DICOM PS3.3 2018b - Information Object Definitions

C.34.9 Defined CT Acquisition Module
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### C.34.9 Defined CT Acquisition Module

Table C.34.9-1 contains a specification of acceptable values and ranges of acquisition parameters for an imaging procedure.

Table C.34.9-1. Defined CT Acquisition Module Attributes

Attribute Name | Tag | Type | Attribute Description
--- | --- | --- | ---
Acquisition Protocol Element Specification Sequence | (0018,991F) | 1 | Specification of the acquisition parameters for acquisition protocol elements in an imaging procedure. There shall be one Item in this Sequence for each Acquisition Protocol Element in the Protocol. See Section C.34.9.1. One or more Items shall be included in this Sequence.
>Protocol Element Number | (0018,9921) | 1 | The Protocol Element Number of the Acquisition Protocol Element being specified in this Item.
>Parameters Specification Sequence | (0018,9913) | 3 | Constraints on one or more acquisition parameters. One or more Items are permitted in this Sequence.
>>Include Table 10.25-1 “Attribute Value Constraint Macro Attributes” | Only Attributes defined in Table C.34.10-1 (i.e., in the Acquisition Protocol Element Sequence (0018,9920) in the Performed CT Acquisition Module) and Private Data Elements associated with this acquisition protocol element may be specified as Selector Attributes. The semantics of values of Constraint Violation Significance (0082,0036) in the Macro are assigned in Section C.34.9.3. The same Attribute shall not appear in more than one Item in the Sequence with the same values for Selector Sequence Pointer (0072,0052) and Selector Sequence Pointer Items (0074,1057).
>>Modifiable Constraint Flag | (0082,0038) | 1C | Specifies whether this constraint may be encoded in a derived instance with a different value. See Section C.34.9.4. Enumerated Values: YES The constraint may be modified. NO The constraint may not be modified. Required if the constraint may not be modified, may be present otherwise.

### Note

The Performed CT Acquisition Module in the CT Performed Procedure Protocol will generally be "fully populated". The Defined CT Acquisition Module in the CT Defined Procedure Protocol Object may be "sparsely populated" (i.e., contains only the Attributes the system that is specifying the protocol "cares about").

Attributes that might commonly be specified here include:

- KVP (0018,0060)

- X-Ray Tube Current in mA (0018,9330)

- Revolution Time (0018,9305)

- Single Collimation Width (0018,9306)

- Total Collimation Width (0018,9307)

- Spiral Pitch Factor (0018,9311)

- Exposure Modulation Type (0018,9323)

- CTDIvol Notification Trigger (0018,9942) (See Section C.34.9.2)

- DLP Notification Trigger (0018,9943)

- CTDIvol (0018,9345)

#### C.34.9.1 Acquisition Protocol Elements

A CT Protocol usually includes more than one Acquisition Protocol Element. For example, a chest Protocol might include three elements in total: two localizer CT radiographs (AP and Lateral), and a single helical scan.

Since all parameters will be nested inside an Acquisition Protocol Element Sequence (0018,9920) and some of the parameters will be further nested inside a CT X-Ray Details Sequence (0018,9325), close attention must be paid to the use of the Selector Sequence Pointer (0072,0052) in the Attribute Value Constraint Macro. Illustrative examples are provided in Table C.34.9-2.

Table C.34.9-2. Example Usage of Selector Macro Attributes for Acquisition Constraints

Example | Selector Attribute (0072,0026) | Selector Value Number (0072,0028) | Selector Sequence Pointer (0072,0052) | Selector Sequence Pointer Items (0074,1057) | Example Constraint
--- | --- | --- | --- | --- | ---
Constrain the value of Element Name (0018,9922) of the first Item in the Acquisition Protocol Element Sequence (0018,9920) | (0018,9922) | 1 | (0018,9920) | 1 | EQUAL"Localizer (AP) "
Constrain the value of Table Speed (0018,9309) of the second Item in the Acquisition Protocol Element Sequence (0018,9920) | (0018,9309) | 1 | (0018,9920) | 2 | EQUAL14 mm/sec
Constrain the value of KVP (0018,0060) of the first beam in the CT X-Ray Details Sequence (0018,9325) of the second Item in the Acquisition Protocol Element Sequence (0018,9920) | (0018,0060) | 1 | (0018,9920), (0018,9325) | 2\1 | RANGE_INCL(120,140)
Constrain the first and second value of Exposure Modulation Type (0018,9323) of the second beam in the CT X-Ray Details Sequence (0018,9325) of the third Item in the Acquisition Protocol Element Sequence (0018,9920) | (0018,9323) | 1 | (0018,9920), (0018,9325) | 3\2 | EQUAL"ANGULAR"
(0018,9323) | 2 | (0018,9920), (0018,9325) | 3\2 | EQUAL"ORGAN_BASED"

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DICOM PS3.3 2018b - Information Object Definitions