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1 .. _orthanc-ids:
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2
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3 Orthanc identifiers
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4 ===================
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5
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6 In Orthanc, each patient, study, series and instance is assigned with
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7 an unique identifier that is derived from the DICOM identifiers.
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8 Contrarily to the :ref:`identifiers of the DICOM standard
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9 <dicom-identifiers>`, the Orthanc identifiers are formatted as a
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10 `SHA-1 hash <http://en.wikipedia.org/wiki/Sha-1>`__ with a fixed
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11 length, so as to be more Web-friendly. More specifically:
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12
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13 * Patients are identified as the SHA-1 hash of their PatientID tag
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14 (0010,0020).
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15 * Studies are identified as the SHA-1 hash of the concatenation of
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16 their PatientID tag (0010,0020) and their StudyInstanceUID tag
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17 (0020,000d).
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18 * Series are identified as the SHA-1 hash of the concatenation of
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19 their PatientID tag (0010,0020), their StudyInstanceUID tag
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20 (0020,000d) and their SeriesInstanceUID tag (0020,000e).
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21 * Instances are identified as the SHA-1 hash of the concatenation of
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22 their PatientID tag (0010,0020), their StudyInstanceUID tag
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23 (0020,000d), their SeriesInstanceUID tag (0020,000e), and their
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24 SOPInstanceUID tag (0008,0018).
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25
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26 Because the DICOM standard guarantees the StudyInstanceUID,
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27 SeriesInstanceUID and SOPInstanceUID tags to be globally unique, the
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28 Orthanc identifiers for studies, series and instances are also
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29 globally unique (provided no hash collision occurs, which is highly
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30 improbable).
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31
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32 The patient-level identifiers are *not* guaranteed to be globally
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33 unique, and might collide between different hospitals. For this
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34 reason, you should **always do queries at the study level** as soon as
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35 you deal with an application that handles patients from different
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36 hospitals.
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37
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38 The actual implementation of the hashing is carried on by the
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39 `DicomInstanceHasher class
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40 <https://bitbucket.org/sjodogne/orthanc/src/default/Core/DicomFormat/DicomInstanceHasher.cpp>`_.
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41
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42
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43 The "Inexistent Tag" error
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44 --------------------------
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45
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46 If you use an old version of Orthanc (< 0.7.4) and you receive the
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47 "*Exception while storing DICOM: Inexistent tag*" error while storing
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48 a DICOM instance into Orthanc, please make sure that all the 4
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49 following tags do exist in the DICOM file:
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50
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51 * PatientID (0010,0020),
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52 * StudyInstanceUID (0020,000d),
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53 * SeriesInstanceUID (0020,000e),
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54 * SOPInstanceUID (0008,0018).
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55
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56 These tags are all used to index the incoming DICOM instances. The
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57 error message is more explicit starting with Orthanc 0.7.4.
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