A simple image domain ray tube integration formula is presented to efficiently compute the inverse synthetic aperture radar (ISAR) image of a complex target by the shooting and bouncing ray (SBR) technique. Contrary to the conventional approach where the ISAR image is obtained by inverse Fourier transforming the computed scattered field data over frequency and aspect, this image domain formula gives the contribution of each ray to the overall ISAR image directly. Under the small angle approximation and utilizing the bistatic-monostatic equivalence, the image domain ray tube integration formula is determined in closed form. Simulation results using the SBR-based code ''Xpatch'' show that the direct image domain method results in good image quality and superior time performance when compared to the conventional frequency aspect approach. ¿ American Geophysical Union 1995