Apparatus and methods are described for receiving into at least one processor at least one image of blood vessels of a subject. Using image processing, discontinuous lines that correspond to paths of at least a portion of the blood vessels in the image are automatically derived, the lines having end points at discontinuities in the lines. Continuous lines that correspond to the paths of the portion of the blood vessels in the image are automatically generated by bridging the discontinuities in the discontinuous lines. An output is generated by the processor, based on the continuous lines. Other embodiments are also described.