How can I use the fourier transform of a sequence of x-ray scan images to segment it?
Cited from wiki:

"a function (of time) is a representation of a signal with perfect time resolution, but no frequency information, while the Fourier transform has perfect frequency resolution, but no time information."

In your case, the Fourier transform has no spatial information. So for segmentation, you'd better try time-frequency analysis, such as wavelet transform