1. A system for analyzing medical images, said system comprising: a set of markers visible in the medical images, which markers are located upon a surgical tool and have known spatial relationships between them; receiving circuitry adapted to receive a given medical image, which given medical image includes the tool and an organ of a subject; image processing circuitry adapted to: (a) identify said markers in the given medical image; (b) estimate a dimension of a spatial relationship between the tool, or a point on the tool, and the organ, based on the locations of the markers in the given medical image.