Table A.91-2 specifies the use of the Functional Group Macros used in the Multi-frame Functional Groups Module for the Height Map Segmentation IOD.
Table A.91-2. Height Map Segmentation Functional Group Macros
The Derivation Image Macro shall identify one or more Image SOP Instances that are the source for the volumetric space to which the Height Map Segmentation frame applies. Referenced derivation images shall have the same Frame of Reference UID (0020,0052).
Each Item of the Derivation Image Sequence (0008,9124) shall specify a number of frames equal to the value of Rows (0028,0010) in the Height Map Segmentation SOP Instance. The Derivation Image Macro for a Height Map Segmentation frame with multiple rows shall enumerate the referenced frame for each row in Referenced Frame Number (0008,1160). Alternatively, if the Functional Group references a single derivation image with a number of frames equal to the number of Height Map Segmentation rows, the Referenced Frame Number (0008,1160) Attribute may be omitted, and the Height Map Segmentation rows shall correspond to the derivation image frames in their storage order in the pixel data.
The value of Purpose of Reference Code Sequence (0040,A170) in Source Image Sequence (0008,2112) in Derivation Image Sequence (0008,9124) shall be (121322, DCM, "Source Image for Image Processing Operation"). The value of Derivation Code Sequence (0008,9215) in Derivation Image Sequence (0008,9124) shall be (113076, DCM, "Segmentation").
Note
The referenced derivation image is the source of the pixel/voxel matrix extent in which the Height Map Segmentation is defined. It might technically not be a source image from which the segmentation is derived, e.g., if both the referenced image and the segmentation are derived from a raw acquisition data set. Use of (121322, DCM, "Source Image for Image Processing Operation") is specified to maintain consistency with the Segmentation IOD (see Section A.51.5.1). Other source data SOP Instances can be identified in the Referenced Image Macro.
Pixel Spacing (0028,0030) in the Pixel Measures Macro specifies the real-world physical distance in the imaging target (patient) as row spacing and column spacing in mm (see Section C.7.6.16.2.1 and Section 10.7.1.3). The pixel spacing of the Height Map Segmentation is determined by the pixel measures of the referenced derivation image (see Figure A.91.5-1). As each height map row corresponds to a derivation image frame with the same number of columns, value 2 (column spacing) of Pixel Spacing (0028,0030) in the Height Map Segmentation will equal value 2 of Pixel Spacing (0028,0030) in the referenced derivation image.
Height Map Segmentation frames with more than one row correspond to a set of parallel derivation image frames, i.e., whose Image Orientation (Patient) (0020,0037) values are identical. As the height map frame is orthogonal to those derivation image frames, value 1 (row spacing) of Pixel Spacing (0028,0030) in the Height Map Segmentation will equal the spacing between derivation image frames, computed from differences in Image Position (Patient) (0020,0032) of the referenced derivation image frames.
Note
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As specified in Section 10.7.1.3, if there is only a single row in the Height Map Segmentation frames, the row spacing value may be zero.
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Height Map Segmentation is defined only for cases where the rows of height map data correspond to the top rows of derivation image frames, and the columns of the Height Map Segmentation correspond to the frames of the derivation image. It is not defined for cases where the derivation image frames are parallel to the Height Map Segmentation frame, or for the 90 degree rotation with the Height Map Segmentation columns corresponding to the derivation image rows, or for the Height Map Segmentation to be aligned to the bottom of the derivation image frames.
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The value of Spacing Between Slices (0018,0088) in the derivation image might be used to determine the row spacing of the Height Map Segmentation, but that Attribute is Type 1C in the Ophthalmic Tomography Image IOD, and might not be present. Even if present, it would not be valid if decimated frames of the derivation image are referenced.
The Plane Position (Patient) and Plane Orientation (Patient) Functional Groups shall be present in a Height Map Segmentation with multiple rows. The values of Image Position (Patient) (0020,0032) and Image Orientation (Patient) (0020,0037) are derived from the values in the derivation image.
Note
The value of Image Position (Patient) (0020,0032) will be equal to the value of Image Position (Patient) (0020,0032) in the first referenced frame of the Derivation Image. The value of Image Orientation (Patient) (0020,0037) will have row direction cosines equal to the row direction cosines of the referenced derivation image, and column direction cosines equal to the cross product of the column direction cosines and row direction cosines of the referenced derivation image. If the coordinate system associated with the Frame of Reference is deformed (e.g., see Section A.52.4.3), then the orientation will be the nominal real world orientation.
Image Position (Patient) (0020,0032) and Image Orientation (Patient) (0020,0037) might not be present in the derivation image, in particular for non-volumetric (e.g., circular) scans, where the derivation image is located in space by reference to points on a localizer image rather than by Plane Position and Plane Orientation.
The Real World Value Mapping Functional Group shall provide the mapping of Height Map Segmentation pixel values to real world distance in the volume defined by the derivation image. Height Map values are floating point numbers representing vertical pixel distances with sub-pixel resolution in the pixel matrix of the derivation image. The value of Measurement Units Code Sequence (0040,08EA) shall be (mm, UCUM, "mm"). Values in the pixel padding range, i.e., between the values of Float Pixel Padding Value (0028,0122) and Float Pixel Padding Range Limit (0028,0124), shall not be mapped.
Note
The value of Real World Value Slope (0040,9225) will typically be equal to first value (row spacing) of the Pixel Spacing (0028,0030) Attribute in the Pixel Measures Functional Group of the referenced derivation image. If the coordinate system associated with the Frame of Reference is deformed (e.g., see Section A.52.4.3), then the value mapping will be the nominal real world distance.
The value of Real World Value First Value Mapped (0040,9216) or Double Float Real World Value First Value Mapped (0040,9214) will typically be 0. The value of Real World Value Last Value Mapped (0040,9211) or Double Float Real World Value Last Value Mapped (0040,9213) will typically be equal to the number of rows in the derivation image.