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.

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Figure 5.1-1 ENTITY CONVENTION

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.

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Figure 5.1-2RELATIONSHIP CONVENTION

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:

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

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

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

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

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

f. (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 which 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.

5.2 Sequences

Certain Tables in this Standard describe Sequences of Items by using the symbol: '>'. The symbol '>' precedes the Attribute (or Module) Name of the members of an Item. All marked Attributes (or Modules) belong to the generic description of an Item which may be repeated to form a Sequence of Items. This Sequence of Items is nested in the Attribute (or Module) which precedes in the table the first member marked with a '>'.

Note: The following table describes the "Referenced Series Sequences" Attribute as a Sequence of one or more Items where each Item contains the three Attributes marked by a '>'. The Sequence of Items is nested inside the value of the Referenced Series Sequence Attribute. The following Attribute (not marked) is not part of the Items of the Sequence.

...... .....
Referenced Series Sequence .....
>Series Date .....
>Series Time .....
>Series Instance UID .....
Modality ....

This notation may be used to create nested hierarchical structures by using '>>' at the second level of nesting and so on.

The Type of the Sequence attribute defines whether the Sequence attribute itself must be present, and the Attribute Description of the Sequence attribute may define whether and how many Items shall be present in the Sequence. The Types of the attributes of the Data Set included in the Sequence, including any conditionality, are specified within the scope of each Data Set, i.e., for each Item present in the Sequence. See PS 3.5.

For describing the number of items in the attribute description the following sentences are preferred:

Sequence Attribute Type Number of Items Sentence
1 or 1C 1 Only a single Item shall be included in this sequence.
1 or 1C 1-n One or more Items shall be included in this sequence.
2 or 2C 0-1 Zero or one Item shall be included in this sequence.
2 or 2C 0-n Zero or more Items shall be included in this sequence.
3 1 Only a single Item is permitted in this sequence.
3 1-n One or more Items are permitted in this sequence.

Note: The encoding of empty sequence attributes is decribed in PS 3.5.

In a number of cases for Normalized IODs, the Data Element Type and Conditions are defined in the appropriate Service definition in PS 3.4, in other cases in the attribute description in PS 3.3.It is not necessary to specify for any Attribute within a Sequence the condition that it is “required if a Sequence Item is present”, since this is always implicit, whether or not there are additional requirements.

5.3 Triplet Encoding of Structured Data (Retired)

This section has been retired. See Section 8.

5.4 Attribute Macros

Some tables contain references to Attribute Macros. This convention is used in cases where the same Attributes are used in multiple tables or multiple places in one Module. The reference means that the Attributes of the Attribute Macro shall be included in the Module in place of the row that contains the reference to the Attribute Macro.

In some cases, the Attribute Macro is used in a Sequence (the VR of the Data Element in which the Attribute is encoded is SQ, see PS 3.5). When this is done, the reference is preceded by one or more ">" characters. The number of ">" characters indicates the level in the sequence that all of the Attributes in the Attribute Macro occupy.

There may be specialization of the description of the Attributes in the Attribute Macro. In these cases, this specialization is described in the Description column of the Module.

Following is an example of this convention.

Table 5.4-1 is an example of a Module table using the Attribute Macro convention.

Table 5.4-1Example Module Table

Attribute Name Tag Type Attribute Description
Attribute A (aaaa,aaaa) 1 This is an example.
Attribute B Sequence (bbbb,bbbb) 1 This is an example of a Sequence Attribute
>Include 'Example Macro' Table 5.4-2 In this Module, Attribute D (dddd,dddd) is Type 1

Table 5.4-2 is an example of the Attribute Macro referenced in Table 5.4-1.

Table 5.4-2Example Macro

Attribute Name Tag Type Attribute Description
Attribute C (cccc,cccc) 1 This is an example.
Attribute D (dddd,dddd) 3 This Attribute is generally a Type 3.

The contents of the Example Module Table, if it had not been described with the Example Macro would have been as shown in Table 5.4-3

Table 5.4-3 Example Module Table without the Use of an Attribute Macro

Attribute Name Tag Type Attribute Description
Attribute A (aaaa,aaaa) 1 This is an example.
Attribute B Sequence (bbbb,bbbb) 1 This is an example of a Sequence Attribute.
>Attribute C (cccc,cccc) 1 This is an example.
>Attribute D (dddd,dddd) 1 In this Module, this Attribute has been specialized to Type 1 as indicated in Table 5.4-1.

5.5 Types and Conditions in Normalized IODs

When a Normalized Information Object Definition in PS 3.3 invokes Modules (e.g., the SOP Common Module) or Attribute Macros that are specified with Data Element Types, those specified Data Element Types and Conditions do not apply. Rather, the Data Element Types and Conditions have to be specified for each Attribute for both SCU and SCP in the appropriate Service definition in PS3.4.