Comparison of routine and on-demand prescription of chest radiographs in mechanically ventilated adults: a multicentre, cluster-randomised, two-period crossover study. - Hejblum, G.; Boëlle, P.-Y.; Vibert, J.-F.; Guidet, B.; Simon, T.; Chalumeau-Lemoine, L.; Ioos, V.; Guider, B.; Salomon, L. (Lancet, 2009)