An entity is used in an Entity-Relationship (E-R) model to represent a Real-World Object, class of Real-World Objects, or DICOM data representation (such as IOD or Module). An entity is depicted as a box within this Part of the DICOM Standard as shown in Figure 5-1.
A relationship, which defines how entities are related, is depicted as a diamond within this Standard as shown in Figure 5-2.
The relationship is read from source to destination entity as indicated by the arrows. The a and b show the source and destination cardinality of the relationship respectively. The following cardinalities are permitted:
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(a = 1, b = 1) -one source entity is related to one destination entity
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(a = 1, b = 0-n) -one source entity is related to zero or more destination entities
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(a = 1, b = 1-n) -one source entity is related to one or more destination entities
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(a = 1-n, b = 1) -one or more source entities are related to one destination entity
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(a = 1-n, b = 0-n) -one or more source entities are related to zero or more destination entities
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(a = 1-n, b = 1-n) -one or more source entities are related to one or more destination entities
In a relationship where (a = 1-n, b = 1-n) the values of the source and destination cardinalities may be different. The value "n" simply denotes one or more.
Note
DICOM has added the use of arrows to the E-R diagramming conventions often used in other literature. This has been done to avoid the possibility of inferring an incorrect relationship that can result from reading a relationship in the reverse order of that intended. For example, a relationship "Cat Catches Mouse" could be read "Mouse Catches Cat" if the arrows were not present.
A relationship may be bi-directional (i.e., the relationship is true in both directions). In such a case, the convention used is arrows pointing toward both the source and the destination entities.