SC & Pixel Spacing
Marc-Andre Bordeleau
I would like to keep the resolution (dpi) of a digitized film in a data-set.
But, the
acquired image is stored in a Secondary Capture IOD. So, SC doesn't allow us
to insert
the field "Pixel Spacing" (0028,0030) (from the Image Plane Module, wich one
is not referenced
by SC (PS 3.3 table A.8-1)). How can I keep the resolution in a data-set ?
Thanks.
David Clunie
This is an important question, and various people have solved it in
different ways up until now by adding attributes from other IODs to
create "standard extended" SOP classes: review various scanner
suppliers conformance statements to see what they do. In summary:
(0028,0030) Pixel Spacing - the meaning is only defined for cross-
sectional modalities like CT and MR (distance "in the patient") and
so using it to specify the size of a pixel on a piece of film is
probably inappropriate; however some people use it for this.
(0018,1164) Imager Pixel Spacing - defined at the front of the image
receptor housing (added in XA/CRF, later added to CR and used in DX);
better perhaps, but really only meaningful for projection radiographs,
therefore OK if scanning a film of a projection radiography or a "true
size" printed CR image, but not good for scanning sheets of printed
CT, MR, US or NM where its definition is not meaningful
In order to provide a definition that is strictly in terms of the size
of the media that is being scanned that someone can subsequently map
to an equivalent (known) distance on a scanned projection radiograph
or a scanned cross-sectional image (e.g. by scaling based on measuring
printed calipers), the new SC objects have added:
(0018,2010) Nominal Scanned Pixel Spacing: Type 1C ="Physical distance
on the media being digitized or scanned between the center of each pixel,
specified by a numeric pair - adjacent row spacing (delimiter) adjacent
column spacing in mm.
Required if Conversion Type (0008,0064) is DF (Digitized Film). May also
be present if Conversion Type (0008,0064) is SD (Scanned Document) or
SI (Scanned Image).
Shall be consistent with Pixel Aspect Ratio (0028,0034), if present."
As you can see, we have tried to make this new attribute mandatory when
it is relevant, as well as to nail down the definition to resolve problems
that have been encountered before.
The new attribute is included in the new SC family of objects that are
currently out for ballot and should be officially part of the standard
around April or May. Be warned that changes may occur after the ballot
and during the preparation of final text, so don't go using these objects
(or this attribute added to another SOP class as an extension) in products
just yet.
For further details, see:
ftp://medical.nema.org/medical/dicom/supps/sup57_lb.pdf
david
--
David A. Clunie mailto:dcl...@comview.com
Development Director, Medical Imaging Products http://www.comview.com/
ComView Corporation Work 914-332-4800 Fax 208-445-5867
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