1. Apparatus for identifying blood vessels in a thermal image of a section of a living body, the apparatus comprising: a convolution unit, for convolving intensity data representing the thermal image with a predetermined vessel shapes filter thereby to provide filtered data; a derivative calculator, for calculating, for each picture-element of said thermal image, calculating at least a first local derivative of said filtered data along a first dimension and a second local derivative of said filtered data along a second dimension, and selecting a highest of said first and said second local derivatives, thereby providing derivative data; a local intensity extrema searcher, for searching in said derivative data for local intensity extrema; and an interpolator, for applying an interpolation procedure for generating contours between at least a portion of said local intensity extrema, and identifying said contours as blood vessels.