# 5 Conventions

## 5.1 Entity-Relationship Model

### 5.1.1 Entity

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 an IOD or Module). An entity is depicted as shown in Figure 5.1-1.

### 5.1.2 Relationship

A relationship, which defines how entities are related, is depicted as a diamond within this Part of the DICOM Standard as shown in Figure 5.1-2.

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:

1. (a = 1, b = 1) - one source entity is related to one destination entity

2. (a = 1, b = 0-n) - one source entity is related to zero or more destination entities

3. (a = 1, b = 1-n) - one source entity is related to one or more destination entities

4. (a = 1-n, b = 1) - one or more source entities are related to one destination entity

5. (a = 1-n, b = 0-n) - one or more source entities are related to zero or more destination entities

6. (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.