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.
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.
Figure C.8.24.2-1: Volume 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.
Figure C.8.24.2-2: Transducer 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).
Figure C.8.24.2-3: Table Frame of Reference