How can I use the fourier transform of a sequence of x-ray scan images to segment it? Sorry if this question seems to trivial, but I am a bit of a novice when it comes to signal and image processing and I need some guidance.  

I have a 3D stack of about 256 grey scale images, each with pixel resolution 256x256, coming from an x-ray tomography scan of a rock sample.  I want to segment the data to isolate the void space from the solid space.  Typically I use ImageJ software to segment it using its 'auto' feature based upon a histogram of the grey-scale color values.  I am wondering if using fourier transform of the grey-scale data can somehow help me in the process.  What can the FT tell me about the image data that I can use for segmentation purposes?
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