How can i get the radial frequency and the fourie index after FFT？

[ylongwu]

Hi:

   I have a 2D array of real,after FFT I get  a 2D array of complex.

   the question is how can i get the  radial frequency and the fourie index from the complex number?  Mybe the question is so easy,  but i am so confused.  please help me!