Measuring distance of periodic structure in biological sample with FFT in MATLAB
i am not an image processing guy.  but i know what the 2-dimensional Discrete Fourier Transform is.

normally, in two dimensions, the way we measure period is with an operation related to the autocorrelation and you can get to the autocorrelation $R_x[k,\ell]$ with the DFT and magnitude-squaring the result $|X[m,n]|^2$ and inverse DFT to get the autocorrelation.  mathematically, you should have half of the 2-dim buffer (like quadrants II and IV) set to zero, to prevent overlap in the correlation, then you have a triangular envelope on the peak values, even if it were a perfectly periodic function.

so, in the $k$ dimension of the result there will be an apparent period, call it $p_k$ and similar for the $\ell$ dimension.  the period will be $p = \sqrt{p_k^2 + p_\ell^2}$ .  actually, when viewing $R_x[k,\ell]$ as a surface, there should be a circular ridge of radius $p$.