The Threshold is a common parameter in the preprocessing settings of most models. It serves for background masking during the pixel-wise calculations.
The masking criterion is based on the histogram of the integrated signal energy (squared TAC values), assuming that pixels with high uptake are the target. The specified percentage defines what fraction of the image pixels are excluded by removing the lower part in the histogram. As an example, a 30% threshold means that the 70% pixels in the upper range of the histogram are retained.
During pixelwise calculation, excluded pixels are not process and are represented by a NaN value in the parametric maps.
Note: In the presence of an explicit mask image, threshold-based masking is not applied.