# How XNAT Scans DICOM to Map to Project/Subject/Session

XNAT gathers information from the DICOM header fields to retrieve important metadata that can be used to automatically associate the objects with the appropriate project, subject and session. Because these fields are used differently under various conditions, XNAT makes several passes over the headers to gather the required information. If sending data from the scanner console, attempts should be made to satisfy the first or second pass rules. From tools like XNAT’s DICOM Browser and DICOM Batch Editor, the third or fourth pass rules may be easier.

First Pass

In the first pass over the DICOM header content, XNAT looks for all relevant metadata in the Patient Comments field (0010,4000). This is a freeform field that technicians can populate at the console. On Siemens’ Syngo system, the field can be populated from the “Additional Information” box below the patient information (name, date of birth, etc) section. The metadata should be entered as follows:

Project:Project_ID (or Project Alias will work as well) Subject:Subject_ID Session:Session_ID

Eg:  Project:BobsProj Subject:subj001 Session:subj001_MR1

• To force the bypass the prearchive, add “AA:TRUE” in this field as well.
• The metadata fields should be separated by whitespace (as above), commas, or semi-colons. All other content is ignored. The metadata values should have no spaces or special characters in them (underscores are ok).
• Project_ID should match an existing project in the XNAT host to which the images are being sent.
• If one or more of the Project, Subject or Session fields are found, then no further passes are necessary. Otherwise, XNAT continues with its second pass.

Second Pass

Same as above, except using the Study Comments (0032,4000) field. This field is another freeform field entered at the Syngo console under “Study comments”

Third Pass

In the third pass, XNAT looks for each metadata field in individual DICOM fields, as below:

DICOM Tag

Tag name

XNAT Field
(0008,1030)Study DescriptionProject ID
(0010,0010)Patient NameSubject Label
(0010,0020)Patient IDSession Label

Fourth Pass

Same as above, except Accession Number (0008,0050) is used as Project ID.

Fifth Pass

There is no fifth pass. If after the four passes described above, XNAT has not satisfactorily matched or identified the metadata, it leaves these fields marked as invalid in the generated XML document. If no Project was identified, the DICOM files will be placed in the “Unidentified” prearchive box. If Project was identified, but Subject and/or Session was not, the DICOM files will be placed in the appropriate project prearchive box. However, the automatic matching tool will not be functional, and additional data entry will be required.