Segmentation of thin vessels

Thank you for the suggestion. Can you give me tips for smaller vessels? Some arteries are very thin and hypoplastic. More automated methods tend to miss these types of vessels.

You can use Grow from seeds effect for smaller vessels, too, you just need to paint more seeds: with your initial seeds some vessel segments will be missed, so you paint a few more, which will result in some noise to be detected as vessels, so you need to paint scribbles there with the “non-vessel” segment, and do this iteratively until you get all the vessels you need, without noise.

If the vessels are very small then you can simplify segmentation by doing subtraction. See this segmentation recipe.