Glad to know that you figured out the issue!
I assume you mean reading dicom tags/headers? Right now itk.js does not expose any official means of reading dicom tags, but we are working on it. (FYI @thewtex) However, it is possible with some custom C++ cross-compiled to webassembly, thought it is a lot more difficult to set up. Check out https://github.com/InsightSoftwareConsortium/itk-js/pull/252 for a discussion of dicom tag/header reading.