Choosing the threshold values for hysteresis
Such image statistics as mean, std etc. are not sufficient to answer the question, and canny may not be the best approach; it all depends on characteristics of the image. To learn about those characteristics and approaches, you may google for a survey of image segmentation / edge detection methods. And this kind of problems often involve some pre-processing and post-processing steps.
I'm trying to choose the best parameters for the hysteresis phase in the canny function of OpenCV. I found some similar questions in stackoverflow but they didn't solve my problem. So far I've found that there are two main approaches:

Compute mean and standard deviation and set the thresholds as: lowT = mean - std, highT = mean+std
Compute the median and set the thresholds as: 0.6*median, 1.33*median

However, any of these thresholds is the best fit for my data. Manually, I've found that lowT=100, highT=150 are the best values. The data (gray-scale image) has the following properties:
median=202.0, mean=206.6283375, standard deviation = 35.7482520742
Does anyvbody know where is the problem? or knows where can I found more information about this?