Color and DICOM: What Does DICOM Standard Say?
All display vendors know it. Grayscale is out and color is in. Why?
Because the technical development in LCD displays and increasing output of LCD panels (even driven from the TV market) make shrink the advantage of the grayscale display. Color LCDs today offer high resolution, high luminance and high contrast ratios. And obviously who wants to be limited to shades of gray when there is so much more to see on a PACS workstation? Color overlays have been an integral part of Doppler flow imaging in ultrasound for decades, and most nuclear medicine images are better viewed using color scales. The need for color display in radiology images has only increased with the growth of PET-CT imaging, functional MRI, and computer-aided detection (CAD). The use of color in graphical user interfaces—things like buttons, scroll bars, and menus—makes them a lot easier to see and understand how to use. Color displays make so much sense, but how does the DICOM standard handle color? And how does one calibrate a color display?
The DICOM standard has adressed color and DICOM in the supplement 100 and defines that it uses ICC profiles to describe color spaces.
Supplement 100 states in part:
"In addition to the storage of presentation states and the definition of a color pixel transformation pipeline, an important objective of this supplement is color-rendering consistency. This is achieved by methods similar to those used for grayscale consistency. A device-independent color space is specified, and the output of the color pixel transformation pipeline is defined in that space. Since achieving device-independent representation of color is a non-trivial problem, and one that has already been addressed by the color printing and photographic industries, existing industry standard mechanisms are re-used. Specifically, rather than directly encoding color pixel values in a deviceindependent space, they are instead defined by reference to an embedded color profile, as defined by the International Color Consortium (ICC). These ICC profiles define the transformation from the device-dependent space into the device-independent Profile Connection Space (PCS) defined by the ICC. As it happens, this PCS is the CIEXYZ or CIELAB space, though the ICC’s choice of space is not exposed directly to the DICOM encoding. The use of the ICC profiles avoids the need to define a DICOM-specific device-independent color space, and leverages the large installed base of support for ICC profiles in existing operating systems and non-medical image handling tools. The choice of ICC profiles over the simpler approach of requiring all images to be in the sRGB space was made because the gamut of the sRGB space is too constrained to cover many real world medical images. The term Profile Connection Space Value (PCS-Value) is introduced to refer to the device-independent color values for the purpose of describing the color pixel transformation pipeline, analogous to the P-Value for the grayscale pixel transformation pipeline. Every DICOM color presentation state instance contains an ICC profile. In addition, an optional ICC profile is added to all existing color image IODs. This allows the embedding of color consistency information without the need for separate presentation states. It is expected that all new color image IODs will make this information mandatory, just as all grayscale image IODs added since the GSDF was defined have had their output defined in P-Values."
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QUBYX took note of this new requirement and integrated ICC profile generation into the medical display calibration software PerfectLum. Users can calibrate to DICOM GSDF or CIE L luminance response, at the same time correct color temperature over the entire dynamic range and create an ICC profile for the display.
Developers who want to add ICC profile support to their imaging product can contact QUBYX to discuss how QUBYX can help for integration.