C.8.19.6.13 X-Ray Isocenter Reference System Macro
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##### C.8.19.6.13 X-Ray Isocenter Reference System Macro

Table C.8.19.6-13 specifies the attributes of the X-Ray Isocenter Reference System Functional Group Macro.

Table C.8.19.6-13. X-Ray Isocenter Reference System Macro Attributes

Attribute Name | Tag | Type | Attribute Description
--- | --- | --- | ---
Isocenter Reference System Sequence | (0018,9462) | 1 | A sequence that describes the Isocenter Reference Coordinate System (O, X, Y, Z). Only a single Item shall be included in this sequence.
>Positioner Isocenter Primary Angle | (0018,9463) | 1 | Position of the X-Ray center beam in the isocenter reference system in the X direction (deg). See Section C.8.19.6.13.1.2 for further explanation.
>Positioner Isocenter Secondary Angle | (0018,9464) | 1 | Position of the X-Ray center beam in the isocenter reference system in the Z direction (deg). See Section C.8.19.6.13.1.2 for further explanation.
>Positioner Isocenter Detector Rotation Angle | (0018,9465) | 1 | Rotation of the X-Ray detector plane (deg). See Section C.8.19.6.13.1.2 for further explanation.
>Table X Position to Isocenter | (0018,9466) | 1 | X position of the Table Reference Point with respect to the Isocenter (mm). See Section C.8.19.6.13.1.3 for further explanation.
>Table Y Position to Isocenter | (0018,9467) | 1 | Y position of the Table Reference Point with respect to the Isocenter (mm). See Section C.8.19.6.13.1.3 for further explanation.
>Table Z Position to Isocenter | (0018,9468) | 1 | Z position of the Table Reference Point with respect to the Isocenter (mm). See Section C.8.19.6.13.1.3 for further explanation.
>Table Horizontal Rotation Angle | (0018,9469) | 1 | Rotation of the table in the horizontal plane. See Section C.8.19.6.13.1.3 for further explanation.
>Table Head Tilt Angle | (0018,9470) | 1 | Angle of the head-feet axis of the table in degrees relative to the horizontal plane. See Section C.8.19.6.13.1.3 for further explanation.
>Table Cradle Tilt Angle | (0018,9471) | 1 | Angle of the left-right axis of the table in degrees relative to the horizontal plane. See Section C.8.19.6.13.1.3 for further explanation.

###### C.8.19.6.13.1 Isocenter Reference System Attribute Description

The Isocenter Reference System Attributes describe the 3D geometry of the X-Ray equipment composed by the X-Ray positioner and the X-Ray table.

These attributes define three coordinate systems in the 3D space:

- Isocenter coordinate system

- Positioner coordinate system

- Table coordinate system

The Isocenter Reference System attributes describe the relationship between the 3D coordinates of a point in the table coordinate system and the 3D coordinates of such point in the positioner coordinate system (both systems moving in the equipment), by using the Isocenter coordinate system that is fixed in the equipment.

### Note

Annex FFF “Enhanced XA/XRF Encoding Examples (Informative)” in PS3.17 describes the transformations necessary to transpose between coordinate systems.

###### C.8.19.6.13.1.1 Isocenter Coordinate System

The Isocenter coordinate system (O,X,Y,Z) of the equipment is defined as follows:

- Origin O is on the System Isocenter

- +Y DOWNWARD (gravity)

- +X, +Z directions in the horizontal plane (gravity plane). Directions arbitrarily defined by the manufacturer

Figure C.8.19.6-4. Isocenter Coordinate System

###### C.8.19.6.13.1.2 Positioner Coordinate System

The Positioner Coordinate System (Op, Xp, Yp, Zp) is defined as follows:

- Origin Op, is the origin of the Isocenter coordinate system O

- Xp axis is parallel to the horizontal scan-lines of the detector (rows). Positive direction from left to right of the detector plane looking towards the source.

- Zp axis is parallel to the vertical scan-lines of the detector (columns). Positive direction from bottom to top of the detector plane looking towards the source.

- Yp is the axis from the isocenter to the X-Ray source. Positive direction from the Isocenter to the X-Ray Source. This axis is so-called the X-Ray center beam.

Figure C.8.19.6-5. Positioner Coordinate System

### Note

The quantities SID and ISO are specified by the attributes Distance Source to Detector (0018,1110) and Distance Source to Isocenter (0018,9402) respectively.

The Positioner Coordinate System (Op, Xp, Yp, Zp) is characterized, with respect to the Isocenter Coordinate System (O, X, Y, Z), by two angles describing the X-Ray center beam, and a third angle describing the rotation of the X-Ray detector plane. These angles are relative to the Isocenter reference system, and independent from the patient position on the equipment.

Positioner Isocenter Primary Angle (0018,9463) (so-called Ap1 in Figure C.8.19.6-6) is defined in the plane XY, as the angle between the plane YZ and the plane YpZ. The axis of rotation of this angle is the Z axis. Angle from -Y to +X is positive. The valid range of this angle is -180 to +180 degrees.

Positioner Isocenter Secondary Angle (0018,9464) (so-called Ap2 in Figure C.8.19.6-6) is defined in the plane YpZ, as the angle of the X-Ray Center Beam (i.e., Yp) relative to the XY plane. The axis of rotation of this angle is perpendicular to the plane YpZ. Angle from the plane XY to +Z is positive. The valid range of this angle is -180 to +180 degrees.

Positioner Isocenter Detector Rotation Angle (0018,9465) (so-called Ap3 in Figure C.8.19.6-6 and in Figure C.8.19.6-7) is defined in the detector plane, as the angle of the vertical scan-lines of the detector (i.e., Zp) relative to the intersection between the detector plane and the plane YpZ. The sign of this angle is positive clockwise when facing on to the detector plane (see Figure C.8.19.6-7). The valid range of this angle is -180 to +180 degrees.

Figure C.8.19.6-6. Positioner Isocenter Angles

Figure C.8.19.6-7. Positioner Isocenter Detector Rotation Angle when Ap1 = 0 and Ap2 = 0

###### C.8.19.6.13.1.3 Table Coordinate System

The table coordinate system (Ot, Xt, Yt, Zt) is defined as follows:

- Origin Ot, so-called Table Reference Point, is on the Table Top plane

- +Xt direction to the TABLE LEFT

- +Zt direction to the TABLE HEAD

- +Yt direction to the TABLE DOWN

The table coordinate system (Ot, Xt, Yt, Zt) is characterized, with respect to the Isocenter coordinate system (O, X, Y, Z), by a 3D translation and 3 angles describing the tilting and rotation:

Table X Position to Isocenter (0018,9466) (so-called TX in Figure C.8.19.6-8) is defined as the translation of the Table Reference Point Ot with respect to the Isocenter Coordinate System in the X direction. Table motion toward +X is positive.

Table Y Position to Isocenter (0018,9467) (so-called TY in Figure C.8.19.6-8) is defined as the translation of the Table Reference Point Ot with respect to the Isocenter Coordinate System in the Y direction. Table motion toward +Y is positive.

Table Z Position to Isocenter (0018,9468) (so-called TZ in Figure C.8.19.6-8) is defined as the translation of the Table Reference Point Ot with respect to the Isocenter Coordinate System in the Z direction. Table motion toward +Z is positive.

### Note

A translation of ( TX,TY,TZ ) = (0, 0, 0) means that the Table Reference Point Ot is at the System Isocenter.

Table Horizontal Rotation Angle (so-called At1 in Figure C.8.19.6-9) is defined in the horizontal plane XZ, as the angle of the projection of the +Zt axis in the XZ plane relative to the +Z axis. The axis of rotation of this angle is the vertical axis crossing the Table Reference Point Ot. Zero value is defined when the projection of +Zt in the XZ plane is equal to +Z. Angle from +Z to +X is positive. The valid range of this angle is -180 to +180 degrees.

Table Head Tilt Angle (so-called At2 in Figure C.8.19.6-9) is defined in the vertical plane containing Zt (i.e., YZt), as the angle of the +Zt axis relative to the horizontal plane XZ. The axis of rotation of this angle is defined as the intersection between the horizontal plane XZ and the plane XtYt. Zero value is defined when +Zt is contained in the horizontal plane XZ. Angle from horizontal (plane XZ) to -Y direction (upwards) is positive,indicating that the head of the table is above the horizontal plane. The valid range of this angle is -45 to +45 degrees.

Table Cradle Tilt Angle (so-called At3 in Figure C.8.19.6-9) is defined in the XtYt plane, as the angle of the +Xt axis relative to the intersection between the XtYt plane and the horizontal plane XZ. The axis of rotation of this angle is the axis Zt. Zero value is defined when +Xt is contained in the horizontal plane XZ. Angle from horizontal (plane XZ) to -Y direction (upwards) is positive,indicating that the left of the table is above the horizontal plane. The valid range of this angle is -45 to +45 degrees.

### Note

The angles At1 , At2 and At3 are independent from any specific mechanical design of the table rotation axis defined by a manufacturer. In particular, they don't require the three rotation axis to cross on a single point. If a mechanical rotation axis does not cross the Table Reference Point Ot, a mechanical rotation around this axis will generate a change in one or more table angles as well as a translation of the Table Reference Point.

Figure C.8.19.6-8. Table Translation with respect to the Isocenter Reference System

Figure C.8.19.6-9. Table Angulations with respect to the Isocenter Reference System

###### C.8.19.6.13.2 Relationship Patient Coordinate System

The Isocenter Reference System attributes allow expressing the positioner angulations (i.e., X-Ray Center Beam direction) as a vector in the table coordinate system. If the relationship between the X-Ray table and the patient is known, it is possible to express any vector of the table coordinate system as a direction in the patient.

Therefore, the Isocenter Reference System attributes allow calculating the positioner angulations in the patient-based coordinate system if the following attributes are present:

- Patient Orientation Code Sequence (0054,0410)

- Patient Orientation Modifier Code Sequence (0054,0412)

Further, the Isocenter Reference System attributes allow calculating the patient anatomical directions (i.e., left, right, head, feet, anterior, posterior) of the rows and columns of the stored image, if the following attributes are present:

- Patient Orientation Code Sequence (0054,0410)

- Patient Orientation Modifier Code Sequence (0054,0412)

- Field of View Rotation (0018,7032)

- Field of View Horizontal Flip (0018,7034)

For registration purposes, a given point fixed in the patient (object of interest) that is defined in the table coordinate system can be expressed as row and column coordinates of the stored image if the relationship between the positioner coordinate system and the stored image is fully characterized. Therefore, the Isocenter Reference System attributes allow calculating the projection of a point of the patient as row and column coordinates of the stored image, if the following attributes are present:

- Frame of Reference UID (0020,0052) and must be equal for all images involved in the registration

- Field of View Rotation (0018,7032)

- Field of View Horizontal Flip (0018,7034)

- Imager Pixel Spacing (0018,1164)

- Distance Source to Isocenter (0018,9402)

- Distance Source to Detector (0018,1110)

In addition for a system equipped with a digit al detector the following attributes need to be present:

- Detector Element Spacing (0018,7022)

- Field of view Origin (0018,7030)

- Position of Isocenter Projection (0018,9430)

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