Perfusion imaging plays a crucial role in the diagnosis of acute cerebral infarction, particularly in the evaluation of the area of penumbra. However, two aspects must be considered with regard to the display of color-coded perfusion parameter maps.

Firstly, there is a wide variation among workstations and vendors with regard to the look-up table (LUT), that correlate parameter value with color code. Generally, a reddish color is assigned to a higher value while a bluish color is assigned to lower value, however, it appears that there is no common system for assigning colors for intermediate values.

Secondly, the appearance of the parameter maps is highly dependent on the window width (WW) and window level (WL) settings; this aspect is similar to that observed in the DWI display. An inappropriate WW/WL setting may lead to erroneous visual interpretation of the hypoperfused area, resulting in incorrect assessment of the penumbra area.

Standard normalization regarding these two aspects is mandatory in order to make a reliable comparison of perfusion maps obtained from different scanners, workstations, or institutions.

ASIST-Japan proposes a standard LUT (a-LUT) as shown below; this a-LUT incorporates the common denominator of the existing LUTs to the maximum extent, but with some modification, so that the difference in the intermediate parameter values can be easily identified. The implementation of a-LUT has already begun on a commercial scale in the workstations of several vendors. a-LUT can be downloaded from here．

ASIST-Japan also proposes a novel, and simple method to normalize the parameter map by changing the WW/WL setting using the region of interest (ROI) placed in the contralateral cerebral hemisphere [2]. That is,