# Lookup Tables

There are 4 types of lookup table (LUT) which can be found **within** DICOM images are part of the standard, and one further type which exists in DicomObjects.

## Palette Colour LUTs

These are only used when the Photometric_Interpretation is PALETTE COLOUR. They can be 8 or 16 bit, and 3 copies exist, one each for red, blue and green.

## Main DICOM LUTs

### Modality LUT

The Modality LUT transformation transforms the manufacturer dependent pixel values into pixel values which are meaningful for the modality and which are manufacturer independent.

In the case of a linear transformation, the Modality LUT is described by the Rescale Slope (0028, 1053) and Rescale Intercept (0028, 1052). In the case of a non-linear transformation, the Modality LUT is described by the Modality LUT Sequence (0028, 3000).

As Modality LUT is a pre-windowing transformation, Image's Window Centre and Width can still be changed after Modality LUT is applied.

### Values of Interest (VOI) LUT

The Value Of Interest(VOI) LUT transformation transforms the modality pixel values into pixel values which are meaningful for the user or the application.

In the case of a linear transformation, the VOI LUT is described by the Window Centre (0028, 1050) and Window Width (0028, 1051). In the case of a non-linear transformation, the VOI LUT is described by the VOI LUT Sequence (0028, 3010).

In general, VOI LUT is the best supported. It serves as a Replacement for Window Centre and Width therefor it is NOT technically allowed to window an image with a VOI LUT applied.

### Presentation LUT

The Presentation LUT transformation transforms the pixel values into P-Values, a device independent perceptually linear space. It may be an identity function if the output of the VOI LUT transformation is in P-Values.

In the case of a linear transformation, the Presentation LUT is described by the Presentation LUT Shape (2050, 0020). In the case of a non-linear transformation, the Presentation LUT is described by the Presentation LUT Sequence (2050,0010).

Please note, **NOT** every Viewer supports all the LUTs. VOI LUT is best supported, as we mentioned above, then comes the Modality LUT and finally the Presentation LUT (This is from our experience). Even if all are individually supported, some viewer may not "chain" them correctly. For those reasons, sometimes it is better to apply any LUTs that your images need into the **Pixel Data** (i.e. replace the pixel data by a post-LUT equivalent).

## DicomObjects ColourTable

This 2D-Array property allows a false-colour lookup table to be applied to a monochrome image. This property is used as a Final LUT when the image is being rendered for any purposes. The first dimension of the array must have lower bound of 1 and upper bound of 3. The second dimension of the array may be any size and may have any maximum value (up to 64K), as both input and output are scaled to fit the required ranges.

An article about Grayscale Transformation Sequences is also available within this knowledgebase.