DICOM PS3.3 2018b - Information Object Definitions

C.8.31.6 Breast X-Ray Isocenter Reference System Macro
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#### C.8.31.6 Breast X-Ray Isocenter Reference System Macro

Table C.8.31.6-1 specifies the Attributes of the Breast X-Ray Isocenter Reference System Functional Group Macro.

Table C.8.31.6-1. Breast 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.
>X-Ray Source Isocenter Primary Angle | (0018,9543) | 1 | Primary angle of the X-Ray source in the isocenter reference system (deg). See C.8.31.6.1.2 for further explanation.
>X-Ray Source Isocenter Secondary Angle | (0018,9544) | 1 | Secondary angle of the X-Ray source in the isocenter reference system (deg). See C.8.31.6.1.2 for further explanation.
>Breast Support Isocenter Primary Angle | (0018,9545) | 1 | Primary angle of the Breast Support in the isocenter reference system (deg). See C.8.31.6.1.3 for further explanation.
>Breast Support Isocenter Secondary Angle | (0018,9546) | 1 | Secondary angle of the Breast Support in the isocenter reference system (deg). See C.8.31.6.1.3 for further explanation.
>Breast Support X Position to Isocenter | (0018,9547) | 1C | X position of the Breast Support Reference Point with respect to the Isocenter (mm). See C.8.31.6.1.3 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.
>Breast Support Y Position to Isocenter | (0018,9548) | 1C | Y position of the Breast Support Reference Point with respect to the Isocenter (mm). See C.8.31.6.1.3 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.
>Breast Support Z Position to Isocenter | (0018,9549) | 1C | Z position of the Breast Support Reference Point with respect to the Isocenter (mm). See C.8.31.6.1.3 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.
>Detector Isocenter Primary Angle | (0018,9550) | 1 | Primary angle of the Detector in the isocenter reference system (deg). See C.8.31.6.1.4 for further explanation.
>Detector Isocenter Secondary Angle | (0018,9551) | 1 | Secondary angle of the Detector in the in the isocenter reference system (deg). See C.8.31.6.1.4 for further explanation.
>Detector X Position to Isocenter | (0018,9552) | 1C | X position of the Detector Reference Point with respect to the Isocenter (mm). See C.8.31.6.1.4 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.
>Detector Y Position to Isocenter | (0018,9553) | 1C | Y position of the Detector Reference Point with respect to the Isocenter (mm). See C.8.31.6.1.4 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.
>Detector Z Position to Isocenter | (0018,9554) | 1C | Z position of the Detector Reference Point with respect to the Isocenter (mm). See C.8.31.6.1.4 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.
>Detector Active Area TLHC Position | (0018,9557) | 1C | The x, y, and z coordinates in mm of the center of the top left hand corner detector element of the detector active area in the Detector coordinate system. See C.8.31.6.1.5 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.
>Detector Active Area Orientation | (0018,9558) | 1C | The direction cosines of the first row and the first column with respect to the Detector coordinate system. See C.8.31.6.1.5 for further explanation. Required if Presentation Intent Type (0008.0068) = FOR PROCESSING. May be present otherwise.

##### C.8.31.6.1 Isocenter Reference System Attribute Descriptions

The Isocenter Reference System Attributes describe the 3D geometry of the X-Ray equipment composed by the X-Ray Source, Breast Support and the Detector.

These Attributes define four coordinate systems in the 3D space:

- Isocenter coordinate system

- X-Ray Source coordinate system

- Breast Support coordinate system

- Detector coordinate system

The Isocenter Reference System Attributes describe the relationship between the 3D coordinates of a point in the Breast Support coordinate system and the 3D coordinates of such point in the X-Ray Source and Detector coordinate systems (which may move in the equipment), by using the Isocenter coordinate system that is fixed in the equipment.

Unlike in X-Ray Angiography, in stereotactic biopsy and breast tomosynthesis the X-Ray source does not move in lock-step with the detector. The detector may be stationary, may translate, or may rotate as the X-Ray source moves. Thus for the purpose of stereotactic biopsy and breast tomosynthesis, the X-Ray Source and Detector Coordinate Systems are defined separately, and each of the potential detector positionings is considered separately.

### Note

Annex Z “X-Ray Isocenter Reference Transformations (Informative)” in PS3.17 describes the transformations necessary to transpose between coordinate systems.

The detector is anterior to the patient standing upright and facing the front of the gantry, as shown in Figure C.8.31.6-1.

Figure C.8.31.6-1. Patient/Detector Relationship (Patient Standing)

As breast projection X-Ray imaging traditionally uses devices with similar physical appearance, the following terms are defined for the purpose of helping later convey the geometry associated with breast projection X-Ray imaging:

GANTRY FRONT | The side of the equipment closest to the patient when the patient is standing, positioned for breast projection X-Ray imaging. | Figure C.8.31.6-2. Gantry Sides
GANTRY RIGHT | The right-hand side of the equipment when the patient is standing facing GANTRY FRONT.
GANTRY LEFT | The left-hand side of the equipment when the patient is standing facing GANTRY FRONT.
GANTRY REAR | The side of the equipment furthest from the patient when the patient is standing, positioned for breast projection X-Ray imaging.

###### C.8.31.6.1.1 Isocenter Coordinate System

The Isocenter coordinate system (O,X,Y,Z) of the equipment is defined as follows (see Figure C.8.31.6-3) :

- Origin O is the System Isocenter, which is the point where the plane of motion of the X-Ray Source spot intersects the axis of rotation of the X-Ray Source.

- +X direction is from GANTRY LEFT to GANTRY RIGHT in the horizontal plane (gravity plane).

- +Y direction is from GANTRY FRONT to GANTRY REAR in the horizontal plane (gravity plane).

- +Z direction is upward (anti-gravity).

- The isocenter coordinate system does not rotate when the tube head or C-arm rotates.

Figure C.8.31.6-3. Isocenter Coordinate System

###### C.8.31.6.1.2 X-Ray Source Coordinate System

The X-Ray Source coordinate system (Os, Xs, Ys, Zs) is defined as follows (see Figure C.8.31.6-4):

- Origin Os is identical to the origin of the Isocenter coordinate system O.

- Xs axis is in the plane normal to the axis of rotation of the X-Ray source through Os and is perpendicular to the X-Ray source. Positive direction is from GANTRY LEFT to GANTRY RIGHT when the X-Ray source is overhead (As1 equals 0°).

- Ys axis is the axis of rotation of the X-Ray source. Positive direction is from GANTRY FRONT to GANTRY REAR.

- Zs axis is normal to the Xs Ys plane. The +Zs direction is generally toward the X-Ray source, defined by the right-hand rule from +Xs and +Ys.

Figure C.8.31.6-4. X-Ray Source Coordinate System

The X-Ray Source coordinate system (Os, Xs, Ys, Zs) is characterized with respect to the Isocenter coordinate system (O, X, Y, Z), by two angles describing the X-Ray center beam. These angles are relative to the Isocenter reference system, and independent from the patient position on the equipment.

X-Ray Source Isocenter Primary Angle (0018,9543) (so-called As1 in Figure C.8.31.6-5) is defined as the angle between the YZ plane and the YZs plane. The angle from +Zs toward +Xs is positive.

X-Ray Source Isocenter Secondary Angle (0018,9544) (so-called As2 in Figure C.8.31.6-5) is defined as the angle between the XZ plane and the XZs plane. The angle from +Zs toward +Ys is positive.

Figure C.8.31.6-5. X-Ray Source Isocenter Angles

###### C.8.31.6.1.3 Breast Support Coordinate System

The Breast Support (see Figure C.8.31.6-6) is the housing over the detector onto which the breast is placed before imaging. For the purpose of this description, Breast Support top surface refers to the patient contact surface of the Breast Support, closest to the X-Ray source.

Figure C.8.31.6-6. Breast Support Orientation

The Breast Support coordinate system (Ob, Xb, Yb, Zb) is defined as follows (see Figure C.8.31.6-7):

- Origin Ob, the so-called Breast Support Reference Point, is an arbitary point, as defined by the equipment manufacturer, on the axis of the center of rotation of the Breast Support.

- Xb axis lies parallel to the Breast Support top surface, passes through Ob, and is parallel to X when the Breast Support top surface is normal to gravity. The +Xb direction is toward BREAST SUPPORT RIGHT.

- Yb axis lies parallel to the Breast Support top surface, passes through Ob, and is perpendicular to the Xb axis. The +Yb direction is toward BREAST SUPPORT ANTERIOR.

- Zb axis is normal to the Breast Support top surface and passes through Ob. The +Zb direction is toward BREAST SUPPORT HEAD.

The Breast Support coordinate system (Ob, Xb, Yb, Zb) is characterized, with respect to the Isocenter coordinate system (O, X, Y, Z), by two rotations that describe the Breast Support tilt and a 3D translation (see Figure C.8.31.6-7). It is the most common convention in breast projection X-Ray system design to have the Breast Support rotate identically and in synchronization with the system C-arm (they are most often rigidly integrated):

Breast Support Isocenter Primary Angle (0018,9545) (so-called Ab1) is defined as the angle between the YZ plane and the YZb plane. The angle is positive when the Breast Support Right end of the Breast Support is lower than the Breast Support Left end of the Breast Support, and is 0° when the Breast Support surface is normal to the +Z direction. In digital breast tomosynthesis, because the X-Ray source moves independently of the Breast Support, the Breast Support Isocenter Primary Angle is the principal place where the user's perception of the C-arm angle is encoded.

Breast Support Isocenter Secondary Angle (0018,9546) (so-called Ab2) is defined as the angle between the XY plane and the XYb plane. The angle is positive when the Breast Support Anterior side of the Breast Support is lower than the Breast Support Posterior end of the Breast Support, and is 0° when the Breast Support surface is normal to the +Z axis. In most breast projection X-Ray imaging equipment the value of Ab2 is 0°.

Figure C.8.31.6-7. Breast Support Angles with respect to the Isocenter Reference System

Breast Support X Position to Isocenter (0018,9547) (so-called BX) is defined as the translation of the Breast Support Reference Point Ob with respect to the Isocenter coordinate system in the +X axis direction. Breast support translation toward +X is positive.

Breast Support Y Position to Isocenter (0018,9548) (so-called BY) is defined as the translation of the Breast Support Reference Point Ob with respect to the Isocenter coordinate system in the +Y axis direction. Breast support translation toward +Y is positive.

Breast Support Z Position to Isocenter (0018,9459) (so-called BZ) is defined as the translation of the Breast Support Reference Point Ob with respect to the Isocenter coordinate system in the +Z axis direction. Breast support translation toward +Z is positive.

### Note

A translation of (BX, BY, BZ) = (0, 0, 0) means that the Breast Support Reference Point Ob is at the System Isocenter.

Figure C.8.31.6-8. Breast Support Translation with respect to the Isocenter Reference System

###### C.8.31.6.1.4 Detector Coordinate System

A detector coordinate system is defined that allows the detector to be either stationary or in motion. For clarity of understanding and interoperability, detector motion terms that may be used during image acquisition are defined, as indicated in Type of Detector Motion (0054,0202):

- Stationary Detector

- Rotating Detector

- Translating Detector

- Complex Detector

An infinite variation of detector motions is possible through time- and angle- dependent combinations of detector rotation and detector translation.

At any given point in time during image acquisition the detector origin Od may be translated with respect to the system isocenter and/or the detector coordinate system rotated with respect to the isocenter coordinate system. The detector coordinate system (Od, Xd, Yd, Zd) is defined as follows (see Figure C.8.31.6-9):

- Detector Reference Point Od, is an arbitrary point, defined by the equipment manufacturer, that lies on the axis of rotation of the detector, if any.

- Xd axis lies in the plane parallel to the detector surface, through the point Od, and perpendicular to the axis of rotation. The +Xd direction is in the same direction as the +X direction when the detector surface is normal to gravity.

- Yd axis is the axis of rotation of the detector and is parallel to Y. The +Yd direction is in the same direction as the +Y direction when the detector surface is normal to gravity.

- Zd axis is normal to the detector surface and through the point Od. The +Zd direction is upward, defined by the right-hand rule from +Xd and +Yd.

Figure C.8.31.6-9. Detector Coordinate System

The Detector coordinate system (Od, Xd, Yd, Zd) is characterized with respect to the Isocenter coordinate system (O, X, Y, Z), by two angles describing the tilt of the detector. These angles are relative to the Isocenter reference system, and independent from the patient position on the equipment.

Detector Isocenter Primary Angle (0018,9550) (so-called Ad1) is defined as the angle between the YZ plane and the YZd plane. The angle is positive when the +Xd axis is lower than the -Xd axis with respect to Z, and is 0°when the detector surface is normal to the +Z axis.

Detector Isocenter Secondary Angle (0018,9551) (so-called Ad2) is defined as the angle between the plane XZ and the plane XZd. The angle is positive when the +Yd axis is lower than the -Yd axis with respect to Z. In most cases this angle is 0°, inferring that the XZ plane is parallel to the Xd plane.

Figure C.8.31.6-10. Detector Isocenter Angles

Detector X Position to Isocenter (0018,9552) (so-called DX) is defined as the translation of the Detector Reference Point Od with respect to the Isocenter coordinate system in the X direction. A translation of DX toward Breast Support Right from Detector Od is positive.

Detector Y Position to Isocenter (0018,9553) (so-called DY) is defined as the translation of the Detector Reference Point Od with respect to the Isocenter coordinate system in the Y direction. A translation of DY toward Breast Support Anterior from Detector Od is positive.

Detector Z Position to Isocenter (0018,9554) (so-called DZ) is defined as the translation of the Detector Reference Point Od with respect to the Isocenter coordinate system in the Z direction. A translation of DZ toward Breast Support Head from Detector Od is positive.

Figure C.8.31.6-11. Translation of the Detector Coordinate System

Although not shown in Figure C.8.31.6-11, translation of the detector moves the Detector Origin Od relative to the Isocenter. Thus during translation of the detector, at least one of DX, DY and DZ will change.

### Note

A translation of (DX, DY, DZ) = (0, 0, 0) means that the Detector Reference Point Od is at the System Isocenter.

The case where the detector is stationary during image acquisition is a degenerate case, but still defined in the same manner.

### Note

1. An angulation of (Ad1, Ad2) = (0, 0) means that the Detector is not rotated relative to the System Isocenter coordinate system.

2. A translation of (DX, DY, DZ) = (0, 0, 0) means that the Detector Reference Point Od is at the System Isocenter.

###### C.8.31.6.1.5 Detector Active Area Relationship

The image and planes in conventional mammography and digital breast tomosynthesis are expressed in relation to Detector FOV and Detector Active area (see C.8.11.4.1.1). As manufacturers design their systems differently, the detector may be mounted or positioned differently in different breast imaging systems. As a result, the TLHC position of the Detector Active Area, together with row and column directions, must be specified (see Figure C.8.31.6-12). The relationship between the Detector Active Area TLHC, the row and column directions of the Active Area, and the Field of View and Detector Coordinate System (defined in C.8.31.6.1.4) is defined by two Attributes.

- Detector Active Area TLHC Position (0018,9557) specifies the x, y and z cooordinates in mm of the center of the pixel defined as Detector Active Area TLHC within the Detector coordinate system, where the Z-direction value is always 0 since the detector itself is in XdYd plane. A value of Detector Active Area TLHC Position (0018,9557) = (0,0,0) means that the Detector Active Area TLHC Position is at the origin Od of the Detector coordinate system.

- Detector Active Area Orientation (0018,9558) specifies the direction cosines of the first row followed by the direction cosines of the first column within the Detector coordinate system. The first three values contain the cosine of the angle between the first row and the Xd, Yd and Zd axes, respectively. The next three values contain the cosine of the angle between the first column and the Xd, Y d and Zd axes, respectively.

Figure C.8.31.6-12. Examples of Detector Active Area TLHC Position and Orientation

In Figure C.8.31.6-12, the left picture illustrates Detector Active Area Orientation (0018,9558) = (0,-1,0,-1,0,0), while the right picture illustrates Detector Active Area Orientation (0018,9558) = (0,1,0,1,0,0).

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DICOM PS3.3 2018b - Information Object Definitions