FFF.2.1.3.1 Rotational Acquisition

This section provides information on the encoding of the movement of the X-Ray Positioner during the acquisition of a rotational angiography.

The related image presentation parameters of the rotational acquisition that are defined in the Enhanced XA SOP Class, such as the mask information of subtracted display, are described in further sections of this annex.

FFF.2.1.3.1.1 User Scenario

The multi-frame image acquisition is performed during a continuous rotation of the X-Ray Positioner, starting from the initial incidence and acquiring frames in a given angular direction at variable angular steps and variable time intervals.

Typically such rotational acquisition is performed with the purpose of further 3D reconstruction. The rotation axis is not necessarily the patient head-feet direction, which may lead to images where the patient is not heads-up oriented.

There may be one or more rotations of the X-Ray Positioner during the same image acquisition, performed by following different patterns, such as:

  1. One rotation for non-subtracted angiography;

  2. Two rotations in the same or in opposite angular directions, for subtracted angiography;

  3. Several rotations at different time intervals for cardiac triggered acquisitions.

FFF.2.1.3.1.2 Encoding outline

The XA SOP Class encodes the absolute positioner angles as the sum of the angle of the first frame and the increments relative to the first frame. The Enhanced XA SOP Class encodes per-frame absolute angles.

In the XA SOP Class, the encoding of the angles is always with respect to the patient, so-called anatomical angles, and the image is assumed to be patient-oriented (i.e. heads-up display). In case of positioner rotation around an axis oblique to the patient, not aligned with the head-feet axis, it is not possible to encode the rotation of the image necessary for 3D reconstruction.

The Enhanced XA SOP Class encodes the positioner angles with respect to the patient as well as with respect to a fixed coordinate system of the equipment.

FFF.2.1.3.1.3 Encoding details

This section provides detailed recommendations of the key attributes to address this particular scenario.

Table FFF.2. 1-28 ENHANCED X-RAY ANGIOGRAPHIC IMAGE IOD MODULES

IE Module PS 3.3 Reference Usage
Image XA/XRF Acquisition C.8.19.3 Specifies the type of positioner.

Table FFF.2. 1-29 ENHANCED XA IMAGE FUNCTIONAL GROUP MACROS

Functional Group Macro PS 3.3 Reference Usage
X-Ray Positioner C.8.19.6.10 Specifies the anatomical angles per-frame.
X-Ray Isocenter Reference System C.8.19.6.13 Specifies the angles of the positioner per-frame in equipment coordinates for further applications based on the acquisition geometry (e.g., 3D reconstruction, registration…).

FFF.2.1.3.1.3.1 XA/XRF Acquisition Module Recommendations

The usage of this module is recommended to define the type of positioner.

Table FFF.2.1-30 XA/XRF ACQUISITION MODULE Example

Attribute Name Tag Comment
Positioner Type (0018,1508) The value CARM is used in this scenario.
C-arm Positioner Tabletop Relationship (0018,9474) Both values YES and NO are applicable to this scenario. Note: On mobile systems where this attribute equals NO, it is possible to do rotation and 3D reconstruction. In such case, the table is assumed to be static during the acquisition.

FFF.2.1.3.1.3.2 X-Ray Positioner Macro Recommendations

This macro is used in the per-frame context in this scenario.

Table FFF.2.1-31 X-RAY POSITIONER MACRO Example

Attribute Name Tag Comment
Positioner Position Sequence (0018,9405)
>Positioner Primary Angle (0018,1510) Angle with respect to the patient coordinate system.
>Positioner Secondary Angle (0018,1511) Angle with respect to the patient coordinate system.

FFF.2.1.3.1.3.3 X-Ray Isocenter Reference System Macro Recommendations

If the value of the C-arm Positioner Tabletop Relationship (0018,9474) is NO, the following macro may not be provided by the acquisition modality. This macro is used in the per-frame context in this scenario.

Table FFF.2.1-32 X-RAY ISOCENTER REFERENCE SYSTEM MACRO Example

Attribute Name Tag Comment
Isocenter Reference System Sequence (0018,9462)
>Positioner Isocenter Primary Angle (0018,9463) Angle with respect to the Isocenter coordinate system, independent of table angulations and how the patient is positioned on the table.
>Positioner Isocenter Secondary Angle (0018,9464) Angle with respect to the Isocenter coordinate system, independent of table angulations and how the patient is positioned on the table.
>Positioner Isocenter Detector Rotation Angle (0018,9465) Angle with respect to the Isocenter coordinate system, independent of table angulations and how the patient is positioned on the table.

FFF.2.1.3.1.4 Example

In this example, the patient is on the table, in position “Head First Prone”. The table horizontal, tilt and rotation angles are equal to zero.

The positioner performs a rotation of 180 deg from the left to the right side of the patient, with the image detector going above the back of the patient, around an axis parallel to the head-feet axis of the patient.

[pic]

Figure FFF.2.1-14Detector Trajectory during Rotational Acquisition

Below are the encoded values of the key attributes of this example:

Positioner Type (0018,1508) = CARM
C-arm Positioner Tabletop Relationship (0018,9474) = YES
Per-Frame Functional Groups Sequence (5200,9230)    
  Item 1     Frame 1
    >Positioner Position Sequence (0018,9405)    
      Item 1      
        >>Positioner Primary Angle (0018,1510)
      Item 1    
        >>Positioner Isocenter Primary Angle (0018,9463) =
  Item "N/2"   Frame "N/2"
    >Positioner Position Sequence (0018,9405)  
      Item 1    
        >>Positioner Primary Angle (0018,1510)
      Item 1    
        >>Positioner Isocenter Primary Angle (0018,9463) =
  Item "N"   Frame "N"
    >Positioner Position Sequence (0018,9405)  
      Item 1    
        >>Positioner Primary Angle (0018,1510)
      Item 1    
       
Image XA/XRF Acquisition C.8.19.3 Specifies the relationship between the table and the positioner.

Table FFF.2. 1-34 ENHANCED XA IMAGE FUNCTIONAL GROUP MACROS

Functional Group Macro PS 3.3 Reference Usage
X-Ray Table Position C.8.19.6.11 Specifies the table position per-frame in three dimensions.
X-Ray Isocenter Reference System C.8.19.6.13 Specifies the position and the angles of the table per-frame in equipment coordinates, for further applications based on the acquisition geometry (e.g., registration…).

FFF.2.1.3.2.3.1 XA/XRF Acquisition Module Recommendations

The usage of this module is recommended to specify the relationship between the table and the positioner.

Table FFF.2.1-35 XA/XRF ACQUISITION MODULE Example

Attribute Name Tag Comment
C-arm Positioner Tabletop Relationship (0018,9474) Both values YES and NO are applicable to this scenario. Note: On mobile systems where this attribute equals NO, it is possible to do table stepping. In such case, the system is not able to determine the absolute table position relative to the Isocenter, which is necessary for 2D-2D registration.

FFF.2.1.3.2.3.2 X-Ray Table Position Macro Recommendations

This macro is used in the per-frame context in this scenario.

Table FFF.2.1-36 X-RAY TABLE POSITION MACRO Example

Attribute Name Tag Comment
Table Position Sequence (0018,9406)
>Table Top Vertical Position (300A,0128) The same value for all frames.
>Table Top Longitudinal Position (300A,0129) The same value for all frames.
>Table Top Lateral Position (300A,012A) Different values per frame, corresponding to the “stepping” intervals in the table plane.
>Table Horizontal Rotation Angle (0018,9469) The same value for all frames.
>Table Head Tilt Angle (0018,9470) The same value for all frames.
>Table Cradle Tilt Angle (0018,9471) The same value for all frames.

FFF.2.1.3.2.3.3 X-Ray Isocenter Reference System Macro Recommendations

If the value of the C-arm Positioner Tabletop Relationship (0018,9474) is NO, the following macro may not be provided by the acquisition modality. This macro is used in the per-frame context in this scenario.

Table FFF.2.1-37 X-RAY ISOCENTER REFERENCE SYSTEM MACRO Example

Attribute Name Tag Comment
Isocenter Reference System Sequence (0018,9462)
>Table X Position to Isocenter (0018,9466) X-position of a fixed point in the table top, it changes per-frame if table horizontal rotation is not zero
>Table Y Position to Isocenter (0018,9467) Vertical position of a fixed point in the table top, it changes per-frame if table head tilt is not zero
>Table Z Position to Isocenter (0018,9468) Z-position of a fixed point in the table top, it changes per-frame
>Table Horizontal Rotation Angle (0018,9469) The same value for all frames.
>Table Head Tilt Angle (0018,9470) The same value for all frames.
>Table Cradle Tilt Angle (0018,9471) The same value for all frames.

FFF.2.1.3.2.4 Example

In this example, the patient is on the table in position “Head First Supine”. The table is tilted of -10 degrees, with the head of the patient below the feet, and the image detector is parallel to the tabletop plane. The table cradle and rotation angles are equal to zero.

The image acquisition is performed during a movement of the X-Ray Table in the tabletop plane, at constant speed and of one meter of distance, acquiring frames from the abdomen to the feet of the patient in one stepping movement for non-subtracted angiography.

The table is related to the C-arm positioner so that the coordinates of the table position are known in the isocenter reference system. This allows determining the projection magnification of the table top plane with respect to the detector plane.

[pic]

Figure FFF.2.1-16Table Trajectory during T able Stepping

[pic]

Figure FFF.2.1-17Example of table positions per-frame during table stepping

Below are the encoded values of the key attributes of this example:

Positioner Type (0018,1508) = CARM
C-arm Positioner Tabletop Relationship (0018,9474) = YES
Per-Frame Functional Groups Sequence (5200,9230)  
  Item 1   Frame 1
    >Table Position Sequence (0018,9406)  
      Item 1    
        >>Table Top Vertical Position (300A,0128)
      Item 1    
        >>Table X Position to Isocenter (0018,9466)
  Item "N"   Frame "N"
    >Table Position Sequence (0018,9406)  
      Item 1    
        >>Table Top Vertical Position (300A,0128)
      Item 1    
     
X-Ray Exposure Control Sensing Regions C.8.19.6.3 Specifies the shape and size of the sensing regions in pixels, as well as their position relative to the top left pixel of the image.

FFF.2.1.4.1.3.1 X-Ray Exposure Control Sensing Regions Macro Recommendations

This macro is recommended to encode details about sensing regions.

If the position of the sensing regions is fixed during the multi-frame acquisition, the usage of this macro is shared.

If the position of the sensing regions was changed during the multi-frame acquisition, this macro is encoded per-frame to reflect the individual positions.

The same number of regions is typically used for all the frames of the image. However it is technically possible to activate or deactivate some of the regions during a given range of frames, in which case this macro is encoded per-frame.

Table FFF.2.1-39 X-RAY EXPOSURE CONTROL SENSING REGIONS MACRO Recommendations

Attribute Name Tag Comment
Exposure Control Sensing Regions Sequence (0018,9434) As many items as number of regions.

FFF.2.1.4.1.4 Example

In this section, two examples are given.

The first example shows how three sensing regions are encoded: 1) central (circular), 2) left (rectangular) and 3) right (rectangular).

[pic]

Figure FFF.2.1-19Example of X-Ray Exposure Control Sensing Regions inside the Pixel Data matrix

Below are the encoded values of the key attributes of this example:

Shared Functional Group Sequence (5200,9229)
Item 1 All frames
Other functional groups
>Exposure Control Sensing Regions Sequence (0018,9434)
Item 1

Figure FFF.2.1-20Attributes of first the example of the X-Ray Exposure Control Sensing Regions

The second example shows the same regions, but the field of view region encoded in the Pixel Data matrix has been shifted of 240 pixels right and 310 pixels down, thus the left rectangular sensing region is outside the Pixel Data matrix as well as both rectangular regions overlap the top row of the image matrix.

[pic]

Figure FFF.2.1-21Example of X-Ray Exposure Control Sensing Regions partially outside the Pixel Data matrix

Below are the encoded values of the key attributes of this example:

Shared Functional Group Sequence (5200,9229)
Item 1 All frames
Other functional groups
>Exposure Control Sensing Regions Sequence (0018,9434)
Item 1

Figure FFF.2.1-22Attributes of the second example of the X-Ray Exposure Control Sensing Regions