The exposure index (EI) gives a feedback to radiographers on the image quality in digital radiography, but its estimation on clinical images raises many challenges. In this paper we provide a critical overview of state of the art methods that address this problem and we show that more robust results can be obtained by detecting anatomical structures. This new approach implicitly manages the presence of multiple structures in the field-of-view. Moreover, we propose a landmark-based method that, by exploiting redundancy of local estimates, is more robust to potential detection errors.