Does an entire DICOM file has just one transfer syntax?

Question

Sorry if this is quite basic, I am new to DICOM. I know a DICOM file has multiple parts like: Patient, Study, Series and Instance (Image).

Now to communicate with a device it needs a Transfer Syntax, which tells the mode of communication, like Little-Endian, Big-Endian, JPEG-Lossless, lossy etc.

So, does each of the DICOM file parts (Patient, Study, Series and Instance (Image)) have their own transfer syntax? Like Patient can communicate as Little-Endian, Study might use JPEG-Lossless or MPEG-4 (if it is video) etc?

OR does the entire DICOM file just use one transfer syntax.

Answer 1

A single transfer syntax is used through all the entire DICOM file (except for the first group with ID=0002, which is written with low endian/explicit VR transfer syntax)

When sending DICOM messages through a network then you can have a different transfer syntax for each message: there you can define different Presentation Contexts during the association negotiation, and each Presentation Context can have a different Transfer Syntax.

After the association negotiation, you can transmit messages with different transfer syntaxes by selecting the proper presentation context/transfer identifier in the message header

Answer 2

Your question does not entirely make sense with how DICOM is organized.

DICOM is composed of various SOP Classes. A SOP Class is Service-Object-Pair. Example services are the Storage Service Class (a service for network storage of messages (typically modality images) or the Media Service Class (for writing of files to media or just saving them to disk).

The Object portion of the SOP Class is defined in an IOD (Information Object Definition). IODs are defined by multiple Modules. The Modules in turn are composed of DICOM tags. Each Module usually groups tags together, and are typically related to an "Entity" in the DICOM model. The module might be associated with the Patient, Series, or Image level of the DICOM model. The IOD consists of all the tags defined in the various Modules. When encoding the IOD, the context of the module that the tags are defined in doesn't matter.

The DICOM Service defines how the tags within an IOD are encoded. Both a DICOM message for network transfer services (in its Group 0x0000 elements) and a DICOM file for media (in its Group 0x0002 elements) contain meta data that describe the encoding and a data set which contains the IOD tags. The group 0x0000 elements in a DICOM Message are always encoded in Implicit VR Little Endian, and the Group 0x0002 elements in a DICOM file are always encoded in the Explicit VR Little Endian transfer syntax. The datasets are always encoded in a single transfer syntax.

Hope this helps a bit.