Domain specific ontologies can be used to improve both precision and recall of information retrieval systems. One approach in this regard is using query expansion techniques and the other would be introducing a semantic similarity measure for concepts in ontology. Although each approach has its own benefits and drawbacks, query expansion techniques are preferred when the corpus volume is so huge that examining concept pairs between query and documents is not reasonable. In this paper a semantic query expansion algorithm for medical information retrieval is introduced. Proposed approach consists of identifying MeSH (Medical Subject Headings) concepts in user's query and applying expansion algorithm to them. Expansion algorithm is based on the location of concepts in MeSH hierarchy, number of synonyms of each concept and number of terms the concept is made of. Results show improvements over classic method, query expansion using general purpose ontology and a number of other approaches.
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