Image to Equipment Mapping Matrix Attribute
| Tag | (0028,9520) |
|---|---|
| Type | Required (1) |
| Keyword | ImageToEquipmentMappingMatrix |
| Value Multiplicity | 16 |
| Value Representation | Decimal String (DS) |
A rigid, homogeneous 4x4 transformation matrix that maps the patient coordinate space in the Frame of Reference used for the patient model to the coordinate system defined by the equipment. Matrix elements shall be listed in row-major order.
See Section 10.39.1.1, Section 10.39.1.2 and Section C.7.6.21.1.
Section 10.39.1.1
10.39.1.1 Equipment Coordinate System
A piece of equipment has an Equipment Coordinate System which can be used for expressing geometric concepts such as locations and orientations. The coordinate system is characterized by the location of the origin and the orientation of coordinate axes with respect to the equipment. The Equipment Coordinate System is a right-handed coordinate system.
Equipment Coordinate Systems are typically based on a standardized definition of axes. The choice of origin is often device-specific or device-type-specific. It may be any significant location on the machine such as the manufacturer-dependent machine isocenter.
The Equipment Coordinate System can be used as the parent for derived coordinate systems.
Section 10.39.1.2
10.39.1.2 Image to Equipment Mapping Matrix and Patient Support Position Macro
The Image to Equipment Mapping Matrix (0028,9520) describes the relationship between the Patient-oriented coordinate system and an Equipment Coordinate System. This matrix AM Bdescribes a rigid transformation of a point ( Bx, By, Bz) with respect to the Patient coordinate system into ( Ax, Ay, Az) with respect to the Equipment Coordinate System as defined in Section C.7.6.21.1.
The Equipment Coordinate System is identified by the Equipment Frame of Reference UID (300A,0675). For further information on the definition of the Equipment Frame of Reference, see Section 10.39.1.1. The patient-oriented coordinate system is identified by the Frame of Reference UID (0020,0052) of the SOP Instance it is used within. Both coordinate systems are expressed in millimeters.
The Patient Support Position Macro invoked by Patient Support Position Sequence (3006,00CB) allows the exchange of device-specific parameters for the patient support device. Applications designed to guide a specific patient support device will be able to de-compose the transformation into device-specific parameters or derive a transformation matrix out of these parameters. Applications that are unable to know the decomposition of the transformation to those parameters and vice versa will still be able to display the native labels and numerical values of those parameters to human readers.
The Patient Support Position Sequence (3006,00CB) may be present to annotate the matrix and display the decomposed matrix contents. The content of the Patient Support Position Macro shall be used for display purposes only. It shall not be used for other purposes. The content of this Macro shall not be used as a substitute for the Image to Equipment Mapping Matrix (0028,9520). In general, there is more than one way to reach the point in space that is described by the Image to Equipment Mapping Matrix (0028,9520). Hence it is explicitly not implied how this position is reached.
In some cases (e.g., emergency treatments in Radiotherapy), the Patient Frame of Reference is not defined by an image series. In this case an arbitrary Frame of Reference is used for the patient coordinate system in the Frame of Reference Module of the SOP Instance. The Image to Equipment Mapping Matrix (0028,9520) has the same meaning as in the case of image-based Patient Frame of Reference.
When a Frame of Reference of a patient model is not available, the well-known Frame of Reference of a patient support device may be used.
The well-known Frame of Reference and its origin and orientation with respect to the device shall be documented in the Conformance Statement. Note that the orientation of the axes of the well-known Frame of Reference are tied to the device and not to the patient.
As an example, the initial position needs to be specified for the patient setup, prior to any imaging. For a treatment table whose origin and orientation are defined by [IEC 61217], the well-known Frame of Reference UID "1.2.840.10008.1.4.3.3" may be used.
If the Image to Equipment Mapping Matrix (0028,9520) and the Patient Support Position Sequence (3006,00CB) are both present, the information in both locations shall be consistent.
Section C.7.6.21.1
C.7.6.21.1 Image to Equipment Mapping Matrix
The Image to Equipment Mapping Matrix (0028,9520) is used to describe the relationship between the patient oriented coordinate system and a modality specific equipment coordinate system. This mapping can only be used with systems that have a well-defined equipment coordinate system (such as XA, etc.).
The Image to Equipment Mapping Matrix AMB describes how to transform a point (Bx,By,Bz) with respect to the Patient-Based Coordinate System into (Ax,Ay,Az) with respect to the equipment coordinate system according to the equation below.
Equation C.7.6.21.1-1.
The Image to Equipment Mapping Matrix is a rigid transformation that involves only translations and rotations. Mathematically, the matrix shall be orthonormal and can describe six degrees of freedom: three translations, and three rotations.
Note
Both the Patient-Based Coordinate System and the Equipment-Based Coordinate System are expressed in millimeters.