http://www.nlm.nih.gov/factsheets.dir/semantic_net.html (Einblicke ins Internet, 10/1995)

UMLS® Semantic Network

The UMLS Semantic Network is one of four knowledge sources under development by the National Library of Medicine as part of the Unified Medical Language System® (UMLS) project. The purpose of the Semantic Network is to provide a consistent categorization of all concepts represented in the UMLS Metathesaurus® and to provide a set of useful relations between these concepts. All information about specific concepts is found in the Metathesaurus; the Network provides information about the set of basic semantic types, or categories, which may be assigned to these concepts.

The semantic types are the nodes in the Network, and the relations between them are the links. The primary link is the `isa' link. This establishes the hierarchy of types within the Network and is used for deciding on the most specific semantic type available for assignment to a Metathesaurus concept. In addition, a set of potentially useful non-hierarchical relations between the types has been identified.

The Semantic Network serves as an authority for the semantic types that are assigned to concepts in the Metathesaurus and that are assigned to databases in the Information Sources Map (ISM). The Network defines these types, both with textual descriptions and by means of the information inherent in its hierarchies. It defines the set of relationships that hold between high level semantic types and, in conjunction with the other UMLS knowledge sources, makes these available for the purpose of enhancing existing information retrieval methods.

Structure and Content of the Semantic Network

The 1994 version of the Semantic Network includes 133 semantic types. There are major groupings for organisms, anatomical structures, biologic function, chemicals, events, physical objects, and concepts or ideas. An important aspect of the Semantic Network is that the level of granularity varies across the Network. The intent is to establish a set of semantic types which will be useful for a variety of tasks, without introducing undue complexity. We expect that in subsequent versions, certain areas will continue to be modified and expanded based on actual use of the Network in research applications. The current scope of the UMLS semantic types is quite broad, allowing for the semantic categorization of a wide range of terminology in multiple domains.

In addition to the `isa' link, 49 other relations which link pairs of semantic types are included. These are grouped into five major categories, which are themselves relations: `physically related to,' spatially related to,' `temporally related to,' `functionally related to,' and `conceptually related to.'

The relations are stated between high level nodes in the Network whenever possible and are generally inherited via the `isa' link by all the children of those nodes. Thus, for example, the relation `process of' is stated to hold between the semantic types `Biologic Function' and Organism.' Therefore, it also holds between `Organ or Tissue Function' (which is a `Physiologic Function,' which is, in turn, a `Biologic Function') and `Animal' (which is an `Organism'). Tables containing the fully inherited set of non-hierarchical links have been provided with the 1994 release of the Network.

The relations are stated between semantic types and do not necessarily apply to all instances of concepts that have been assigned to those semantic types. That is, the relation may or may not hold between any particular pair of concepts. So, though the relation `evaluation of' holds between the semantic types `Sign' and `Organism Attribute,' a particular sign or a particular attribute may not be linked by this relation. Thus, signs such as "overweight" and "fever" are evaluations of the organism attributes "body weight" and "body temperature," respectively. However, "overweight" is not an evaluation of "body temperature," and "fever" is not an evaluation of "body weight."

In some cases there will be a conflict between the placement of types in the Network and the link to be inherited. If this is so, the inheritance of the link is said to be blocked. For example, by inheritance the type `Mental Process' would be `process of' `Plant.' Since plants are not sentient beings, this link is explicitly blocked. In other cases the nature of the relation is such that it should not be inherited by the children of the types that it links.

In that case, the relation is defined for the two semantic types it explicitly links, but blocked for all the children of those types. For example, `conceptual part of' links `Body System' and `Fully Formed Anatomical Structure,' but it should not link `Body System' to all the children of `Fully Formed Anatomical Structure,' such as "Cell" or "Tissue."

The relations provided in the Network do not represent all possible relations between semantic types. Rather, they are intended as an initial step in the development of a set which will be useful for retrieval of information from a variety of computerized biomedical databases.

Several portions of the MeSHr hierarchy have been labelled with child to parent semantic relationships. All of the Anatomy, Diseases, and Psychiatry and Psychology sections have been labelled, as well as a portion of the Biological Sciences section. The `isa' relation and all non-hierarchical relations in the Semantic Network are valid for use in this labelling.

The links that are expressed between MeSH terms are, with a few exceptions, reflected in the Semantic Network. That is, if two MeSH terms are linked by a certain relation, then that link is expressed in the Network as a link between the semantic types that have been assigned to those MeSH terms. For example, "Amniotic Fluid," which is a `Body Substance,' is a child of "Embryo," which is an `Embryonic Structure.' The labelled relationship is `surrounds.' Therefore, the link "Body Substance surrounds Embryonic Structure" is represented in the Network.

Assigning Semantic Types

Each concept in the Metathesaurus has been assigned at least one semantic type to reflect its meaning. The notion of "meaning" is relative, first to the scope and granularity of the Network itself. For example, the concept "Macaca" receives the semantic type ` Mammal' because there is not a more specific type `Primate' available in the Network. In all cases, the most specific semantic type available in the hierarchy is assigned to the concept. The variable granularity of the semantic types has important implications for interpreting the meaning (i.e., semantic type) that has been assigned to a Metathesaurus concept. For example, a sub-tree under the node `Physical Object' is `Manufactured Object.' It has only two child nodes, `Medical Device' and `Research Device.'

It is obvious that there are manufactured objects other than medical devices and research devices. Rather than proliferate the number of semantic types to encompass multiple additional subcategories for these objects, concepts that are neither medical devices nor research devices are simply assigned the more general semantic type `Manufactured Object.'

Second, the notion of meaning is relative to the source vocabulary. The Metathesaurus consists of terms from its controlled source vocabularies. he meaning of each term is defined by its source, explicitly by definition or annotation; by context (its place in a hierarchy); by synonyms and other stated relationships between terms; and also by its usage in description, classification, or indexing.

Distribution Formats

The UMLS Semantic Network is provided in two formats; a relational table format and a unit record format. The information associated with each semantic type includes a unique identifier, a tree number indicating its position in an `isa' hierarchy, a definition, and its immediate parent and children. The information associated with each relation includes a unique identifier, a tree number, a definition, and the set of semantic types that can plausibly be linked by this relation. A Macintosh application for viewing and browsing the Semantic Network is also provided.

Application Procedures

Those who wish to obtain copies of UMLS products are required to sign a one-year experimental agreement with the NLM. Sample records, documentation, and copies of the experimental agreement, are available from the NLM anonymous ftp file service at nlmpubs.nlm.nih.gov (UMLS files and documents are located in the /nlmpubs/umls directory) or write to:

Betsy L. Humphreys
UMLS Project Officer
National Library of Medicine
8600 Rockville Pike
Bethesda, MD 20894
FAX 301/496-4450

Other fact sheets in the UMLS series:

Printed Copies are available from:

Office of Public Information
National Library of Medicine
8600 Rockville Pike
Bethesda, Maryland 20894

NLM HyperDOC / UMLS Semantic Network / June 1994