An approach to take diffraction into account in acoustic nonlinear parameter imaging is described. In this type of tomography, two collinear planar waves with different frequencies are used to insonify the object. The complex-amplitude of the generated secondary wave at the difference frequency is detected for producing the projections. The algorithmic formulation relies on the use of two simultaneous governing wave equations for the primary and difference-frequency waves. Three algorithms are presented respectively for reconstructing weakly, moderately, and strongly scattering objects. Illustrations of quantitative tomograms of the nonlinear parameter generated from simulated data show good correlations with theoretical results.