Using ImageMagick 6.6.9-6 2011-04-14 Q16, I simply ran "convert 4026778101315.DIC 4026778101315.PNG".
Then I did "identify.exe -verbose " on both files. The DICOM file was reported as having "Background color: white", and the PNG file "Background color: black".
Here is what the above DICOM file looks like. Notice the "TR" in the bottom right, along with the extra bright white tooth surfaces.
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And this is what the corresponding PNG file looks like:

Please help. I've tried setting the background colour explicitly, but it has no effect on the resulting PNG image.