In this paper, an optimization of 3D ray tracing for wideband multiple-input multiple-output (MIMO) indoor channel is presented. First, the evolution of the predicted wideband parameters with the number and type of interactions of the simulated channel is analyzed and the relevant phenomena to take into account during simulations are specified. Then, and for a better performance of 3D ray tracing, an efficient approach to consider the diffuse scattering is presented. It extends 3D ray tracing by associating a scattering cluster with each interaction point of the ray. Scattering clusters are defined by analyzing the simulated responses of each object while taking into account its surfaces roughness or small details. They vary with the type of interaction point and the kind of object where the interaction occurs. Comparisons with measurements performed in a residential area show that adding scattering into simulations substantially improves the accuracy of the prediction of spatio-temporal channel parameters.