# DICOM Standard Browser

Built with by Innolitics, a team of medical imaging software developers.

Data synced with official DICOM standard on 18 April 2024. The DICOM Standard is under continuous maintenance, and the current official version is available at http://www.dicomstandard.org/current/. DICOM Parts 3, 4, and 6, © NEMA. Please note that the most recent PDF version of the standard is the official reference, and should checked when making technical decisions.

## X-Ray Source Isocenter Secondary Angle Attribute

Tag (0018,9544) Required (1) XRaySourceIsocenterSecondaryAngle 1 Double (FD)

Secondary angle of the X-Ray source in the isocenter reference system (deg).

See C.8.31.6.1.2 for further explanation.

### Note

Part of the Breast X-Ray Isocenter Reference System Functional Group Macro with usage: M

## C.8.31.6.1.2

###### 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