Tags "hidden" inside sequences

[EllenWasbo]

I am converting an application from IDL to Python. For multiframe images, information pr image can be found within sequences. In IDL I did not have to know anything about the sequence when searching for a specific tag. IDL would return all matches. pydicom return None if the tag is "hidden" within a sequence and I have to specify the sequence first. Is there a way, in pydicom, to get tags that are nested inside sequences without knowing the sequence tag number or name?

[mrbean-bremen]

I converted this to a discussion issue, as this comes up every now and then.

There is no build-in way to do this, but to get the common tags for enhanced image classes, you could write a convenience function, as these tags are always in a nested sequence inside the SharedFunctionalGroupsSequence:

def get_tag(ds, tag):
    if tag in ds:
        return ds[tag]
    if "SharedFunctionalGroupsSequence" in ds:
        item = ds.SharedFunctionalGroupsSequence[0]
        for element in item:
            if element.VR == "SQ":
                if element.value and tag in element.value[0]:
                    return element.value[0][tag]

This will return a DataElement object:

>>> ds = dcmread("my_enhancedmr.dcm")
>>> element = get_tag(ds, "RepetitionTime")
>>> element
(0018, 0080) Repetition Time                     DS: '3500.0'
>>> element.value
3500

Note that this will show only common tags. Tags that differ between frames can be looked up in a similar way in the PerFrameFunctionalSequence, with the difference that this sequence has an item for each frame instead of only one item.

[EllenWasbo]

Thank you for the quick response although it was not the answer I hoped for :) I will not find all the relevant tags in those sequences. At least I will have to add DetectorInformationSequence and RotationInformationSequence for NM multiframe and I guess the list will grow longer as I work dig deeper and work with files from different modalities and vendors. At least, now I know that I will have to create that code myself and find the balance between flexible/slow vs limited/hardcoded/fast.

[mrbean-bremen]

True, but there should not be too many of these. Old style multi-frame classes have the tags in the dataset root, all the Enhanced XX classes are handled, and there should be very few multi-frame SOP classes that have some of the standard tags inside a sequence. I actually can't think of any besides NM Images, but I may miss some, of course.