how to extract black lines form images

[Rinto Anugraha]

Hello

I have 16bit grayscale images, and in that image there are some black lines.
How to extract black lines from the image? In other word, how to know the
property of that black lines in the image? For example, their length.
Thank you.

Best wishes,

Rinto Anugraha
Kyushu University, JAPAN