Dear sir/madam,
I am trying to understand the concepts behind DICOM tag data. It is highly appreciated if someone could explain following structure of tag data.Further I am confused whether these tags can be repeated.
(gggg,eeee) means Group and Element in hexadecimal format.
Then follow two capital letters, meaning the data-type. For example US is an unsigned short integer and SQ is a sequence.
The above Group/Element is also called a tag.
Then follows the data-length, and the data themselves.
In the header-dump you posted, e.g.:
(0008,0008) CS ORIGINAL\PRIMARY\AXIAL # 3, 22 Image Type
gggg eeee datatype data # ?, data-length GroupElementDictionaryName
The ? is probably how many back-slash separated values are in the data, but I'm not sure.
Yes, tags can be repeated. This is done in sequences. If you are familiar with C: a sequence is comparable to a struct array. They can be nested.
Lambert Zijp ,Thank you so much, and because of tag repeating in sequences, a specific tag can two values. (according to the file attached (0028,0010) have 128 and 512) Will this need to be considered when designing an engine to render DICOM images.
Tharika Weerakoon, you could have a look at the following discussion:
https://www.researchgate.net/post/What_is_DICOM_file_format
which includes some details about DICOM parsing.
There are indeed two blocks of tags that describe and contain the pixeldata. The last block (512 x 512 x 2 = 524288) is the CT slice. The first block is put in a 'private' sequence. Odd Groups are not part of the dicom standard; they can be used to put whatever you want. In this case, Toshiba has put there a jpeg; probably a thumbnail of the slice.
Because 128 x 128 x 2 is much more than 5310.
And if you look at the first bytes of the private pixeldata...
D8 FF E0 FF 10 00 46 4A 46 49 01 00 00 01 01 00
... and byte swap them...
FF D8 FF E0 00 10 4A 46 49 46 00 01 01 00 00 01
... you can recognize the jpeg signature.
Lambert Zijp and Olaf Dietrich thank you so much for these insights.
Tags can indeed be repeated in some circumstances. For example, this is commonly done in DICOM Structured Reports using code sequences. The overall structure of DICOM files is similar in many respects to XML and the binary encoding is quite similar to TIFF. "Digital Imaging and Communications in Medicine (DICOM): A Practical Introduction and Survival Guide" by Oleg Pianykh provides a helpful introduction to the topic.
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