C.21 Spatial Fiducials
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## C.21 Spatial Fiducials

### C.21.1 Spatial Fiducials Series Module

Table C.21.1-1 defines the general Attributes of the Spatial Fiducials Series Module.

Table C.21.1-1. Spatial Fiducials Series Module Attributes

Attribute Name | Tag | Type | Attribute Description
--- | --- | --- | ---
Modality | (0008,0060) | 1 | Modality type. Enumerated Values: FID

### C.21.2 Spatial Fiducials Module

Table C.21.2-1 defines the general Attributes of the Registration.

Table C.21.2-1. Spatial Fiducials Module Attributes

Attribute Name | Tag | Type | Attribute Description
--- | --- | --- | ---
Content Date | (0008,0023) | 1 | The date the content creation started.
Content Time | (0008,0033) | 1 | The time the content creation started.
Include Table 10-12 “Content Identification Macro Attributes” |
Fiducial Set Sequence | (0070,031C) | 1 | A sequence of items, each of which is a fiducial set. One or more Items shall be included in this sequence.
>Frame of Reference UID | (0020,0052) | 1C | Identifies a Frame of Reference that may or may not be an image set (e.g., an atlas or physical space). See Section C.7.4.1.1.1 for further explanation. Required if Referenced Image Sequence (0008,1140) is absent. May be present otherwise.
>Referenced Image Sequence | (0008,1140) | 1C | Identifies the set of images in which the fiducials are located. One or more Items shall be included in this sequence. Required if Frame of Reference UID (0020,0052) is absent. May be present otherwise. All referenced images shall have the same Frame of Reference UID if present in the images.
>>Include Table 10-3 “Image SOP Instance Reference Macro Attributes” |
>Fiducial Sequence | (0070,031E) | 1 | A sequence that specifies fiducials, one item per fiducial. One or more Items shall be included in this sequence.
>>Fiducial Identifier | (0070,0310) | 1 | A fiducial assignment identifier that is unique within this Fiducial Sequence item but may match the fiducial identifier of an equivalent feature in another item.
>>Fiducial Identifier Code Sequence | (0070,0311) | 1C | A code sequence for a term that identifies a well-known fiducial type (potentially including methodology, anatomy, tools, etc.). Only a single Item shall be included in this sequence. Required if Identifier (0070,0310) is absent. May be present otherwise.
>>>Include Table 8.8-1 “Code Sequence Macro Attributes” | Defined CID 7101 “Brain Atlas Fiducials”.
>>Fiducial UID | (0070,031A) | 3 | Globally unique identifier for the fiducial instance of this fiducial assignment.
>>Fiducial Description | (0070,030F) | 3 | User description or comments about the fiducial.
>>Shape Type | (0070,0306) | 1 | See Section C.21.2.1.1 for Defined Terms.
>>Number of Contour Points | (3006,0046) | 1C | Number of points (triplets) in Contour Data (3006,0050). Required if Contour Data is present.
>>Contour Data | (3006,0050) | 1C | Specifies the coordinates of this item's fiducial. One triplet (x,y,z) shall be present for each point in the fiducial. See Section C.21.2.1.2 for further explanation. Required if Frame of Reference UID (0020,0052) is present in this item of the Fiducial Set Sequence (0070,031C). Shall not be present otherwise. ### Note Contour Data may not be properly encoded if Explicit VR transfer syntax is used and the VL of this attribute exceeds 65534 bytes.
>>Contour Uncertainty Radius | (0070,0312) | 3 | The estimated uncertainty radius for the Contour Data in mm. See Section C.21.2.1.3
>>Graphic Coordinates Data Sequence | (0070,0318) | 1C | The image pixel locations of the fiducial's points. One or more Items shall be included in this sequence. More than one item shall be present only if a fiducial spans more than one image. Required if Contour Data is not present. May be present otherwise.
>>>Graphic Data | (0070,0022) | 1 | Graphic point coordinates of the fiducial points in the image of the Referenced Image Sequence. If Fiducial's Contour Data (3006,0050) is present, these points correlate to the points in the Contour Data, one row-column pair for each point and in the same order. See Section C.10.5.1.2 for further explanation.
>>>Referenced Image Sequence | (0008,1140) | 1 | A sequence that specifies the image containing the fiducial's graphic coordinates. Only a single Item shall be included in this sequence. Shall be an image within the set of the images in the Referenced Image Sequence (0008,1140) of the encapsulating Fiducial Set Sequence (0070,031C) item.
>>>>Include Table 10-3 “Image SOP Instance Reference Macro Attributes” |

#### C.21.2.1 Spatial Fiducials Module Attribute Descriptions

##### C.21.2.1.1 Shape Type

For convenient registration, correlated Fiducials exist in each image set of the Registration Sequence. Correlated Fiducials are identified with either Fiducial Identifier (0070,0310) or Fiducial Identifier Code Sequence (0070,0311).

Shape Type (0070,0306) defines the geometric interpretation of Contour Data (3006,0050) and Graphic Data (0070,0022). A point is defined as a triplet (x,y,z) in the case of spatial data or a pair (x,y) in the case of graphic data.

Defined Terms:

POINT

  a single point designating a single fiducial point.

  ### Note

  A point may be the epicenter of a more complex shape such as sphere.

LINE

  two points that specify a line or axis such as the inter-orbital line. The point locations have no significance other than identifying the line, i.e., they are not line segment end points.

PLANE

  three points that identify a plane such as the laterality plane

SURFACE

  three or more points (usually many) that reside on, or near, a region of a curved surface. The surface may be flat or curved, closed or open. The point order has no significance.

RULER

  two or more evenly spaced collinear points ordered sequentially along the line, such as a physical ruler placed in the imaging field.

L_SHAPE

  three points of two perpendicular line segments, AB and BC, having a common end point B. The order of the points is: ABC. May represent an L-shaped marker placed in the imaging field.

T_SHAPE

  three points of two perpendicular line segments AB and CD, such that C bisects AB. The order is ABD.

SHAPE

  three or more points that specify the shape of a well-known fiducial type. The term in the Fiducial Identifier Code Sequence (0070,0311) defines the shape and the order of the points that represent it.

##### C.21.2.1.2 Contour Data

Contour Data (3006,0050) is an ordered set of triplets that defines a shape. The triplets (x,y,z) denote points in the Reference Coordinate System of the Registration Instance.

### Note

Contours may associate observational data with a set of Image features or specify coordinates that are input data for a measurement.

##### C.21.2.1.3 Contour Uncertainty Radius

The uncertainty is an estimate of the standard deviation of the fiducial location process.

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