We present a fast 3-D scattering center extraction algorithm using the shooting and bouncing ray technique. The proposed algorithm generates a 3-D inverse synthetic aperture radar image from which a set of 3-D scattering centers is then extracted using the CLEAN algorithm. In the conventional extraction algorithm, computation time is improved using the fast Fourier transform (FFT)-based scheme. However, the memory requirement is greatly increased because of the high over-sampling required to mitigate the interpolation error. Evaluation of memory-time tradeoffs indicates that the conventional algorithm is still too time consuming given the constraints of practical memory. Thus, a modified FFT-based scheme is proposed to improve computation time without increasing the memory requirement. Through modifying the ray spread function, the distortion from the interpolation error is mitigated without the use of high oversampling. Implementation based on the proposed FFT-based scheme accelerates the image formation process significantly. Numerical simulations of realistic targets are presented to demonstrate the performance of the proposed algorithm.