Probably you want to increase “Variance” value to reduce the number of contours. For example try something like 2.0, 2.0, 2.0.
Canny edge detection result with default parameters for MRHead sample data set:
After setting Variance to 2.0, 2.0, 2.0:
After setting Upper threshold to 10:
After choosing the grayscale volume as background, Canny output as foreground, foreground opacity to 0.5, and in Volumes module setting 0.5 threshold and a color lookup table:
