Lookup Tables (LUT)

How to handle Color and DICOM LUTs

A Lookup Table (LUT) maps each input pixel value to an output value used somewhere along the rendering pipeline — turning raw acquisition numbers into the contrast, the color, and the brightness you actually see on screen. The DICOM rendering pipeline chains four kinds of LUT, each applied at a different stage:

1. Modality LUT — converts raw pixel values into physical modality values (e.g. Hounsfield units for CT).
2. Values of Interest (VOI) LUT — maps the modality values into a visible intensity range, enhancing specific anatomical structures or pathological conditions (this is what the Window / Level controls).
3. Presentation LUT — converts the intensity values into P-Values: device-independent values calibrated to human perception (used to render consistently across monitors).
4. Palette Color LUT — replaces grayscale intensity values with colors, producing a pseudo-color rendering.

Windowing and Rendering

Windowing and Rendering is a panel in Image Tools of the 2D viewer, exposing the user-controllable parts of the LUT pipeline. The same controls are also accessible from the Lookup Table toolbar, the main menu, and the right-click menu.

• Window — width of the input range mapped to the displayed values. Change it via the slider, or by selecting Window / Level in the mouse actions.
• Level — center of the range defined by Window. Same input options as above.
• Preset — picks a predefined Window / Level pair. The dropdown is ordered as:
  • An empty entry, shown while Window and Level are being adjusted manually (slider or mouse drag).
  • Window / Level pairs or VOI LUT data carried by the DICOM file (suffixed with [DICOM]). The default selection is the first [DICOM] entry when present, otherwise Auto Level [Image].
  • Auto Level [Image] (always shown) — Window and Level computed from the full pixel-value range of the current image.
  • Modality-specific presets (e.g. Lung, Bone, Soft Tissue for CT).
• LUT Shape — the transfer function used between input and display: linear (default), sigmoid, or logarithmic.
• LUT — pseudo-color LUT applied on top of the grayscale image. Default (image) keeps the original image color model. Picking a LUT from the toolbar or the menus is usually easier because each entry is shown with a preview.
• Invert LUT — flips the LUT direction (dark↔bright, or reversed color mapping).
• Filter — 2D image filter applied before the LUT, useful for enhancing image quality or highlighting specific structures. Default: None.