DICOM PS3.3 2019a - Information Object Definitions
C. Coordinate Systems

Where explicitly specified, the coordinate systems defined by IEC 61217 shall be applied, with the exception of the IEC PATIENT Coordinate System.

In addition, the following sections define the coordinate systems to be used in situations where IEC 61217 coordinate systems are not applicable. No other coordinate systems shall be used.

C. Fixed Beam Line

The direction of fixed beam-line can be described as a gantry system, provided that the position of the (virtual) gantry bearing is defined. The relation between their patient support coordinate system axes and the choice of the 'gantry' angle, e.g., 90 or 270 deg, shall be consistent with a standard gantry coordinate system. All coordinate systems derived from the IEC GANTRY coordinate system (BEAM LIMITING DEVICE, WEDGE, X-RAY IMAGE RECEPTOR) automatically follow in the same way as defined in a 'real' gantry system.

The IEC PATIENT SUPPORT system is linked to the IEC GANTRY coordinate system through its common parent system, the IEC FIXED coordinate system. The Y-axis of IEC GANTRY points towards the (virtual) gantry bearing. The Y-axis of the IEC FIXED coordinate system has to point in the same direction. Z-axis in IEC FIXED coordinate system is always pointing upwards. With Y and Z-axes defined, the X-axis of IEC FIXED is also given.

Figure C.8.8.25-2a and Figure C.8.8.25-2b show IEC FIXED (F), GANTRY (G) and PATIENT SUPPORT (S) coordinate systems for a horizontal fixed beam-line.

Fixed Beam Line - View Along IEC FIXED Y-axis

Figure C.8.8.25-2a. Fixed Beam Line - View Along IEC FIXED Y-axis

Fixed Beam Line - View From Top (Along IEC FIXED Z-axis)

Figure C.8.8.25-2b. Fixed Beam Line - View From Top (Along IEC FIXED Z-axis)

C. Table Top Pitch and Table Top Roll

For further information, see Section C.

C. Seated Treatments

RT Ion Plan contains an Attribute Patient Support Type (300A,0350), which can be CHAIR or TABLE. The patient support type CHAIR does not change the coordinate axes of the patient support coordinate systems relative to their parent systems. It is more an Attribute of the type like the patient position in imaging (i.e., HFS, HFP, …).

The orientation of the treatment chair shall be defined with the chair positioned in such way, that the patient looks towards the gantry bearing (or along the Y axis of the IEC FIXED system) if all angles, especially IEC PATIENT SUPPORT angle are 0°. All other parameters follow straight forward, once this definition is accepted. i.e., chair rotation is a rotation of IEC PATIENT SUPPORT coordinate system; a backward tilt of the chair is a positive rotation of the PITCHED TABLE TOP coordinate system. A translation of the chair is a translation of the IEC TABLE TOP system.

The roll angle is typically 0º.

For a seated treatment on a horizontal beam-line, the following angles are therefore defined:If IEC GANTRY angle is 90º (270º), IEC PATIENT SUPPORT angle is 270º (90º) for the position where the patient looks into the beam port.

C. Ocular Treatments
C. Gantry Beam Line

Eye treatments on the gantry shall use all existing IEC coordinate systems with their standard definition. This applies especially to IEC BEAM LIMITING DEVICE, IEC WEDGE FILTER, IEC X-RAY IMAGE RECEPTOR.

IEC PATIENT SUPPORT, and IEC TABLE TOP coordinate systems are defined as above. Additionally, a rotation of the head fixation device is possible. The Head Fixation Angle (300A,0148) shall be defined as the angle of the head fixation device with respect to the TABLE TOP coordinate system. Positive head fixation angle is in the same direction as positive PATIENT SUPPORT pitch, i.e., backwards.

Proton eye treatments require an additional coordinate system for the placement of the fixation light. Since it is usually mounted onto the beam port the 'natural' coordinate system for devices mounted there is the IEC BEAM LIMITING DEVICE coordinate system. The angles for the fixation light positions shall therefore be defined as follows:

Rotation of the fixation light about the IEC BEAM LIMITING DEVICE Z-axis (Zb) is defined as Azimuthal Angle. The Azimuthal Angle is equal to 0° when the fixation light is positioned on the axis Xb of the IEC BEAM LIMITING DEVICE coordinate system. An increase in the value of the Azimuthal Angle corresponds to clockwise rotation of the fixation light as view along the axis Zb towards the virtual source.

The polar angle is always positive and defined as the angle between IEC BEAM LIMITING DEVICE Z-axis and the line connecting isocenter with the fixation light position.

Proton eye treatments require the wedge thin edge position as one additional. The wedge thin edge position allows the specification of a wedge, which does not cover the full open field. The wedge thin edge position is positive, if the wedge does not cover the isocenter position and negative, if it does cover.

Figure C.8.8.25-5 and Figure C.8.8.25-6 show the angles and Attributes as described above.

Patient's eye view

Figure C.8.8.25-5. Patient's eye view

Lateral view along the positive axis Xb

Figure C.8.8.25-6. Lateral view along the positive axis Xb

C. Fixed Beam Line

The coordinate systems for the treatment chair are defined above and shall also be applied to seated eye treatments.

In this case, it is recommended that a beam limiting device angle of 90º be formally applied (provided the gantry angle is defined to be 90º (and not 270º). This results in the same coordinates of the fixation light and wedge relative to the patient as in the treatment situation with the patient lying on the table.

C. Gantry Pitch Angle

The Gantry Pitch Angle is not defined in IEC 61217. This angle is defined in the DICOM Standard in a way compatible with the current notion of IEC by introducing it as rotation of the IEC GANTRY System as indicated below.

The Gantry Pitch Angle is defined as the rotation of the coordinate axes Yg, Zg about axis Xg by an angle ψg; See Figure C.8.8.25-7. An increase in the value of angle ψg corresponds to the clockwise rotation as viewed from the isocenter along the positive Xg axis

Gantry Pitch Angle

Figure C.8.8.25-7. Gantry Pitch Angle

DICOM PS3.3 2019a - Information Object Definitions