The editors of this journal have decided that the number of authors of original articles and guidelines may in the future exceed the current limit of six. For other articles, e.g. clinical lessons, case reports and review articles, the limit remains. Contributions to the series 'Diagnostic image' will have a maximum of two authors. For a number of years now, authors have been asked to provide specific information as to the nature of their contribution to the article that carries their name. This enables the editors and the reviewers to assess whether the criteria for authorship have been met. Information on authors' contributions will not be printed alongside the articles but will be kept available to readers for three months after the article has appeared.