This paper discusses a new approach for 3D optical imaging of tissue using a very low power, narrow-pulse, plane-wave illuminator, and a receiver which takes 2D images of the backscattered light in a number of widely spaced, narrow time-gates. An optical parameter of a single voxel is estimated using only the average backscattered energy in a single time-gate from a single detector. Thus a solution to the `inverse problem' becomes practical even for 3D images with spatial resolution on the order of a millimeter. The tissue parameters are computed one slab at a time using a `layer peeling' process. A multiple-pass imaging process using later time gates on later passes allows the image quality to be greatly improved. An analytical model has been developed for the accuracy with which the optical parameters of a given voxel can be computed as a function of voxel depth. This paper describes the computation of optimum gate parameters and optical tissue parameter estimation error versus tissue depth, image resolution, and receiver accuracy.