<p>I worked on retina vessel detection for a bit few years ago, and there are different ways to do it:</p>

<ul>
<li>If you don't need a top result but something fast, you can use oriented openings, <a href="https://www.researchgate.net/publication/3327391_Segmentation_of_vessel-like_patterns_using_mathematical_morphology_and_curvature_evaluation" rel="nofollow">see here</a> and <a href="http://cmm.ensmp.fr/Anciens/zana/" rel="nofollow">here</a>.</li>
<li>Then you have an other version using mathematical morphology <a href="https://www.google.com/url?sa=t&amp;rct=j&amp;q=&amp;esrc=s&amp;source=web&amp;cd=1&amp;ved=0ahUKEwiJ1qDb-cXMAhUMOsAKHaXXBcoQFggkMAA&amp;url=http%3A%2F%2Fcmm.ensmp.fr%2F~walter%2Farticles_walter%2Fwalterklein.pdf.gz&amp;usg=AFQjCNG3e1Ueke67eY9JEw6ub00Y8roQ-A&amp;sig2=dzVYD_5_2BKMaOQpRLjjOQ&amp;cad=rja" rel="nofollow">version here</a>.</li>
</ul>

<p>For better results, here are some ideas:</p>

<ul>
<li>Personally, I used combination of Gabor filters, and results where pretty good. See <a href="http://www.thibault.biz/StackOverflow/RetinaVesselsGabor.png" rel="nofollow">the segmentation result here on the first image of drive</a>.</li>
<li>And <a href="https://arxiv.org/pdf/cs/0510001.pdf" rel="nofollow">Gabor can be combined with learning for a good result</a>, or <a href="http://www.icmlc.org/icmlc2011/014_icmlc2011.pdf" rel="nofollow">here</a>.</li>
<li>Few years ago, <a href="https://www.researchgate.net/publication/5896442_Retinal_Blood_Vessel_Segmentation_Using_Line_Operators_and_Support_Vector_Classification" rel="nofollow">they claimed to have the best algorithm</a>, but I've never had the opportunity to test it. I was sceptic about the performance gap and the way they thresholded the line detector results, it was kind of obscure.</li>
<li>But I know that nowadays, many people try to tackle the problem using CNN, but I've not heard about significant improvements.</li>
</ul>

<p>[EDIT] To answer your specific question, you can erase the bright ring, and then apply a histogram stretching. But I think that the methods I introduced before will work better than the filter you are using.</p>
Having extracted the blood vessels, how to brighten them?
<p>I've used Kirsch filter to try and obtain the blood vessels, but the result isn't the best, as shown below:</p>

<p><a href="https://i.stack.imgur.com/ATcPV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ATcPV.jpg" alt="enter image description here"></a></p>

<p>Although the vessels have been obtained, they aren't bright enough. How do I go about making them 'more visible'? </p>