Spatial Registration IOD is ill-defined - DICOM

This is a discussion on Spatial Registration IOD is ill-defined - DICOM ; The new Spatial Registration IOD is ill-defined. A registration matrix transforms points in one frame of reference to a point in another frame of reference. The Spatial Registration IOD doesn't specify which frame is which. The IOD has a Frame ...

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Thread: Spatial Registration IOD is ill-defined

  1. Spatial Registration IOD is ill-defined

    The new Spatial Registration IOD is ill-defined. A registration matrix
    transforms points in one frame of reference to a point in another frame
    of reference. The Spatial Registration IOD doesn't specify which frame
    is which.

    The IOD has a Frame Of Reference module, which we can call the FOR of
    the registration. The Spatial Registration Module has a Registration
    Sequence, each item of which contains a frame of reference (ignoring
    the Reference Image Sequence for now) and a matrix sequence. Let's call
    this second frame of reference the FOR of the item.

    Section C.20.2.1.1 carefully describes how use the transformation
    matrix to map a point in RCS-B to RCS-A. However, it nowhere states
    which is frame A: is it the FOR-of-the-registration or the
    FOR-of-the-item?

    We are directed to PS 3.17 for more information. Unfortunately, Annex
    O of PS 3.17 is inconsistent. The second paragraph on Page 151 states:

    The Spatial Registration instance specifies a transformation from
    its own local Reference Coordinate System to that of the
    referenced image sets or atlas.

    That makes it sound like the matrix maps from FOR-of-the-registration
    to FOR-of-the-item. However, the first line of Page 154 says:

    A Spatial Registration instance consists of one or more instances
    of a Registration. Each Registration specifies a transformation
    from the RCS of the Referenced Image Set, to the RCS of this
    Spatial Registration instance (see PS 3.3) identified by the Frame
    of Reference UID (0020,0052).

    which is precisely the opposite; i.e. from FOR-of-the-item to
    FOR-of-the-registration.

    Which is it?

    -Steve


  2. Re: Spatial Registration IOD is ill-defined

    Hi Steve,

    The short answer is "FOR of the item to FOR of the registration".
    This is taken from the "Frame of Reference Transformation Matrix"
    description on page 805 which says "A 4x4 homogeneous transformation
    matrix that registers the referenced images to the local RCS."

    That being said, I agree that this isn't very precise and there seems
    to be inconsistencies as you pointed out. Furthermore, since these
    matrices are to be concatenated, they cannot all go from the references
    images to the local RCS... We need to sort this out.

    I'm hoping this can be addressed at the next meeting in a couple of
    weeks, and we'll try to make things clearer.

    Best,

    Alex.


  3. Re: Spatial Registration IOD is ill-defined

    Hi Alex,

    I'm happy to hear that there will shortly be an opportunity to address
    this. I hope that the inconsistency in Part 17 can be addressed. But
    more importantly, Part 3 should be made unambiguous.

    Alexandre Guimond wrote:

    > The short answer is "FOR of the item to FOR of the registration".
    > This is taken from the "Frame of Reference Transformation Matrix"
    > description on page 805 which says "A 4x4 homogeneous transformation
    > matrix that registers the referenced images to the local RCS."


    The problem for me is that many people treat "registers" as a symmetric
    relation, almost as a synonym for "matches". So you cannot know which
    way the mapping goes.

    Once you start talking about a spatial transformation, however, you
    most certainly can specify the frames of reference. Part 3 talks about
    frames "A" and "B"; all the standard needs, IMHO is to specify which is
    A and which is B.

    > That being said, I agree that this isn't very precise and there seems
    > to be inconsistencies as you pointed out. Furthermore, since these
    > matrices are to be concatenated, they cannot all go from the references
    > images to the local RCS... We need to sort this out.


    Yes. So specify the identities of "A" and "B" for the CONCATENATED
    matrix.

    Regards,
    Steve


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