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 C.8.24.2.1 for an overview of the Ultrasound Frame Of Reference Module.

Table C.8.24.2-1ULTRASOUND 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 Mapping Matrix (0020,9309) 1 A 4x4 homogeneous transformation matrix that maps the Volume Frame of Reference coordinate system (XV, YV, ZV) to the Transducer Frame of Reference 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 (eg, 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 homogeneous transformation matrix that maps the Volume Frame of Reference coordinate system (XV,YV, ZV) to the Table Frame of Reference 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 X V , Y V , and Z V 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 X V axis and columns are parallel to the Y V axis and whose origins lie on the Z V axis.

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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 X X , Y X , and Z X 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 Y X axis is normal to the transducer face and the “direction reference” ( i.e. transducer tactile marker or zero reference) is aligned with the positive X X 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 (x A , y A , z A ) is the apex (or phase center) of the acquisition volume geometry in the Volume Frame of Reference. The apex (x A , y A , z A ) may be located in the volume or exterior to it.

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Figure C.8.24.2-2: Transducer 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 X T , Y T , and Z T 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).

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Figure C.8.24.2-3: Table Frame of Reference

C.8.24.2.2 Ultrasound Frame of Reference Module Attributes

C.8.24.2.2.1 Volume to Transducer Mapping Matrix

The Volume to Transducer Mapping Matrix (0020,9309) is used to describe the relationship between the Transducer Frame of Reference coordinate system and the Volume Frame of Reference coordinate system.

The Volume to Transducer Mapping Matrix ([M TV ]=[P]*[Q]) describes how to transform a point (X V , Y V , Z V ) in the Volume coordinate system into (X X ,Y X , Z X ) in the Transducer coordinate system according to the equation below.

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Where:

X V Y V Z V The voxel location (in mm) in the Volume Frame of Reference

X X Y X Z X The voxel location (in mm) in the Transducer Frame of Reference

Pij,Pij,Pij A 3x3 matrix of direction cosine values as measured to the Transducer origin from the volume origin.

Qx,Qy,QzThe translation values (in mm) describe the location in mm of the Transducer Frame of Reference (X X ,Y X , Z X ) origin from the Volume Reference Origin (X V ,Y V , Z V )measured in millimeters along the Volume axes i.e. to the transducer origin from the volume origin.

C.8.24.2.2.2 Volume to Table Mapping Matrix

The Volume to Table Mapping Matrix (0020,930A) is used to describe the relationship between the Volume Frame of Reference coordinate system and a modality specific equipment coordinate system. This mapping can be used only with systems that have a well-defined equipment coordinate system.

The Volume to Table Mapping Matrix ([M VG ]=[R]*[S]) describes how to transform a point (X V Y V Z V ) in the Volume coordinate system into (X T , Y T , Z T ) in the Table coordinate system according to the equation below.

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Where:

X V Y V Z V The voxel location (in mm) in the Volume Frame of Reference

X T Y T Z T The voxel location (in mm) in the Table Frame of Reference

Rij,Rij,Rij A 3x3 matrix of direction cosine values as measured to the gantry origin from the volume origin.

Sx,Sy,SzThe translation values (in mm) describe the location in mm of the Table Frame of Reference (X T , Y T , Z T ) origin from the Volume Reference Origin (X V ,Y V , Z V )measured in millimeters along the table axes I.e. to the table origin from the volume origin.

Note: The Mapping Matrices are rigid transformations that involve only translations and rotations. Mathematically, the matrix is orthonormal and describes six degrees of freedom: three translations, and three rotations.