Grid Frame Offset Vector (3004,000C) shall be provided if a dose distribution is encoded as a multi-frame image. Values of the Grid Frame Offset Vector (3004,000C) shall vary monotonically and are to be interpreted as follows:
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If Grid Frame Offset Vector (3004,000C) is present and its first element is zero, this Attribute contains an array of n elements indicating the plane location of the data in the right-handed image coordinate system, relative to the position of the first dose plane transmitted, i.e., the point at which Image Position (patient) (0020,0032) is defined, with positive offsets in the direction of the cross product of the row and column directions.
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If Grid Frame Offset Vector (3004,000C) is present, its first element is equal to the third element of Image Position (Patient) (0020,0032), and Image Orientation (Patient) (0020,0037) has the value (1,0,0,0,1,0), then Grid Frame Offset Vector contains an array of n elements indicating the plane location (patient z coordinate) of the data in the Patient-Based Coordinate System.
In future implementations, use of option a) is strongly recommended.
This Attribute is conditional since the RT Dose Module may be included even if pixel doses are not being transmitted, or the image may be a single-frame image. If the Multi-frame Module is present, Frame Increment Pointer (0028,0009) shall have the Enumerated Value of 3004000C (Grid Frame Offset Vector).
Note
Option (a) can represent a rectangular-parallelepiped dose grid with any orientation with respect to the patient, while option (b) can only represent a rectangular-parallelepiped dose grid whose planes are in the transverse patient dimension and whose x- and y-axes are parallel to the patient x- and y-axes.
Example: Figure C.8.8.3-1 shows an example of plane positions for a dose grid with transverse planes.
For this example, Table C.8-39b gives the values of elements in the Grid Frame Offset Vector (3004,000C) for both relative (option (a)) and absolute (option (b)) interpretations, under the following conditions:
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The value of Image Orientation (Patient) (0020,0037) is (1,0,0,0,1,0). I.e., the dose grid is transverse with x- and y-axes parallel to the patient x- and y-axes;
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The value of Image Position (Patient) (0020,0032), i.e., the position of the first element of the dose grid, is (4, 5, 6); and
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The spacing between adjacent dose grid planes is 2mm (uniform).
Table C.8-39b. Values of Dose Grid Frame Offset Vector Under Relative (A) and Absolute (B) Interpretations