We introduce a fast and accurate numerical method for simulating the propagation of an ultrasound beam inside inhomogeneous tissue for mapping beam absorption, refraction and diffraction in the body. The technique, called the hybrid angular spectrum method, is an extension of the angular spectrum method to inhomogeneous tissue. Inhomogeneous tissue is modeled using voxels, each with its own speed of sound, density and absorption coefficient. The proposed technique produces very fast simulations, with total calculation times of about one minute for a 201x201x101 model.