C.8.24.2 Ultrasound Frame of Reference Module
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#### C.8.24.2 Ultrasound Frame of Reference Module

Table C.8.24.2-1 specifies the attributes of the Ultrasound Frame Of Reference Module. See Section C.8.24.2.1.

Table C.8.24.2-1. Ultrasound Frame of Reference Module Attributes

Attribute Name | Tag | Type | Attribute Description
--- | --- | --- | ---
Volume Frame of Reference UID | (0020,9312) | 1 | Uniquely identifies this Volume Frame of Reference.
Ultrasound Acquisition Geometry | (0020,9307) | 1 | Characteristic of the ultrasound acquisition geometry. Defined Terms: APEX there exists an apex of the scan lines from which the volume data was acquired.
Apex Position | (0020,9308) | 1C | Position of the apex (or phase center) of the acquisition geometry, encoded as xA, yA, and zA in mm units in the Volume Frame of Reference. The apex (xA, yA, zA) may be located in the volume or exterior to it. Required if value of Ultrasound Acquisition Geometry (0020,9307) is APEX.
Volume to Transducer Relationship | (0020,930B) | 1C | Relationship between the transducer and the acquired volume. Enumerated Values: FIXED The transducer position and orientation relative to the volume is constant and specified by Volume to Transducer Mapping Matrix (0020,9309). POSITION_VAR The transducer position relative to the volume varies during acquisition and the position specified by Volume to Transducer Mapping Matrix (0020,9309) is a nominal value. The transducer orientation relative to the volume is constant and specified by Volume to Transducer Mapping Matrix (0020,9309). ORIENTATION_VAR The transducer orientation relative to the volume varies during acquisition and the orientation specified by Volume to Transducer Mapping Matrix (0020,9309) is a nominal value. The transducer position relative to the volume is constant and specified by Volume to Transducer Mapping Matrix (0020,9309). VARIABLE The transducer position and orientation relative to the volume varies during acquisition and the position and orientation specified by Volume to Transducer Mapping Matrix (0020,9309) are nominal. Required if the transducer position and/or orientation relative to the volume is not constant. May be present otherwise.
Volume to Transducer Mapping Matrix | (0020,9309) | 1 | A 4x4 rigid transformation matrix that maps the Volume Frame of Reference homogeneous coordinate system (XV, YV, ZV) to the Transducer Frame of Reference homogeneous coordinate system (XX,YX, ZX). Matrix elements shall be listed in row-major order. See Section C.8.24.2.1 for details.
Patient Frame of Reference Source | (0020,930C) | 1C | Indicates how the supplied Image Position (Patient) (0020,0032) and Image Orientation (Patient) (0020,0037) values are obtained. Enumerated Values: TABLE A positioning device, such as a gantry, was used to generate these values. ESTIMATED Estimated patient position / orientation (e.g., estimated by the user), or if reliable information is not available. REGISTRATION Acquisition has been spatially registered to a prior image set. Required if either Image Position (Patient) (0020,0032) or Image Orientation (Patient) (0020,0037) is present.
Table Frame of Reference UID | (0020,9313) | 1C | Uniquely identifies this Table Frame of Reference. Required if Patient Frame of Reference Source (0020,930C) is TABLE.
Volume to Table Mapping Matrix | (0020,930A) | 1C | A 4x4 rigid transformation matrix that maps the Volume Frame of Reference homogeneous coordinate system (XV,YV, ZV) to the Table Frame of Reference homogeneous coordinate system (XT,YT, ZT). Matrix elements shall be listed in row-major order. See Section C.8.24.2.2 for details. Required if Patient Frame of Reference Source (0020,930C) is TABLE.

##### C.8.24.2.1 Ultrasound Frame of Reference Module Overview

The Ultrasound Frame of Reference Module is used to relate the image planes to a frame of reference appropriate for the ultrasound modality, most notably a volume-based frame of reference. There are many different transducer scan acquisition geometries used in 3D ultrasound imaging. Regardless of the acquisition geometry, after acquisition of the initial scan images comprising the volume, the ultrasound (US) scanner will assemble (reformat) the data into a proper Cartesian volume with the assumption that the data are related through a Right-Hand Coordinate System (RHCS). x-positions are defined in mm with positive values increasing towards the right. y-positions are defined in mm with positive values in the direction of increasing image depth. z-positions are defined in mm with positive values in the direction as defined in a right-hand coordinate system.

A Cartesian volume will consist of a series of 1 to n parallel planes. The image planes comprising the Cartesian volume are typically oriented during creation of the volume so that the best image quality is in the XY plane. Table C.8.24.2-1 specifies the attributes of the Ultrasound Frame of Reference Module. There are three levels of detail for the Ultrasound Frame of Reference: Volume, Transducer and Table.

###### C.8.24.2.1.1 Volume Frame of Reference

The Volume Frame of Reference is a Right-hand Coordinate System consisting of a Volume Origin at the location (0,0,0) and mutually orthogonal XV, YV, and ZV axes in a Right-Hand Coordinate System. The particular IOD using the Volume Frame of Reference may constrain the alignment of frames with respect to the axes of the Volume Frame of Reference. For example, Figure C.8.24.2-1 illustrates the use of the Volume Frame of Reference with frames whose rows are parallel to the XV axis and columns are parallel to the YV axis and whose origins lie on the ZV axis.

Figure C.8.24.2-1. Volume Frame of Reference

###### C.8.24.2.1.2 Transducer Frame of Reference

The Transducer Frame of Reference is a Right-hand Coordinate System consisting XX, YX, and ZX axes originating at a reference "Transducer Origin" defined as the geometric center of the transducer face.

The orientation of the Transducer Frame of Reference relative to the Volume Origin is such that the YX axis is normal to the transducer face and the "direction reference" (i.e., transducer tactile marker or zero reference) is aligned with the positive XX axis. A transformation is specified between the Volume Frame of Reference and the Transducer Frame of Reference to define the position of the transducer relative to the volume. This transformation is specified by the Volume to Transducer Mapping Matrix (0020,9309).

The Transducer Frame of Reference recognizes two types of scan geometry: 1) a scan geometry with a real apex such as would be the case for a pyramid, toroid or rotational volume acquisition, or 2) a scan geometry for which there is no specific apex. The point (xA, yA, zA) is the apex (or phase center) of the acquisition volume geometry in the Volume Frame of Reference. The apex (xA, yA, zA) may be located in the volume or exterior to it.

Figure C.8.24.2-2. Transducer Frame of Reference

In some acquisition modalities, the transducer position and/or orientation changes during acquisition and a single transducer mapping matrix is insufficient to describe the relationship between transducer and volume. In this case, Volume to Transducer Relationship (0020,930B) is required and specifies the nature of the relationship Transducer Frame of Reference and the Volume Frame of Reference.

###### C.8.24.2.1.3 Table Frame of Reference

There also may exist a fixed equipment reference called the Table Frame of Reference, a Right-hand Coordinate System consisting of XT, YT, and ZT axes originating at a reference "Table Origin". See Figure C.8.24.2-3.

### Note

In this context the Table Frame of Reference refers to a fixed coordinate system in space that may be provided by a variety of source devices such as coordinates from a magnetic position sensor, LED sensor array, a physical scanner gantry, or similar device.

A transformation may be specified between the Volume Frame of Reference and the Table Frame of Reference to define the position and orientation of the volume relative to this external frame of reference. This transformation is specified by the Volume to Table Mapping Matrix (0020,930A).

Figure C.8.24.2-3. Table Frame of Reference

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