DICOM empty, "NULL" values

Hi all,

here is some evidence for the discussion about DICOM empty values and 
how many flavors there are:

It is important to note that DICOM is originally a binary format and the 
definition of a "NULL" value for elements that must be present, but can 
be empty is:
"if a Value for a Type 2 Data Element is unknown it can be encoded with 
zero Value Length and no Value"
(https://dicom.nema.org/medical/dicom/current/output/chtml/part05/sect_7.4.3.html)

If the value is not of Type 2, then the DICOM standard states: "in this 
case the standard explicitly specifies the validity of empty values in 
the list of Defined Terms for each value". Which means you find it 
somewhere in Pt. 3 of the standard, if it is applicable. 
(https://dicom.nema.org/dicom/2013/output/chtml/part05/sect_6.3.html)

This is the most common occurrence of "NULL" in DICOM and officially 
every VR (value representation) is treated like that and there is a 
single representation of "NULL" here.


Obviously, the DICOM standard does not use "empty string" as such, even 
though an empty string is typically implemented in such a way that is 
close to the above definition.

There is a kind of array in DICOM, which has a value multiplicity (VM) 
greater than 1 and in case of strings the backslash is used as 
delimiter. Encoding of empty values is not explicitly defined in the 
standard in this case, but there are some occurrences, where "empty" 
values in Code Strings can occur. So this would be an example of a 
string array with one empty value (between 1.0 and 3.0):
1.0\\3.0\4.0
see: 
https://dicom.nema.org/medical/dicom/current/output/html/part05.html#sect_6.4


There is another set of "NULL" flavors that originates from CDA/HL7 and 
is more or less adopted by DICOM for reports created from DICOM objects 
(https://dicom.nema.org/medical/dicom/current/output/chtml/part02/sect_7.7.html):
https://dicom.nema.org/MEDICAL/Dicom/current/output/chtml/part20/sect_5.3.2.html#:~:text=In%20HL7%20v3%2C%20a%20flavor,represent%20the%20lack%20of%20information.

If I understand that correctly, these are not native DICOM Null flavors, 
but they are from CDA/HL7 realm. And they occur when you transform a 
DICOM object to CDA.


To me it looks like that we have only to deal with one "native" DICOM 
"NULL".

Kind Regards

Detlef


-- 
Detlef Grittner, MSc ISM, M.A.
CTO

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