DICOM File-sets and DICOMDIR

This tutorial is about DICOM File-sets and covers:

  • An introduction to DICOM File-sets and the DICOMDIR file

  • Loading a File-set using the FileSet class and accessing its managed SOP instances

  • Creating a new File-set and modifying existing ones

It’s assumed that you’re already familiar with the dataset basics.


The DICOM File-set

A File-set is a collection of DICOM files that share a common naming space. Most people have probably interacted with a File-set without being aware of it; one place they’re frequently used is on the CDs/DVDs containing DICOM data that are given to a patient after a medical procedure (such as an MR or ultrasound).

The specification for File-sets is given in Part 10 of the DICOM Standard.



Despite its name, a DICOMDIR file is not a file system directory and can be read using dcmread() like any other DICOM dataset.

Every File-set must contain a single file with the filename DICOMDIR, the location of which is dependent on the type of media used to store the File-set. For the most commonly used media (DVD, CD, USB, PC file system, etc), the DICOMDIR file will be in the root directory of the File-set. For other media types, Part 12 of the DICOM Standard specifies where the DICOMDIR must be located.


It’s strongly recommended that you avoid making changes to a DICOMDIR dataset directly unless you know what you’re doing. Even minor changes may require recalculating the offsets for each directory record. Use the FileSet methods (see below) instead.

The DICOMDIR file is used to summarize the contents of the File-set and is a Media Storage Directory instance that follows the Basic Directory IOD.

>>> from pydicom import examples
>>> ds = examples.dicomdir
>>> ds.file_meta.MediaStorageSOPClassUID.name
'Media Storage Directory Storage'

The most important element in a DICOMDIR is the (0004,1220) Directory Record Sequence; each item in the sequence is a directory record, and one or more records are used to briefly describe an available SOP Instance and its location within the File-set’s directory structure. Each record has a record type given by the (0004,1430) Directory Record Type element, and different records are related to each other using the hierarchy given in Table F.4-1.

>>> print(ds.DirectoryRecordSequence[0])
(0004, 1400) Offset of the Next Directory Record UL: 3126
(0004, 1410) Record In-use Flag                  US: 65535
(0004, 1420) Offset of Referenced Lower-Level Di UL: 510
(0004, 1430) Directory Record Type               CS: 'PATIENT'
(0008, 0005) Specific Character Set              CS: 'ISO_IR 100'
(0010, 0010) Patient's Name                      PN: 'Doe^Archibald'
(0010, 0020) Patient ID                          LO: '77654033'

Here we have a 'PATIENT' record, which from Table F.5-1 we see must also contain Patient’s Name and Patient ID elements. The full list of available record types and their requirements is in Annex F.5 of Part 3 of the DICOM Standard.


While it’s possible to access everything within a File-set using the DICOMDIR dataset, making changes to an existing File-set quickly becomes complicated due to the need to add and remove directory records, recalculate the byte offsets for existing records and manage the corresponding file system changes. A more user-friendly way to interact with one is via the FileSet class.

Loading existing File-sets

To load an existing File-set just pass a DICOMDIR Dataset or the path to the DICOMDIR file to FileSet:

>>> from pydicom import dcmread
>>> from pydicom.fileset import FileSet
>>> path = examples.get_path("dicomdir")  # The path to the examples.dicomdir dataset
>>> ds = dcmread(path)
>>> fs = FileSet(ds)  # or FileSet(path)

An overview of the File-set’s contents is shown when printing:

>>> print(fs)
DICOM File-set
  Root directory: /home/user/env/lib/python3.7/site-packages/pydicom/data/test_files/dicomdirtests
  File-set UID:
  Descriptor file ID: (no value available)
  Descriptor file character set: (no value available)
  Changes staged for write(): DICOMDIR update, directory structure update

  Managed instances:
    PATIENT: PatientID='77654033', PatientName='Doe^Archibald'
      STUDY: StudyDate=20010101, StudyTime=000000, StudyDescription='XR C Spine Comp Min 4 Views'
        SERIES: Modality=CR, SeriesNumber=1
          IMAGE: 1 SOP Instance
        SERIES: Modality=CR, SeriesNumber=2
          IMAGE: 1 SOP Instance
        SERIES: Modality=CR, SeriesNumber=3
          IMAGE: 1 SOP Instance
      STUDY: StudyDate=19950903, StudyTime=173032, StudyDescription='CT, HEAD/BRAIN WO CONTRAST'
        SERIES: Modality=CT, SeriesNumber=2
          IMAGE: 4 SOP Instances
    PATIENT: PatientID='98890234', PatientName='Doe^Peter'
      STUDY: StudyDate=20010101, StudyTime=000000
        SERIES: Modality=CT, SeriesNumber=4
          IMAGE: 2 SOP Instances
        SERIES: Modality=CT, SeriesNumber=5
          IMAGE: 5 SOP Instances
      STUDY: StudyDate=20030505, StudyTime=050743, StudyDescription='Carotids'
        SERIES: Modality=MR, SeriesNumber=1
          IMAGE: 1 SOP Instance
        SERIES: Modality=MR, SeriesNumber=2
          IMAGE: 1 SOP Instance
      STUDY: StudyDate=20030505, StudyTime=025109, StudyDescription='Brain'
        SERIES: Modality=MR, SeriesNumber=1
          IMAGE: 1 SOP Instance
        SERIES: Modality=MR, SeriesNumber=2
          IMAGE: 3 SOP Instances
      STUDY: StudyDate=20030505, StudyTime=045357, StudyDescription='Brain-MRA'
        SERIES: Modality=MR, SeriesNumber=1
          IMAGE: 1 SOP Instance
        SERIES: Modality=MR, SeriesNumber=2
          IMAGE: 3 SOP Instances
        SERIES: Modality=MR, SeriesNumber=700
          IMAGE: 7 SOP Instances

The FileSet class treats a File-set as a flat collection of SOP Instances, abstracting away the need to dig down into the hierarchy like you would with a DICOMDIR dataset. For example, iterating over the FileSet yields a FileInstance object for each of the managed instances.

>>> for instance in fs:
...     print(instance.PatientName)
...     break

A list of unique element values within the File-set can be found using the find_values() method, which by default searches the corresponding DICOMDIR records:

>>> fs.find_values("PatientID")
['77654033', '98890234']

The search can be expanded to the File-set’s managed instances by supplying the load parameter, at the cost of a longer search time due to having to read and decode the corresponding files:

>>> fs.find_values("PhotometricInterpretation")
>>> fs.find_values("PhotometricInterpretation", load=True)

More importantly, the File-set can be searched to find instances matching a query using the find() method, which returns a list of FileInstance. The corresponding file can then be read and decoded using FileInstance.load(), returning it as a FileDataset:

>>> for instance in fs.find(PatientID='77654033'):
...     ds = instance.load()
...     print(ds.PhotometricInterpretation)

find() also supports the use of the load parameter:

>>> len(fs.find(PatientID='77654033', PhotometricInterpretation='MONOCHROME1'))
>>> len(fs.find(PatientID='77654033', PhotometricInterpretation='MONOCHROME1', load=True))

Creating a new File-set

You can create a new File-set by creating a new FileSet instance:

>>> fs = FileSet()

This will create a completely conformant File-set, however it won’t contain any SOP instances. Since empty File-sets aren’t very useful, our next step will be to add some SOP instances to it.

Modifying a File-set

FileSet and staging

Before we go any further we need to discuss how the FileSet class manages changes to the File-set. Modifications to the File-set are first staged, which means that although the FileSet instance behaves as though you’ve applied them, nothing will actually change on the file system itself until you explicitly call FileSet.write(). This includes changes such as:

You can tell if changes are staged with the is_staged property:

>>> fs.is_staged

You may also have noticed this line in the print(fs) output shown above:

Changes staged for write(): DICOMDIR update, directory structure update

This appears when the FileSet is staged and will contain at least one of the following:

  • DICOMDIR update or DICOMDIR creation: the DICOMDIR file will be updated or created

  • directory structure update: one or more of the SOP instances in the existing File-set will be moved over to use the pydicom File-set directory structure

  • N additions: N SOP instances will be added to the File-set

  • M removals: M SOP instances will be removed from the File-set

Adding SOP instances

The simplest way to add new SOP instances to the File-set is with the add() method, which takes the path to the instance or the instance itself as a Dataset and returns the addition as a FileInstance.

To reduce memory usage, instances staged for addition are written to a temporary directory and only copied to the File-set itself when write() is called. However, they can still be accessed and loaded:

>>> instance = fs.add(examples.ct)
>>> instance.is_staged
>>> instance.for_addition
>>> instance.path
>>> type(instance.load())
<class 'pydicom.dataset.FileDataset'>

Alternatively, if you want more control over the directory records that will be added to the DICOMDIR file, or if you need to use PRIVATE records, you can use the add_custom() method.

The add() method uses pydicom’s default directory record creation functions to create the necessary records based on the SOP instance’s attributes, such as SOP Class UID and Modality. Occasionally, they may fail when an element required by these functions is empty or missing:

>>> rt_dose = examples.rt_dose
>>> fs.add(rt_dose)
Traceback (most recent call last):
  File ".../pydicom/fileset.py", line 1858, in _recordify
    record = DIRECTORY_RECORDERS[record_type](ds)
  File ".../pydicom/fileset.py", line 2338, in _define_rt_dose
    _check_dataset(ds, ["InstanceNumber", "DoseSummationType"])
  File ".../pydicom/fileset.py", line 2281, in _check_dataset
    raise ValueError(
ValueError: The instance's (0020, 0013) 'Instance Number' element cannot be empty

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../pydicom/fileset.py", line 1039, in add
    record = next(record_gen)
  File ".../pydicom/fileset.py", line 1860, in _recordify
    raise ValueError(
ValueError: Unable to use the default 'RT DOSE' record creator as the instance is missing a required element or value. Either update the instance, define your own record creation function or use 'FileSet.add_custom()' instead

When this occurs, there are three options:

  • Update the instance to include the required element and/or value

  • Override the default record creation functions with your own by modifying DIRECTORY_RECORDERS

  • Use the add_custom() method

According to the exception message above, the Instance Number element is empty. Let’s update the instance and try adding it again:

>>> rt_dose.InstanceNumber = "1"
>>> fs.add(rt_dose)

Removing instances

SOP instances can be removed from the File-set with the remove() method, which takes the FileInstance or list of FileInstance to be removed:

>>> len(fs)
>>> instances = fs.find(PatientID="1CT1")
>>> len(instances)
>>> fs.remove(instances)
>>> len(fs)

Applying the changes

Let’s add a couple of SOP instances back to the File-set:

>>> fs.add(examples.ct)
>>> fs.add(examples.mr)

To apply the changes we’ve made to the File-set we use write(). For new File-sets, we have to supply the path where the File-set root directory will be located:

>>> from pathlib import Path
>>> from tempfile import TemporaryDirectory
>>> t = TemporaryDirectory()
>>> t.name
>>> fs.write(t.name)
>>> fs.is_staged
>>> root = Path(t.name)
>>> for path in sorted([p for p in root.glob('**/*') if p.is_file()]):
...     print(path)

The root directory for existing File-sets cannot be changed, so for those you only need to call write() without any arguments:

>>> instances = fs.find(PatientID="1CT1")
>>> fs.remove(instances)
>>> fs.write()
>>> for path in sorted([p for p in root.glob('**/*') if p.is_file()]):
...     print(path)

For existing File-sets that don’t use the same directory structure semantics as FileSet, calling write() will move SOP instances over to the new structure. However, if the only modification you’ve made is to remove SOP instances or change ID, UID, descriptor_file_id, or descriptor_character_set, then you can pass the use_existing keyword parameter to keep the existing directory structure and update the DICOMDIR file.

First, we need to copy the existing example File-set to a temporary directory so we don’t accidentally modify it:

>>> from shutil import copytree, copyfile
>>> t = TemporaryDirectory()
>>> dst = Path(t.name)
>>> src = examples.get_path("dicomdir").parent
>>> copyfile(src / "DICOMDIR", dst / "DICOMDIR")
>>> copytree(src / "77654033", dst / "77654033")
>>> copytree(src / "98892001", dst / "98892001")
>>> copytree(src / "98892003", dst / "98892003")

Now we load the File-set from the temporary directory, remove instances and write out the changes with use_existing to keep the current directory structure:

>>> fs = FileSet(dst / "DICOMDIR")
>>> instances = fs.find(PatientID="98890234")
>>> fs.remove(instances)
>>> fs.write(use_existing=True)  # Keep the current directory structure
>>> for path in sorted([p for p in dst.glob('**/*') if p.is_file()]):
...     print(path)

If you’d just called write() without use_existing, then it would’ve moved the SOP instances to the new directory structure:

>>> fs.write()
>>> for path in sorted([p for p in dst.glob('**/*') if p.is_file()]):
...     print(path)


In this tutorial you’ve learned about DICOM File-sets and the DICOMDIR file. You should now be able to use the FileSet class to create new File-sets, and to load, search and modify existing ones.