Default DICOM encoding without Specific Character Set

Question

If a DICOM file does not define a Specific Character Set (0008,0005), what character set does it use by default? Is ASCII the default encoding for DICOM files?

TL;DR

A DICOM file contains German ä in one of the tags, but the file does not specify any character set. I assume that in this case the file is allowed to contain only ASCII symbols (the default character set) and report this file as invalid. Before I submit my change, I want to make sure that I understood DICOM correctly.

Answer 1

As specified in the Dicom Data Structures and Encoding

6.1.2.5.4 Levels of Implementation and Initial Designation

a) Attribute Specific Character Set (0008,0005) not present:

7-bit code Implementation level: ISO 2022 Level 1 - Elementary 7-bit code (code-level identifier 1)

Initial designation: ISO-IR 6 (ASCII) as G0. Code Extension shall not be used

Reference:

• http://dicom.nema.org/medical/dicom/current/output/chtml/part05/chapter_6.html#sect_6.1.2.5.4

Answer 2

To add to answer by JonnyQ, DICOM standard also defines mechanisms when confronted with character sets that are unknown to implementations or unsupported (see PS 3.5 section 6.1.2.3). Implementations can print or display such characters by replacing all unknown characters with the four characters "\nnn", where "nnn" is the three digit octal representation of each byte.

An example given in the standard for an ASCII based machine as follows:

Character String: Günther

Encoded representation: 04/07 15/12 06/14 07/04 06/08 06/05 07/02

ASCII based machine: G\374nther

Implementations may also encounter Control Characters which they have no means to print or display. Application may print or display such Control Characters by replacing the Control Character with the four characters “\nnn”, where “nnn” is the three digit octal representation of each byte.