An implicitly stable ratio imaging condition for prestack reverse-time migration (RTM) was defined using excitation criteria. Amplitude maxima and their corresponding occurrence times were saved at each grid point during forward source wavefield extrapolation. Application of the imaging condition involves dividing the amplitudes of the back-propagated receiver wavefield by the precomputed maximum source wavefield amplitude only at the grid points that satisfy the image time at each time step. The division normalizes by the source amplitude, so only the highest signal-to-noise ratio portion of the data is used. Provided that the source and receiver wavefield amplitudes are accurate at the reflection points, the peak wavelet amplitudes in the migrated image are the angle-dependent reflection coefficients and low wavenumber artifacts are significantly reduced compared to those in images calculated by crosscorrelation. Using excitation information and time-binning for the imaging condition improves computational and storage efficiency by three or more orders of magnitude when compared to crosscorrelation with the full source wavefield. Numerical tests with synthetic data for the Marmousi2 model have shown this method to be a cost-effective and practical imaging condition for use in prestack RTM.