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 Datasets 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 (since v1.3) 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 test file ‘CT_small.dcm’:

>>> from pydicom import dcmread
>>> from pydicom.data import get_testdata_files
>>> ct_filename = get_testdata_files("CT_small")[0]
>>> ds = dcmread(ct_filename)
>>> ds
(0008, 0005) Specific Character Set              CS: 'ISO_IR 100'
(0008, 0008) Image Type                          CS: ['ORIGINAL', 'PRIMARY', 'AX
IAL']
...
...
(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 (since v1.3) provides convenient 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

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 convenient 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.