Private Data Elements¶
Accessing or creating private data elements
Introduction¶
The DICOM standard allows DICOM file creators to use private data elements to store information that is not defined by the DICOM standard itself.
Private data elements are stored in a Dataset just
like other data elements. When reading files with pydicom, they will automatically be read
and available for display. pydicom knows descriptive names for some
‘well-known’ private data elements, but for others it may not be able to
show anything except the tag and the value.
When writing your own private data elements, the DICOM standard requires the use of ‘private creator blocks’. pydicom has some convenience functions to make creating private blocks and data elements easier.
The sections below outlines accessing and creating private blocks and data elements using pydicom.
Displaying Private Data Elements in pydicom¶
Here is an example of some private tags displayed for pydicom’s example CT dataset:
>>> from pydicom import examples
>>> ds = examples.ct
>>> ds
Dataset.file_meta -------------------------------
(0002, 0000) File Meta Information Group Length UL: 192
(0002, 0001) File Meta Information Version OB: b'\x00\x01'
(0002, 0002) Media Storage SOP Class UID UI: CT Image Storage
(0002, 0003) Media Storage SOP Instance UID UI: 1.3.6.1.4.1.5962.1.1.1.1.1.20040119072730.12322
(0002, 0010) Transfer Syntax UID UI: Explicit VR Little Endian
(0002, 0012) Implementation Class UID UI: 1.3.6.1.4.1.5962.2
(0002, 0013) Implementation Version Name SH: 'DCTOOL100'
(0002, 0016) Source Application Entity Title AE: 'CLUNIE1'
-------------------------------------------------
(0008, 0005) Specific Character Set CS: 'ISO_IR 100'
(0008, 0008) Image Type CS: ['ORIGINAL', 'PRIMARY', 'AXIAL']
...
(0009, 0010) Private Creator LO: 'GEMS_IDEN_01'
(0009, 1001) [Full fidelity] LO: 'GE_GENESIS_FF'
(0009, 1002) [Suite id] SH: 'CT01'
...
The last two lines in the example above show pydicom’s display of two private data elements. The line preceding those shows the private creator data element that reserves a section of tag element numbers for that creator’s use.
Since the descriptions for private data elements are not part of the DICOM standard, and are thus not necessarily unique, pydicom does not allow you to access data elements using those names. This is indicated by enclosing the text in square brackets, to make it clear it is different from DICOM standard descriptors.
You can still access the private data elements using the tag, remembering that
data elements access by tag number return a full DataElement
instance, and the value attribute is needed to get the value:
>>> ds[0x00091001].value
'GE_GENESIS_FF'
You can also create a PrivateBlock instance and access elements
through it:
>>> block = ds.private_block(0x0009, 'GEMS_IDEN_01')
>>> block[0x01]
(0009, 1001) [Full fidelity] LO: 'GE_GENESIS_FF'
>>> block[0x01].value
'GE_GENESIS_FF'
Using the private block like this is even more useful when creating your own private data elements, as shown in the next section.
Setting Private Data Elements with pydicom¶
The DICOM standard requires a private creator data element to identify and reserve a section of private tags. That name should be unique, and usually has the company name as the first part to accomplish that. pydicom provides convenience functions to manage this:
>>> block = ds.private_block(0x000b, "My company 001", create=True)
>>> block.add_new(0x01, "SH", "my value")
>>> ds
...
(000b, 0010) Private Creator LO: 'My company 001'
(000b, 1001) Private tag data SH: 'my value'
...
Standard Python operations like in and del can also be used when working
with block object:
>>> 0x01 in block
True
>>> 0x02 in block
False
>>> del block[0x01]
>>> 0x01 in block
False
Since v3.0, there’s also a convenience method to add a private tag without creating a private block first:
>>> block = ds.add_new_private("My company 001", 0x000B, 0x01, "my value", VR.SH)
>>> ds
...
(000b, 0010) Private Creator LO: 'My company 001'
(000b, 1001) Private tag data SH: 'my value'
...
Note that for known private tags you don’t need to provide the VR in this function.
Removing All Private Data Elements¶
One part of anonymizing a DICOM file is to ensure that private data elements
have been removed, as there is no guarantee as to what kind of information
might be contained in them. pydicom provides a convenience function
Dataset.remove_private_tags() to recursively remove private elements:
>>> ds.remove_private_tags()
This can also be helpful during interactive sessions when exploring DICOM files, to remove a large number of lines from the display of a dataset – lines which may not provide useful information.
Adding new entries to the DICOM dictionary¶
pydicom contains a dictionary with all known DICOM tags from the latest DICOM standard at release time. It also contains a dictionary with a number of known private tags collected from various sources. Sometimes you may encounter tags unknown to pydicom - either tags defined in a newer version of the standard, or, the more common case, private tags that are not contained in the private tags dictionary.
In this case, you can add these tags to the DICOM dictionary before reading or writing datasets containing these tags. After that, pydicom will correctly handle the type of these tags, and can display their description if needed.
For standard tags, you can use add_dict_entry() or
add_dict_entries() (to add multiple tags at once):
>>> add_dict_entry(tag=0x888800001, VR="SH", keyword="SomeNewTag", description="Some New Tag")
For private tags, the analogous functions are
add_private_dict_entry() and add_private_dict_entries():
>>> add_private_dict_entry(private_creator="ACME 1.1", tag=0x004100001, VR="DA", description="Release Date")
Note that private tags do not have a keyword, as they are not registered in the standard DICOM data dictionary. As a private tag is defined by the tuple of private creator, group ID and tag offset, you always have to provide the private creator to define a new private tag.
An example of how to use add_private_dict_entries() can
be found in this code snippet.