Semi-analytical models are often used for computationally efficient ultrasonic simulations. They typically apply plane-wave or quasi plane-wave approximations to the ultrasonic fields at the location of flaws in order to calculate diffraction coefficients. In favorable cases, the plane-wave approximations used for echo computations in the Civa software yield satisfying results. However, it can lead to inaccuracies in unfavorable cases, such as for wide probe apertures, outside of the focal region, or for beam-splitting or distortion due to irregular geometries. This communication presents an improved model, implemented in a development version of the software. The new approach describes the ultrasonic field as a sum of rays and applies the plane-wave approximation to each ray instead of the entire field. It significantly improves the accuracy of echo computations. However, it implies that the diffraction is calculated for each pair of incident and diffracted rays instead of being calculated only once: a specific algorithm has been developed in order to avoid a significant increase in computation times. Benefits of the new approach are illustrated by comparing its results and computation times to those of the former plane-wave model in several cases of interest. They are also compared to those of a coupled finite element model (Civa-Athena).