VOI LUTs are non-linear look-up tables used to display the image. If one is defined by the imaging modality, the VOI LUT is added to the preset window/level list, preceded by '#'. Since the VOI LUTs can be non-linear, changing the W/L settings of an image with a VOI LUT applied forces the curve to be linear.
To configure the viewer to ignore VOI LUTs when they exist in an image, open the Customize Settings panel from the Settings menu and select the Image tab. For each modality type or for the Default modality type, clear the check mark from the box labeled Apply VOI LUT. Existing VOI LUTs are available from the Window/Level menu in the Window/Level toolbar, but are not automatically applied.