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Rationale for Correction:
The Enhanced US Volume IOD requires the use of equally spaced parallel slices, as
described in the IOD itself.
It also introduced Dimension Organization Type, with Defined Terms for "3D" and
"3D_TEMPORAL", though described them only as parallel slices, whithout mentioning equal
spacing, though it is believed that was intended.
A recipient may want to recognize an equally spaced set of parallel slices from
Dimensions with indices that point to a Stack rather than the position attributes, or to
position attributes that are other than those in the traditional patient-relative
coordinate system (such as the new volume-based coordinate system introduced in the
Enhanced US Volume IOD, or without understanding the details of the coordinate system.
Making the Defined Terms of Dimension Organization Type would make that possible. Note
that a Volumetric Properties value of VOLUME does not imply either parallel or equal
spacing, so is not helpful in this respect.
Also, though the in-plane Pixel Spacing is always conveyed in the Enhanced family of
objects, Spacing Between Slices is not encoded in most Enhanced IODs. If it were,
however, a recipient without knowledge of the coordinate system could make use of the
volume for MPRs and measurements. Since the existing Pixel Measures functional group
macro already includes Slice Thickness, it makes sense to add Spacing Between Slices to
it. Since Spacing Between Slices is encoded optionally in the top-level Data Set in the
Enhanced MR Image IODs, it is removed for consistency in favor of the functional group
macro.
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