Ontology is the study of the nature of being, existence or reality as such, as well as the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology deals with questions concerning what entities exist or can be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences.
There are many ontologies in existence (http://bioportal.bioontology.org/ontologies), but in 2006 the Radiological Society of North America (RSNA) started to create an ontology related specifically to radiology, RadLex.
In radiology, ontological systems help to determine how "entities" are related to each other. For example:
- Lung cancer:
- is a "Cancer"
- is a "Disorder of the Lung"
- has subtype "Small cell lung cancer"
- has subtype "Non small cell lung cancer"
RadLex doesn't just list pathological entities, but also procedural, systems and reporting entities.
An other useful ontology is the 'Human phenotype ontology' (http://bioportal.bioontology.org/visualize/45159/) which aims to list disease and how diseases relate to each other in a semi-hierarchy.