Extract feature area from image
It is going to be a tough task, and you may not achieve an automated process.

Here are some bricks that you could try:


Enhance the contrast of the image so that you can clearly distinguish all the features by looking at the image. Here is a link to an algorithm that shows good results.
In this enhanced image, extract the edges (using for example a standard Canny-Deriche edge detector).
From the edge detector output, you can start adding the required application logic in order to keep the desired vessels: size or length criteria, linear features extraction, etc.