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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.

The Breast Support coordinate system (O_b, X_b, Y_b, Z_b) is defined as follows (see Figure C.8.31.6-7):

The Breast Support coordinate system (O_b, X_b, Y_b, Z_b) 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 YZ_b 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 XY_b 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°.

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