A structure set defines a set of areas of significance. Each area can be associated with a Frame of Reference and zero or more images. Information that can be transferred with each region of interest (ROI) includes geometrical and display parameters, and generation technique.
Table C.8-41—STRUCTURE SET MODULE ATTRIBUTES
|Attribute Name||Tag||Type||Attribute Description|
|Structure Set Label||(3006,0002)||1||User-defined label for Structure Set.|
|Structure Set Name||(3006,0004)||3||User-defined name for Structure Set.|
|Structure Set Description||(3006,0006)||3||User-defined description for Structure Set.|
|Instance Number||(0020,0013)||3||A number that identifies this object instance.|
|Structure Set Date||(3006,0008)||2||Date at which Structure Set was last modified.|
|Structure Set Time||(3006,0009)||2||Time at which Structure Set was last modified.|
|Referenced Frame of Reference Sequence||(3006,0010)||3||Introduces sequence of items describing Frames of Reference in which the ROIs are defined. One or more items are permitted in this sequence. See C.126.96.36.199.|
|>Frame of Reference UID||(0020,0052)||1||Uniquely identifies Frame of Reference within Structure Set.|
|>Frame of Reference Relationship Sequence||(3006,00C0)||3||Introduces sequence of transforms that relate other Frames of Reference to this Frame of Reference. One or more items are permitted in this sequence.|
|>>Related Frame of Reference UID||(3006,00C2)||1||Frame of Reference Coordinate System to be transformed to the current Frame of Reference.|
|>>Frame of Reference Transformation Type||(3006,00C4)||1||Type of Transformation. Defined Terms: HOMOGENEOUS|
|>>Frame of Reference Transformation Matrix||(3006,00C6)||1||Four-by-four transformation Matrix from Related Frame of Reference to current Frame of Reference. Matrix elements shall be listed in row-major order. See C.188.8.131.52.|
|>>Frame of Reference Transformation Comment||(3006,00C8)||3||Comment regarding the transformation between the related and current Frames of Reference.|
|>RT Referenced Study Sequence||(3006,0012)||3||Introduces sequence of Studies containing series to be referenced. One or more items are permitted in this sequence.|
|>>Include ‘SOP Instance Reference Macro' Table 10-11|
|>>RT Referenced Series Sequence||(3006,0014)||1||Introduces sequence of items describing series of images within the referenced study which are used in defining the Structure Set. One or more items shall be included in this sequence.|
|>>>Series Instance UID||(0020,000E)||1||Unique identifier for the series containing the images.|
|>>>Contour Image Sequence||(3006,0016)||1||Introduces sequence of items describing images in a given series used in defining the Structure Set (typically CT or MR images). One or more items shall be included in this sequence.|
|>>>>Include ‘Image SOP Instance Reference Macro’ Table 10-3|
|Structure Set ROI Sequence||(3006,0020)||3||Introduces sequence of ROIs for current Structure Set. One or more items are permitted in this sequence.|
|>ROI Number||(3006,0022)||1||Identification number of the ROI. The value of ROI Number (3006,0022) shall be unique within the Structure Set in which it is created.|
|>Referenced Frame of Reference UID||(3006,0024)||1||Uniquely identifies Frame of Reference in which ROI is defined, specified by Frame of Reference UID (0020,0052) in Referenced Frame of Reference Sequence (3006,0010).|
|>ROI Name||(3006,0026)||2||User-defined name for ROI.|
|>ROI Description||(3006,0028)||3||User-defined description for ROI.|
|>ROI Volume||(3006,002C)||3||Volume of ROI (cubic centimeters).|
|>ROI Generation Algorithm||(3006,0036)||2||Type of algorithm used to generate ROI. Defined Terms: AUTOMATIC = calculated ROI SEMIAUTOMATIC = ROI calculated with user assistance MANUAL = user-entered ROI|
|>ROI Generation Description||(3006,0038)||3||User-defined description of technique used to generate ROI.|
|>Derivation Code Sequence||(0008,9215)||3||A coded description of how this ROI was derived. One or more items are permitted in this Sequence. See C.184.108.40.206 for further explanation.|
|>>Include ‘Code Sequence Macro’ Table 8.8-1||Enumerated Value (113085, DCM, "Spatial resampling").|
The Referenced Frame of Reference Sequence (3006,0010) describes a set of frames of reference in which some or all of the ROIs are expressed. Since the Referenced Frame of Reference UID (3006,0024) is required for each ROI, each frame of reference used to express the coordinates of an ROI shall be listed in the Referenced Frame of Reference Sequence (3006,0010) once and only once.
Notes: 1.As an example, a set of ROIs defined using a single image series would list the image series in a single Referenced Frame of Reference Sequence (3006,0010) item, providing the UID for this referenced frame of reference (obtained from the source images), and listing all pertinent images in the Contour Image Sequence (3006,0016).
2. As an example, a set of ROIs containing ROIs referencing more than one frame of reference would list the referenced images in two or more different Referenced Frame of Reference Sequence (3006,0010) items, providing in each case the UID for this referenced frame of reference (obtained from the source images), and listing all pertinent images in the Contour Image Sequence (3006,0016). Each ROI would then reference the appropriate Frame of Reference UID (0020,0052).
In a rigid body system, two coordinate systems can be related using a single 4 x 4 transformation matrix to describe any rotations and/or translations necessary to transform coordinates from the related coordinate system (frame of reference) to the primary system. The equation performing the transform from a point (X’,Y’,Z’) in the related coordinate system to a point (X,Y,Z) in the current coordinate system can be shown as follows, where for homogeneous transforms M 41 = M 42 = M 43 = 0 and M 44 = 1:
X M 11 M 12 M 13 M 14 X’
Y = M 21 M 22 M 23 M 24 x Y’
Z M 31 M 32 M 33 M 34 Z’
1 M 41 M 42 M 43 M 44 1
If an ROI is created by re-sampling an existing ROI that is either (a) present in a different Frame of Reference (FoR) and has been resampled to align with the image series referenced by the current structure set, or (b) present in the same FoR, but its contours are referenced to a different image series, it is useful to be able to identify that the ROI is resampled. The Derivation Code Sequence (0008,9215) shall be used with code (113085, DCM, "Spatial resampling") to indicate the resampled ROI.