Understanding the complexity of encoding and compression in DICOM communication
DICOM transfer syntax is a set of rules determining how DICOM data is packaged and transmitted between systems. DICOM transfer syntax ensures that both sender and receiver correctly interpret the data. DICOM transfer syntax encodes DICOM data into a stream of bytes for transmission.
DICOM transfer syntax can be either implicit or explicit. The transfer syntax can be negotiated between DICOM nodes during connection establishment.
Explicit DICOM transfer syntax uses a well-defined set of rules that are easy to interpret, while implicit DICOM transfer syntax leaves some interpretation up to the receiving system.
There are many different DICOM transfer syntaxes available, each with its own advantages and disadvantages. Some DICOM transfer syntaxes are more efficient than others, while some DICOM transfer syntaxes are more universally supported.
For more advanced PACS analysts, DICOM transfer syntax selection is an important decision that should be made carefully based on the specific needs of the DICOM system in question. Especially during initial implementation of a new DICOM application that must send and receive to existing DICOM systems.
If you’re just starting with DICOM, you might wonder what all this talk about DICOM transfer syntaxes is about. Don’t worry. We’re here to help. In this module, we’ll give you a brief introduction to DICOM transfer syntaxes. This is a slightly more advanced topic for beginners. So we will be using as simple terms as possible.
By the end of this article, you should better understand what DICOM transfer syntaxes are and how they work. Plus if you’re a prospective PACS admin, hopefully strengthen your chances of becoming one.
What Is A Dicom Transfer Syntax?
What is a DICOM transfer syntax? Simply put, it is a set of rules that dictate how DICOM files are encoded and transmitted. These rules govern everything from the file’s structure to the way in which data is compressed.
Several different DICOM transfer syntaxes exist, each of which has its own advantages and disadvantages. In this module, we’ll take a look at some of the most popular DICOM transfer syntaxes and discuss their key features.
So, without further ado, let’s get started!
DICOM Transfer Syntax #01: Explicit VR Little Endian
Transfer Syntax UID 1.2.840.10008.1.2.1
This is one of the most commonly used DICOM transfer syntaxes. As the name suggests, it uses little-endian byte ordering, and each DICOM element is explicitly labeled with a VR (Value Representation) tag.
One of the key advantages of this transfer syntax is that it is relatively simple to understand and implement. However, one downside is that DICOM files encoded using this syntax can be quite large, as each DICOM element is fully labeled.
DICOM Transfer Syntax #02: Implicit VR Little Endian
Transfer Syntax UID 1.2.840.10008.1.2
Implicit VR Little Endian is the default transfer syntax that should be available on all DICOM systems. This DICOM transfer syntax is very similar to the Explicit VR Little Endian syntax, with one key difference: DICOM elements are not explicitly labeled with a VR tag. Thus the destination application entity must reference against an internal DICOM data library.
This advantage is that DICOM files encoded using this syntax are typically smaller than those encoded using the Explicit VR Little Endian syntax. However, the downside is that it can be more difficult to understand and implement, as DICOM elements are not explicitly labeled.
DICOM Transfer Syntax #03: Explicit VR Big Endian
Transfer Syntax UID 1.2.840.10008.1.2.2
This DICOM transfer syntax is the opposite of the Explicit VR Little Endian syntax in that it uses big-endian byte ordering, and each DICOM element is explicitly labeled with a VR tag.
One advantage of this transfer syntax is that DICOM files encoded using this syntax are typically easier to read and understand, as the DICOM element labels are more clearly visible. However, one downside is that DICOM files encoded using this syntax can be quite large, as each DICOM element is fully labeled. As this is an older transfer syntax, some DICOM applications no longer support it.
DICOM Transfer Syntax #04: JPEG 2000 Image Compression
Transfer Syntax UID 1.2.840.10008.1.2.4.91
This DICOM transfer syntax uses JPEG 2000 image compression, which is a state-of-the-art compression algorithm. DICOM files encoded using this syntax are typically much smaller than those encoded using other transfer syntaxes, making them more suitable for storage and transmission.
However, one downside of this transfer syntax is that not all DICOM systems accept this encoding. Especially older systems.
DICOM Transfer Syntax #05: RLE Image Compression
Transfer Syntax UID 1.2.840.10008.1.2.5
This DICOM transfer syntax uses RLE (Run-Length Encoding) image compression, which is a relatively simple compression algorithm.
However, one downside of this transfer syntax is that it can be less effective at compressing images, resulting in larger file sizes.
DICOM Transfer Syntax #06: Deflate Image Compression
Transfer Syntax UID 1.2.840.10008.1.2.1.99
This DICOM transfer syntax uses Deflate image compression, which is a lossless compression algorithm. Deflate image compression also compresses the DICOM data in addition to the image. Helpful for compressing non-image DICOM files.
The downside of this transfer syntax is for DICOM files with pixel data for images. It does not compress as well as other transfer syntaxes such as JPEG Lossless or JPG EG 2000.
DICOM Transfer Syntax #07: JPEG Lossless, Non-hierarchical (Process 14)
Transfer Syntax UID 1.2.840.10008.1.2.4.57
This DICOM transfer syntax uses JPEG Lossless compression, which is a lossless compression algorithm. In lossless compression, data is not lost in the process.
A downside of this transfer syntax is that it can be more difficult to understand and implement, as it uses a more complex compression algorithm.
What is an association negotiation?
The association negotiation is the first process in a DICOM connection between two application entities (two DICOM systems). In the association negotiation, the method of available communication is presented by each application entity. From here, both systems will agree on the transfer syntax and abstract syntax that will be used. Hence, association negotiation.
What is the abstract syntax in Dicom?
The DICOM abstract syntax is a formal description of the DICOM data model. It defines the structure and content of DICOM files and allows DICOM files to be unambiguously interpreted by computers.
The abstract syntax is a DICOM element that contains a UID (unique identifier) that identifies the specific DICOM file format known as the SOP class.
What is the presentation context in Dicom?
The presentation context in DICOM occurs during an association negotiation. The presentation context defines what information is exchanged and what transfer syntax will be used. The information exchanged is called the abstract syntax as defined previously.
What is the DICOM file format?
The DICOM file format is a standard file format for storing and transmitting medical images. DICOM files can be stored in JPEG, PNG or other formats and can be compressed using either lossless or lossy compression.
DICOM transfer syntaxes are a standard way of compressing and storing medical images. There are several different DICOM transfer syntaxes, each with its own advantages and disadvantages.
In this module, we have looked at seven common DICOM transfer syntaxes and discussed both benefits and drawbacks of each one. The most appropriate DICOM transfer syntax will ultimately depend on the specific needs and requirements of the system. And the ability for interconnected DICOM applications to also support the same transfer syntax.
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