title: Improving image retrieval effectiveness via query expansion using MeSH hierarchical structure.
abstract: OBJECTIVE
  We explored two strategies for query expansion utilizing medical subject headings (MeSH) ontology to improve the effectiveness of medical image retrieval systems. In order to achieve greater effectiveness in the expansion, the search text was analyzed to identify which terms were most amenable to being expanded.
  
  
  METHODS
  To perform the expansions we utilized the hierarchical structure by which the MeSH descriptors are organized. Two strategies for selecting the terms to be expanded in each query were studied. The first consisted of identifying the medical concepts using the unified medical language system metathesaurus. In the second strategy the text of the query was divided into n-grams, resulting in sequences corresponding to MeSH descriptors.
  
  
  METHODS
  For the evaluation of the system, we used the collection made available by the ImageCLEF organization in its 2011 medical image retrieval task. The main measure of efficiency employed for evaluating the techniques developed was the mean average precision (MAP).
  
  
  RESULTS
  Both strategies exceeded the average MAP score in the ImageCLEF 2011 competition (0.1644). The n-gram expansion strategy achieved a MAP of 0.2004, which represents an improvement of 21.89% over the average MAP score in the competition. On the other hand, the medical concepts expansion strategy scored 0.2172 in the MAP, representing a 32.11% improvement. This run won the text-based medical image retrieval task in 2011.
  
  
  CONCLUSIONS
  Query expansion exploiting the hierarchical structure of the MeSH descriptors achieved a significant improvement in image retrieval systems.
authors.affiliation: Department of Information Technology, University of Huelva, Huelva, Spain.
authors.forename: Mariano
authors.initials: M
authors.lastname: Crespo Azcárate
authors.forename: Jacinto
authors.initials: J
authors.lastname: Mata Vázquez
authors.forename: Manuel
authors.initials: M
authors.lastname: Maña López
mesh_terms: D003952:Diagnostic Imaging; D006801:Humans; D016247:Information Storage and Retrieval / Q000379:methods*; D046650:Medical Subject Headings; D012984:Software; D017432:Unified Medical Language System