Transfer Syntax is the language used in DICOM to describe the DICOM file format and the network transfer methods. 3 main variables are contained in the Transfer Syntax:

• VR: Implicit/Explicit
• Endianism: Little-Endian/BigEndian
• Pixel Data Compression

Transfer Syntax is in the File Meta Header and more information about how to control the network transfer using Transfer Syntax can be found _here.

Here is a summary of the most common DICOM Transfer syntaxes:

Uncompressed

Implicit VR Little-endian

• Value : 1.2.840.10008.1.2
• Description: This is the only mandatory DICOM transfer syntax, but as the VR is implicit (requiring all applications to have an up to date dictionary to make sense of the data) it is also the worst, and others should be used wherever possible.

Explicit VR Little-endian

• Value : 1.2.840.10008.1.2.1
• Description : Explicit Value Representation must be included for each single DICOM tag. This transfer syntax is more often used since each data element has it's own explicit value type declaration.

Explicit VR Big-endian

• Value : 1.2.840.10008.1.2.2
• Description : This is identical to Explicit VR little endian, apart from the fact that big-endian byte ordering is used. It is very little used nowadays.

NOTE

All the compressed synatxes below use explicit VR little endian.

Lossless Compressed

Lossless methods are truly lossless, and the output is guaranteed to match the input exactly, and therefore there is no concept of quality.....the degree of compression depends only on the method used, and the nature of the image (noise compresses badly!)

JPEG Lossless

• Values : 1.2.840.10008.1.2.4.57 &
• Description : This is utterly unrelated to the "JPEG" methods listed above, despite the name! It is actually a very simple Pulse Code Modulation scheme, where each pixel is "Predicted" based on previous pixels, and then the much smaller "difference" is stored, after being compressed using Huffman coding.

JPEG Lossless First Order

• Value : 1.2.840.10008.1.2.4.70
• Description : Identical to the main JPEG lossless above, but with a constrained value for the predictor, giving a slightly simplified algorithm, with slightly greater speed, but slightly less compression on most images (2-5% typically)

RLE Lossless

• Value : 1.2.840.10008.1.2.5
• Description : The simplest form of compression technique which is widely supported by most bitmap file formats such as TIFF, BMP, and PCX. The content of the information greatly affects its efficiency in compressing the information, and whilst it is useful for images with large areas of zeros, it is almost useless for images such as radiographs where adjacent pixels rarely have exactly the same value.

JPEG 2000 (lossless)

• Value : 1.2.840.10008.1.2.4.90
• Description : Like JPEG lossless, the use of the name JPEG is confusing, as it bears no resemblance to other "JPEG" compression methods. JPEG 2000 (J2K) is a wavelet based algorithm with the advantage over all the multitude of proprietary wavelets of being standardised and interoperable. In lossless mode (as required for this transfer syntax) the degree of compression is course detemrined by the image itself, but it typically performs about 10% better than JPEG lossless. It is however a much more complicated algorithm, giving significantly slower performance, and unless space or bandwidth are very constrained (e.g. as they may be for teleradiology), the speed penalty is unlikely to make this a popular choice.

Lossy Compressed

JPEG Baseline

• Value : 1.2.840.10008.1.2.4.50
• Description : This provides a DICOM wrapper around "standard" JPEG content, which is equivalent to external JPEG files (though of course still as full DICOM content, rather than than industry standard JFIF header). Note that it is always lossy, irrespective of the quality factor used - so even an image compressed with "100% quality" is still not identical to the original. It is limited to to 8 bits per sample.

JPEG Extended

• Value : 1.2.840.10008.1.2.4.51
• Description : This is equivlent to JPEG baseline as above, but with extension to allow up to 12 bits per sample, allowing use with modalities which require "re-windowing" such as CT. Unlike baseline, this format is not used much (if at all) outside DICOM.

JPEG 2000 (lossy)

• Value : 1.2.840.10008.1.2.4.91
• Description : This is a variation on the JPEG 2000 lossless listed above, but permitting the image to be lossy (infact,m it is legal for it also to be lossless using this transfer syntax). In general, the same issues apply to this as to the lossless variation, but it does have a place in teleradiology etc. for two reasons:
  • It can (with suitable software) be used incrementally, so that a rough image is shown initially, improving as more data is recieved
  • For a given amount of data, the image is said to be better than using the best alternative lossy method (standard JPEG), but some people do have concerns about artifacts looking more "believable" (and therefore dangerous) than with JPEG, where they are easily noticable as "blockiness".