky = 0. This location corresponds to the constant term in the Fourier representation of the image, whose magnitude is the same as the average image brightness. This central point has the highest magnitude of any other point in k-space, best seen in the 3D representation (right).
There are two reasons the central area of k-space is the brightest. First, the central row (ky = 0) is acquired with no phase-encoding gradient (and hence no destructive wave interference caused by phase-encoding steps). Secondly, the central column of k-space (kx = 0) coincides with the peak of the MR echo.
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2D and 3D pictures of k-space
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These features are illustrated below where the MR signals of rows from the center (left) and periphery (right) of k-space are compared.
MR signal from a line near the center of k-space has high amplitude.
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MR signal from a line through periphery of k-space shows low amplitude
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Related Questions
Where do you get the data to fill k-space? How does this relate to MR signals and echoes?
Where do you get the data to fill k-space? How does this relate to MR signals and echoes?