6.2.3.9. Lossy Image Compression (0028,2110)

Compression, in general, aims to reduce file size, with or without decreasing data quality of the original file. DICOM is a medical imaging file standard used to store multiple information such as information of the image itself (resolution, pixel data, bit allocation, compression type, etc.) . With the rising usage of imaging in clinical diagnosis, there is a need for a fast and secure method to share a large number of images between practitioners, and compression is used to facilitate interoperability.

The main purpose of compression techniques is memory efficiency, fast compression, and the generation of the best output. It can be divided into two types, lossless compression and lossy compression. Lossless compression is a type of data compression which does not remove any information from the initial data, while the lossy compression removes some of the information from the initial data.

Compression of image may compromise the diagnostic value of photographs and is therefore not the preferred method of storage. The main reasons to utilize compression in the past were related to storage costs and bandwidth speed, which, over time, have increased by orders of magnitude. In addition, each image is composed of single relatively small file (less then 20MB).

According to DICOM :

The Attribute Lossy Image Compression (0028,2110) conveys that the Image has undergone lossy compression. It provides a means to record that the Image has been compressed (at a point in its lifetime) with a lossy algorithm and changes have been introduced into the pixel data. Once the value has been set to “01”, it shall not be reset.

Note

If an image is compressed with a lossy algorithm, the Attribute Lossy Image Compression (0028,2110) is set to “01”. Subsequently, if the image is decompressed and transferred in uncompressed format, this Attribute value remains “01”. The value of Lossy Image Compression (0028,2110) in SOP Instances containing multiple frames in which one or more of the frames have undergone lossy compression shall be “01”.

For orthodontic purposes, this tag should be used, because of the varied nature of photographic capture equipment. Some cameras are configured to store images in raw and uncompressed format, others will save a compressed JPEG image. In order to correctly encode this information, the following three DICOM tags should be used.

In an orthodontic setting, the following use cases are to be considered:

	Use Case	(0028,2110)	(0028,2112)	(0028,2114)
0 1	Photograph was acquired with a camera which saves the images in RAW uncompressed format, or a lossless compression method. For example: TIFF, RAW or PNG.	00	not present	not present
0 2	Photograph was acquired with a camera which saved the image using a lossy compression scheme, for example JPEG.	01	For a 30:1 compression ratio, set to 30. If unknown, do not add this tag.	See Lossy IMage Compression Method below.
0 3	Photograph was acquired with a camera using an unknown type of image format, and then converted into DICOM.	If image is in a lossy compression format like JPEG, use 01 here and follow same indications as above use case 02. Otherwise don’t specify this tag at all.	If image is in a lossy compression format like JPEG, follow same indications as above use case 02. Otherwise don’t specify this tag at all.	If image is in a lossy compression format like JPEG, follow same indications as above use case 02. Otherwise don’t specify this tag at all.
0 4	An existing DICOM image was modified and compressed with a lossy algorithm.	01	For a 30:1 compression ratio, set to 30. If unknown, do not add this tag.	See Lossy IMage Compression Method below.
0 5	Photograph was taken using an analog camera, and image was then converted into digital using a scanner.	Use SC Secondary Capture IOD. Then set this tag following the same guidelines as use case 01 or 02 above, s ubstituting the camera with whatever imaging device was used to acquire or scan the analog photograph or slide.	Use SC Secondary Capture IOD. Then set this tag following the same guidelines as use case 01 or 02 above, s ubstituting the camera with whatever imaging device was used to acquire or scan the analog photograph or slide.	Use SC Secondary Capture IOD. Then set this tag following the same guidelines as use case 01 or 02 above, s ubstituting the camera with whatever imaging device was used to acquire or scan the analog photograph or slide.

6.2.3.9.1. Lossy Image Compression Attribute (0028,2110)

Set to the string "00" if image is not compressed. Otherwise set to the string "01".

In order to determine if an image is to be considered compressed or not, one needs to be familiar with the algorithm used to store the image. As an example, all forms of JPEG images are considered lossy compression and should thus be tagged as "01".

6.2.3.9.2. Lossy Image Compression Ratio (0028,2112)

6.2.3.9.3. Lossy Image Compression Method (0028,2114)

Check DICOM Standard for most current defined terms.

At time of writing, allowed terms were:

• ISO_10918_1 for JPEG Lossy Compression [ISO/IEC 10918-1],

• ISO_14495_1 for JPEG-LS Near-lossless Compression [ISO/IEC 14495-1]

• ISO_15444_1 JPEG 2000 Irreversible Compression [ISO/IEC 15444-1]

All of the above terms are to be stored in this tag as a simple string (CS Codes String DICOM value representation allows only 16 uppercase characters or numbers here).